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                 OMNI 2 Preparation 
Joe King and Natalia Papitashvili, NASA/SPDF and ADNET Systems, Inc.


Abstract: This file describes the OMNI 2 data set. It identifies the data sources, edits and time shifts, cross regressions, normalizations, selection prioritization, etc. It identifies access paths to the OMNI 2 data set and to the multiple input data sets.It was most recently updated February, 2013


[Note: a complementary OMNI 2 documentation file, addressing time spans of various data types, a format description of the OMNI 2 data records, 1- and 27-day averaged OMNI 2 versions, and other details is found at]

OMNI 2 was created in 2003 as a successor to the "old OMNI" data set first created at the National Space Science Data Center (NSSDC) in the mid-1970's. OMNI 2 was then maintained and extended by NSSDC for several years after which its maintenance and evolution were taken over by the Space Physice Data Facility at Goddard. The OMNI data set is an hourly resolution multi-source data set now spanning the period November, 1963 (IMP 1 launch) to May, 2003. The primary data of OMNI are near-Earth solar wind magnetic field and plasma parameters. Also included are 1973-2001 energetic particle data from IMP 8 and sunspot number and geomagnetic (Dst, Kp, C9) activity indices. AE indices have replaced C9 indices in OMNI 2, and 1967-1972 IMP 4 and 5 particle data have been added.

[Note added 2008] In 2007-2008, we added Geotail solar wind data for 1995-2005 to High Resolution OMNI and then computed hourly averages and inserted them into hourly OMNI 2 for hours for which we did not already have IMP 8, Wind or ACE data. Our preparatory work with the Geotail data is extensively discussed in the HRO documentation file at and will not be repeated here. Only minimal annotations to reflect Geotail data in OMNI 2 are made to this file that was initially created some years ago.]

The solar wind data are from 15 geocentric spacecraft and from 3 spacecraft (ISEE 3, Wind, ACE) typically an hour (as the solar wind flows) upstream from the Earth. The spacecraft, identified below, are mainly NASA spacecraft, although US/DOD (Vela), ESA (HEOS 1 and 2) and Soviet (Prognoz 10) spacecraft also contribute. A priority scheme was established for selecting which spacecraft's data would be used for hours with data from multiple spacecraft. Data from the 3 upstream spacecraft were time- shifted at higher resolution (1-5 min) to Earth and hourly averages then computed in "Earth time" for inclusion in OMNI. The time shift assumed solar wind variation phase fronts were planar and, for old OMNI, that those planes were normal to the ecliptic and aligned with the ideal (Parker) interplanetary (solar wind) magnetic field (IMF).

It is useful to consider early, middle and later periods of OMNI and OMNI 2. The early period, 1963 to 1971, saw the availability of most spacecraft per year. The middle period, 1971 to 1994, was dominated by IMP 8. The later period, 1994 to current, is marked by Wind, ACE and IMP 8. There was virtually no data overlap between the early and middle periods, and the early-period OMNI 2 data are virtually unchanged from the early-period OMNI data.

Extensive cross comparisons of overlapping data sets were made in creating old OMNI. It was determined to normalize plasma densities (N) and temperatures (T) to achieve better uniformity across the multi-source data being interspersed. Post-1971 plasma speeds (V) and IMF values for all times (with the exception of the Prognoz 10 Bx component) did not require normalization. Since the distributions of solar wind densities and temperatures tend to be more log-normal than normal, cross-regressions of density data from one spacecraft (j) against another (k) used the form log N(k) = a(k,j) + b(k,j) * log N(j) and likewise for temperature data. Normalization of N and T values used the same form: log N(norm) = a(j) + b(j) * log N(obsvd by jth s/c). All plasma N and T data from the middle and later periods (since the 1971 IMP 6 launch) were somewhat arbitrarily normalized to Los Alamos National Laboratory (LANL) IMP 7 data in old OMNI. The early- period data were normalized to Explorer 33 (N and T) or to Vela (V), although the first source (IMP 1) overlapped with no other source so its data could not be normalized.

To a very large extent, OMNI was built with the hourly-averaged data provided by PI's after those PI's (and their colleagues) had made judgements about which of their data were, and were not, taken in the solar wind beyond the Earth's bow shock. Only ISEE 3 hourly averages were built at NSSDC prior to the mid-1990's. Software tools were not so readily available in OMNI's early years to identify and eliminate magnetosheath- contaminated data or other bad data.

Much detail about the sources and preparation of the OMNI data set is found at the OMNIWeb site at

OMNI 2 represents a significant advance over old OMNI. The types of data and the hourly resolution of OMNI 2 are basically the same as for OMNI. However OMNI 2 adds solar wind alpha-to-proton density ratios for middle and later periods, the Auroral Electrojet (AE) index, and computed values of solar wind flow pressure, electric field, Alfven Mach number and plasma beta. These computed values are included since the OMNIWeb interface now allows selecting data records based on user-specified numeric ranges of any combination of OMNI's parameters, so users may find intervals of special interest using these new parameters.

Software tools have been used in the building of OMNI 2 to find and eliminate unwanted (e.g., magnetosheath-contaminated) points. Close interactions with data providers have been the rule during this process. Therefore OMNI 2 will be even cleaner than old OMNI, although we cannot guarantee the total absence of inappropriate points.

New normalizations for post-1971 density and temperature values to Wind/SWE values have been made, yielding in OMNI 2 changed and hopefully more reliable absolute values of those parameters, from which are computed parameters like solar wind pressure that affects magnetospheric and heliospheric sizes.

A new approach to time shifting of ISEE 3, Wind and ACE data has been used. This assumes solar wind variation phase fronts are still planar and normal to the ecliptic, but they intersect the ecliptic halfway between the ideal IMF spiral angle and the normal to the Sun-Earth line.

The following sections address: the data sources; the checking and cleaning of input data sets; the time shifting of ISEE 3, Wind and ACE data; the data set intercomparisons; the data set normalizations; the selection prioritizations for multi-source hours. A separate file with current time spans for each parameter, with a detailed format statement (what words, what sequence, what Fx.y representations) in the hourly ASCII records of OMNI 2, and with a discussion of the creation of daily and 27-day averaged versions of OMNI 2, is available at

It should be noted that 1-min and 5-min version of OMNI are now described at and accessible from

The Data Sources

In this section we address, in turn, the sources of IMF data, plasma data, energetic particle data, and solar and geomagnetic activity indices used in OMNI 2.

IMF data sources: Table 1 shows the sources of magnetic field data. The parentheses after the spacecraft names contain the value used in the IMF spacecraft ID field of the OMNI 2 records. Some parentheses also contain an "Exx." These are the corresponding Explorer numbers and may be useful in cross referencing. The PI column gives the Principal Investigator or other lead person. Key additional scientists for recent spacecraft are A. Szabo (IMP 8, Wind), R. Lepping (IMP 8) and C. Smith (ACE). The time span column of Table 1 gives the time range of data included in OMNI 2; it may be less than the time range over which the instrument produced useful data.

As of 2009, Wind and ACE data were being periodically added to OMNI. Information on current latest date of plasma data in OMNI is given at

All instruments were boom-mounted triaxial fluxgate magnetometers except those on IMP's 1 and 3 which were biaxial but flippable (spacecraft spin plus sensor orientation yielded three dimensional vectors) and that on ISEE 3 which was a vector helium magnetometer. Most older data were received as hourly averages of IMF intensities and GSE Cartesian components from the Principal Investigator (PI) teams. For old OMNI, GSM components transformed from, and the magnitude and GSE direction angles of, the IMF vector formed from the hourly GSE Cartesian component averages, were determined at NSSDC or at the data provider's site. For OMNI 2, GSM components were computed at NSSDC from hourly averaged GSE components using the GSM-to-GSE transformations of the Tsyganenko Geopack software package and using time tags at the midpoints of the hours. (Only for 1963-4 IMP 1 data was this not the case since the Geopack code is only relevant to 1965 and forward.) For ISEE 3, Wind and ACE, NSSDC computed hourly averages from time-shifted higher resolution data as explained later in this text.

Standard deviations were provided to NSSDC in Cartesian components and field intensities, except that with the HEOS data standard deviations in the field magnitude and direction angles were provided. These were inserted into the words for standard deviations in Cartesian components in the records of OMNI to which HEOS contributed IMF data. For OMNI 2, we have computed from the HEOS-provided data values of standard deviations of Cartesian components and included these in OMNI 2 instead of the OMNI- included standard deviations in angles.

Table 1

Spacecraft	PI		Time span		Reference

IMP 1 (18, E18)	Ness		11/27/63-02/15/64	Ness et al, 1964
IMP 3 (28, E28)	Ness		06/01/65-01/29/67	Ness et al, 1964
AIMP 1 (33,E33)	Ness		07/04/66-07/13/68	Behannon et al, 1968
IMP 4 (34, E34)	Ness		05/26/67-12/27/68	Fairfield, 1969
AIMP 2 (35,E35)	Ness		07/26/67-11/10/69	Ness et al, 1967
HEOS (1)	Hedgecock	12/11/68-10/28/75	Hedgecock, 1975
IMP 5 (41, E41)	Ness		06/21/69-10/26/72	Fairfield & Ness, 1972
IMP 6 (41, E43)	Ness		03/14/71-07/21/74	Fairfield, 1974
IMP 7 (47, E47)	Ness		09/26/72-04/03/73	Mish & Lepping, 1976
IMP 8 (50, E50)	Ness		10/30/73-05/12/00	Mish & Lepping, 1976
ISEE 3 (13, E59) E.Smith	08/14/78-12/21/82	Frandsen et al, 1978
Prognoz10 (10)  Yeroshenko	04/27/85-11/04/85	Styazhkin et al, 1985
Wind (51)       Lepping		11/21/94-current       	Lepping et al, 1995
ACE (71, E71)   Ness		02/06/98-current	Smith et al, 1998.
Geotail (60)    Nagai          1995/128 - 1997/272

For the past many years, the only IMF data included in OMNI have been from the IMP 8, 
Wind and ACE spacecraft, launched in 1973, 1994 and 1997, respectively.  The IMP 8 
magnetometer failed in June, 2000, after almost 27 years of operations.  The Wind and 
ACE magnetometers remain operational in February 2013. The web pages of the IMP 8, Wind
and ACE magnetometer teams are to be found at:
IMP 8:

Hourly and higher resolution versions of data from these magnetometers are available 
via multiple pathways, mostly cited on the foregoing web pages.  In particular, NSSDC 
makes files ftp-accessible from and makes 
data accessible with graphical display and screen lists with subsetting by parameter 
and by time via CDAWeb and via FTPBrowser at and at

Of special relevance to OMNI 2 preparation is an FTPBrowser-accessible merged hourly 
IMP8-Wind-ACE IMF data set at 
from which one may make overlapping time-series plots.  A second interface at enables one to make scatter plots 
and linear regression fits of user-selected parameter pairs and time span.  Results of
analyses of these data, with this latter tool, are reported below.
A third interface at enables one to 
determine distributions, means and their standard deviations, and medians of any IMF
parameter from any of the spacecraft named, for any time span. 

In October 2011, the Wind/MFI team finished the reprocessing of
all MFI data.  Among other things, well-determined Bz offset values
were used.  The new MFI data were inserted into OMNI 2 when
they became available, replacing the earlier MFI data.

Plasma data sources: Most of the solar wind plasma data used in OMNI 2 are from the MIT Faraday Cups (cf. PI = Bridge in Table 2) or the Los Alamos National Laboratory (LANL) electrostatic analyzers (cf. PI = Bame in Table 2). Table 2 shows the data sources in some detail. The data were mainly provided to NSSDC and used in OMNI as 1-hour averages. Exceptions are (1) the IMP 1, Vela and HEOS data which were provided as 3-hour averages and were assigned to each of three successive one-hour records in OMNI, (2) the ISEE 3, Wind and ACE data whose 1-hour averages were computed at NSSDC from time-shifted higher-resolution data and (3) the LANL IMP 6, 7 and 8 data whose 1-hour averages were computed at NSSDC from higher-resolution data.

As of 2009, Wind and ACE data were being periodically added to OMNI. Information on current latest date of plasma data in OMNI is given at

It should be noted that A.J. Lazarus and J.C. Kasper of MIT contributed the Wind/SWE ion measurements and data processing and analysis thereof. The ACE/SWEPAM instrument was one of the series of LANL electrostatic analyzers to whose success R. Skoug contributed significantly. Additional key scientists contributing to IMP 8 plasma data are have been J. Gosling and J. Steinberg at LANL and A. Lazarus, K. Paularena and J. Richardson at MIT.

Table 2

Spacecraft	PI		Time span		Comments, Reference 

IMP 1 (18)	Bridge		11/27/63-02/22/64	Bridge et al, 1965
Merged Vela (97) Bame	07/21/64-03/18/71	Bame et al, 1971
Vela 3 (3)	Bame		07/26/65-11/13/67	Hundhausen et al, 1967
AIMP 1 (33)	Bridge		07/06/66-09/23/69	Lyon et al, 1968
IMP 4 (34)	Ogilvie	06/03/67-12/16/67	Ogolvie et al, 1968
AIMP 2 (35)	Bridge		07/28/67-07/03/68	Lyon et al, 1967
OGO 5 (5)	Neugebauer	03/05/68-04/29/71	Neugebauer, 1970
HEOS 1 (1)	Bonetti	12/11/68-04/15/70	Bonetti et al, 1969
IMP 6 (43)	Bame		03/18/71-07/21/74	Feldman et al, 1973	
IMP 7 (44)	Bame		10/06/72-09/29/78	Asbridge et al, 1976
IMP 7 (47)	Bridge		01/03/75-09/20/78	Lazarus et al, 1998
IMP 8 (45)	Bame		11/04/73-07/16/00	Asbridge et al, 1976
IMP 8 (50)	Bridge		12/05/73-07/26/01	Lazarus et al, 1998
ISEE 1 (11)	Bame		10/30/77-12/19/79	Bame et al, 1978b
ISEE 3 (13)	Bame		08/16/78-10/12/82 	Bame et al, 1978a
Wind (51)	Ogilvie		01/01/95-       	Kasper, 2002
ACE (71)	McComas		02/05/98-		McComas et al, 1998
Geotail (60)  L.Frank        1995/128 - 2004/341

[2008: As of 2008, the Wind/SWE non-linear fits-based proton
parameters (cf. below) had not been determined past late November,
2004.  Accordingly we have used in OMNI 2 a cleaned version of the
Wind/SWE key parameter data set for later times.  See the
documentation of thisdata set in the High Resolution OMNI
documentation file at]

The plasma parameters included in the early-period OMNI-input data sets (i.e., the 
first 8 data sets of Table 2) are identified in the original OMNI documentation, 
available through OMNIWeb, and will not be repeated here.  For the middle and later 
periods, we have used the following in OMNI 2 from the various input data sets:

			N	V	T      phi-V    theta-V   alpha/prot

IMP 6 			x	x	x	x		    x
IMP7 (LANL)		x	x	x	x		    x
IMP7 (MIT)			x
IMP8 (LANL)		x	x	x	x		    x
IMP8 (MIT)		x	x	x	x	x
ISEE 1				x
ISEE3 (protons)		x	x	x	x
ISEE3 (electrons)	x	x
Wind/SWE		x	x	x	x	x	    x
ACE/SWEPAM		x	x	x	x	x 	    x

The parameters provided by the LANL plasma team were determined by taking moments over distribution functions. The parameters provided by the MIT team were determined by making nonlinear fits of convecting Maxwellian distributions (anisotropic bi-Maxwellians for Wind/SWE) to the observed distributions. The references cited above further describe these approaches. As a special situation, both moments-based and fits-based Wind/SWE parameters were provided by MIT. We chose to include in OMNI 2 the fits-based SWE parameters rather than the moments-based parameter values for consistency with using fits-based parameters for other MIT-provided data sets. That OMNI 2 involves the interspersal of LANL-provided moments-based parameters with MIT- provided fits-based parameters will be at least partly compensated for by the cross normalizations of multi-source data (discussed subsequently).

Readers interested in the differences between the two parameter-determination approaches may access or pla_iwa_s2.html or pla_iwa_s3.html to compare Wind/SWE hourly averages based on time- shifted moments-based 92-s parameters with equivalent fits-based Wind/SWE hourly averages. Figure 1 (V), Figure 2 (N) and Figure 3 (T) show scatter plots and linear regression fits for Wind/SWE parameter pairs. In general, flow speeds are the same to within a percent or so independent of parameter determination approach, while for both densities and temperatures the moments-based parameters are greater than or equal to the fits-based parameter values.

The LANL plasma instruments on ISEE 3 measured ions and electrons separately. The ion instrument failed February 19, 1980. Electron-based flow speeds and densities were used in OMNI 2 after the ion instrument failure until late 1982, but neither electron- based temperatures nor flow direction angles were included into OMNI 2. (This was also done for OMNI years ago.) It should be noted that on two days (July 4,1979, and July 31, 1979), ISEE 3 measured densities so low that for each of several hours, the hourly averaged density was less that 0.05/cc. Given OMNI's use of F6.1 format for densities, this yields an apparent density of 0.0 in OMNI for these few hours.

Only flow speeds are provided from IMP 7 (MIT) and from ISEE 1. In the former case, this limitation to flow speed was at the suggestion of A. Lazarus at MIT. In the case of ISEE 1, there were too few hours (<240) when ISEE 1 data were used in OMNI 2 (only when no other data were available) to prioritize doing new density and temperature normalizations. These may yet be done.

Web pages of the primary contributors of recent plasma data to OMNI 2 are at:


As with the magnetometer data addressed earlier, hourly and higher resolution plasma data are available via multiple pathways, mostly cited on the foregoing web pages. They are available via ftp from and with display and subsetting capabilities via CDAWeb and/or FTPBrowser at and

Several multiple-source hourly data sets were created at NSSDC to aid in data checking and cross-normalization. These are discussed in the "Data cleaning" section to follow and at

For completeness, we mention two additional adjustments we might have made to OMNI plasma densities but did not. First, for values shifted from ~200 Re (~0.01 AU) upstream to Earth (for ISEE 3, ACE and some Wind data as discussed in detail later), we might have multiplied by 0.98 to reflect the fact that densities are expected to fall as the inverse square of heliocentric distance, on average. Second, we might have normalized to 1.00 AU by allowing for the non-circularity of the Earth's annual orbit about the sun. Note that Earth is at about 1.017 AU mid-year each year and at about 0.983 near the start and end of the year. This would contribute a year-end density at Earth ~7% higher than at mid-year. (M. Collier, GSFC, private communication, reports higher long-term-averaged OMNI densities for October-January than for the rest of the year.)

We neglect the first ~2% effect because it is small, and we neglect the second ~7% effect because the objective of OMNI is to reflect the state of the solar wind just outside the bow shock in support of solar wind - magnetosphere coupling studies more so than to support strictly solar wind studies.

Energetic particle data sources: Fluxes of protons above 6 energy thresholds (1, 2, 4, 10, 30, 60 Mev) from the IMP 7 and IMP 8 Charged Particle Measurement Experiment (CPME; Principal Investigator S.M. Krimigis, then R.B. Decker) are included in OMNI and OMNI 2 for the period January 1, 1973, through the end of 2005 shortly after which IMP 8 operations terminated. The data were prepared and provided by CPME Co-Investigator T.P. Armstrong and colleagues at U. Kansas and Fundamental Technologies, LLC. The instrument and data are further described at and at

Fluxes of protons above 1, 10, 30 and 60 MeV for mid-1967 through the end of 1972, from the JHU/APL Solar Proton Monitoring Experiment (SPME) on IMP 4 (1967/150 - 1969/123) and IMP 5 (1969/172 - 1972/358) were added to OMNI 2 shortly after its creation. The fluxes were computed at NSSDC from count rates provided on tape to NSSDC decades earlier. The values are not reliable absolute measures of quiet time galactic fluxes, but are good for solar and shock- accelerated particles. cf. Williams and Bostrom, J. Geophys. Res.,74, 3019, 1969.

Fluxes of protons above 10, 30 and 60 MeV, as measured by NOAA's geosynchronous GOES 11 spacecraft for 2006-2010 (cf. and from GOES 13 for 2011 and later (cf., were added to OMNI 2. (The GOES 13 data added to OMNI 2 are actually averages over the fluxes given at NGDC for eastward-looking and westward looking sensors.) Principal Investigator for the GOES energetic particle instruments is currently T. Onsager, and key responsible NGDC person is D. Wilkinson. Comparisons of IMP 8, GOES 10 and GOES 11 proton flux values obtained between 1999 and 2005 show reasonably good agreement during solar particle flux events; see

Solar and geomagnetic activity indices: OMNI 2 contains daily sunspot numbers (Rz) assigned to each hour of the relevant day, hourly AE and Dst indices and 3-hourly Kp values assigned to each hour of the relevant 3-hour interval. Note that, in the OMNI- to-OMNI-2 transition, AE is added and C9 is deleted. (For Kp whose normal values are of the form 1+, 2-, 2, 2+..., we have used 13, 17, 20, 23,....)

The AE and Dst indices are computed at and obtained from the World Data Center for Geomagnetism, operated by the Data Analysis Center for Geomagnetism and Space Magnetism at Kyoto University, Japan. See Definitive time series of AE, Dst and other indices reach back to 1957 on the Kyoto web site. There are provisional values of AE and Dst also available from Kyoto for periods after the definitive time series that end in mid-1988 (AE) and mid-2002 (Dst). Definitive hourly values of AE and Dst are included in OMNI 2 from 1963 to their ends and are extended when possible. Provisional Dst values are included in OMNI 2 as a continuation of our practice started with OMNI years ago. Given their ready accessibility from Kyoto, we have not included the provisional AE values in OMNI 2. We thank Drs. T. Iyemori and T. Kamei for permission to include these indices in OMNI 2.

The Rz and Kp values are downloaded from NOAA's National Geophysical Data Center. See

Cleaning of source data

It is desirable that OMNI 2 data be as free of "bad data" as possible. Extensive checking of 1971-current magnetic field and plasma data was carried out at NSSDC as part of creating the OMNI 2 data set. Several web-based tools were created and used to review input data sets individually and as compared to each other. Creation and use of such tools was not feasible in OMNI's infancy nearly 30 years ago.

There are two sources of "bad data" in OMNI 2. One is "noise points" in constituent data sets that may arise from transient instrument malfunction or, in the case of plasma parameters, from the time variation of plasma during the accumulation of one distribution function whose subsequent analysis for determining bulk parameters yields meaningless values. Such noise points typically yield single-point upward or downward spikes in parameter profiles. While instrument teams have removed most such points in the data they provided to NSSDC, it has been beneficial to seek out such points here.

The other main source of bad data in OMNI 2 is the inappropriate inclusion of magnetosheath field or plasma data in a data set intended as solarwind-only. This is more significant, the more times the source spacecraft crosses the Earth's bow shock. Thus it is insignificant for ACE which went into an upstream libration point orbit immediately after launch, very significant for IMP 8 in its ~12-day geocentric orbit, and significant to an intermediate extent for Wind with its more complex and time- varying orbit.

The data sets provided to NSSDC and used in OMNI and OMNI 2 were nominally for solar wind periods only. One exception was that, since about 1994 and by agreement between NSSDC and the IMP 8 magnetometer team, IMP 8 magnetic field hourly data were time continuous (exclusive of data gaps that were independent of orbit phase). These were filtered at NSSDC by MIT's plasma-data-based identification of solar wind intervals prior to adding to OMNI.

Magnetosheath-contaminated magnetic field data typically has higher field intensity and variance levels than concurrent data from a nearby spacecraft solely in the solar wind. Magnetosheath-contaminated plasma data typically shows lower flow speed, higher density and temperature and flow direction significantly further from helio-radial than concurrent data from a nearby solarwind-only spacecraft. These signatures are not equally clear in all cases. Further, most of these changes seen upon crossing the bow shock may also be seen upon crossing solar wind structures, especially interplanetary shocks. Thus, they are not always unambiguous markers of bow shock crossings.

Many dataset-specific browse interfaces to OMNI 2-contributing high-resolution data sets from IMP 8, ISEE 3, Wind and ACE are available at These were typically used in dataset cleaning after suspicious hourly averages were found with some of the tools below.

Among the tools developed and used for concurrently screening multi-source data and
summarized at were: for plotting magnetic field intensity 
or components from IMP 8, Wind and ACE for 1994-2000; for plotting plasma parameters and/or 
variances from any pair of the 5 data sets IMP8/MIT, IMP8/LANL, Wind/SWE (fits-
based), Wind/SWE (moments-based) or ACE/SWEPAM for 1995-2001; for plotting plasma parameters 
and/or variances from IMP8/MIT and IMP8/LANL for 1973-2001; for plotting plasma parameters and/or 
variances from IMP8/MIT, IMP8/LANL and ISEE 3 for 1978-1982; for plotting plasma parameters 
and/or variances from IMP6/LANL, IMP7/LANL and IMP8/LANL for 1971-1978

These tools all work with time-shifted (see below) hourly-averaged data. They yield plots with one panel per physical parameter selected, with color-coded intensity-time profiles from each of the data sources. They make noise points visible and they make intervals visible when the apparent solar wind behavior at multiple sources differs significantly. Some cases of the latter correspond to one source being magnetosheath- contaminated while other cases may correspond to real differences in the solar wind plasma domains seen at the two spacecraft.

Another family of tools was also developed that generates scatter plots of parameter values from various pairs of plasma data sources. (These tools also determine regression fits as will be further discussed in the later data comparison and cross- normalization sections of this paper.) This set of tools makes outliers very visible and has led to identification of several magnetosheath-contaminated data-hours. These tools include for 1994-latest_available IMP 8, Wind, ACE and Geotail magnetic field parameters for 1995-2001 IMP 8, Wind and ACE plasma parameters for 1995-latest_available IMP 8, Wind, ACE and Geotail plasma parameters for 1978-1982 IMP8/MIT, IMP8/LANL and ISEE 3 plasma parameters. for 1997-1998 LANL IMP 6, 7 and 8 plasma parameters.

Replacing the "s2" in these url's with "s3" in the last four of these gives a variant of the pages for working with base-10 logarithms of densities and temperatures rather than with densities and temperatures themselves. These tools allow filtering by values of any of the parameters in the relevant data records, including numbers of fine scale points in the hour-averages. The second of them also allows filtering by the impact parameter (transverse separation distance, see below) between any pair of spacecraft.

For many years, the IMP 8 spacecraft was the only OMNI source, and is the dominant OMNI 2 data source from its late-1973 launch to the mid-1978 ISEE 3 launch and again from the late-1982 departure of ISEE 3 from an L1 orbit until the late-1994 Wind launch. In its 12-day, 35-Re near-circular geocentric orbit, IMP makes at least 2 and frequently 10-20 transitions into and out of the solar wind, across the Earth's time-varying bow shock. To enable a more reliable exclusion of magnetosheath-contaminated IMP 8 field or plasma data, and a more reliable inclusion of interesting solar wind intervals (that might once have been excluded by the magnetic field or plasma teams in their NSSDC submissions of hourly solar wind data as being magnetosheath-contaminated), a major effort was undertaken (with support from a NASA/AISRP grant) by the IMP 8 magnetic field team at GSFC and the plasma team at MIT to jointly study the field and plasma data and to identify and characterizeall IMP 8 bow shock crossings. The fruits of this effort is visible at To date (August, 2003), the effort has been completed for 1977-2000.

The bow shock crossing database was used to create a file flagging IMP 8 hours as corresponding to (1) IMP wholly in the solar wind, (2) IMP making one or more bow shock crossings, (3) other hours. We have used this file along with an early-version OMNI 2 file to identify and delete magnetic field or plasma data when IMP 8 was not wholly in the solar wind. (The exception was that, for the case of LANL plasma data wherein hourly averages were created from ~2-min data previously separated at LANL as being in the solar wind vs. magnetosheath, LANL hourly averages were retained in OMNI 2 for hours in which IMP encountered shock crossings and was therefore partly in the solar wind.)

Further, the (a) bow shock-database-derived, region-flagging file was used with (b) OMNI 2 and with (c) a file of IMP 8 magnetic field hourly averages created at NSSDC from 15-s, all-orbit-phases magnetic field data to find hours when IMP was in the solar wind (as per (a)), there were no IMF data in OMNI (as per (b)), and there were IMP IMF data (in (c)). For such hours, IMP 8 IMF data were added to OMNI 2 from (c). For 1977-2000, there were almost 2300 such hours. An equivalent effort was made to capture extra solar wind plasma data, but only 29 not-previously-included hours were found.

Time-shifting of data

In this section we address why, when and how we time shift data of ~minute resolution before building hourly averages for inclusion in OMNI 2.

Why and when to shift: That most of the source spacecraft contributing to OMNI and OMNI 2 make IMF and plasma observations minutes upstream of the magnetosphere (e.g., <= 15 minutes for the moon-orbiting, late-1960's Explorer 35 spacecraft at ~60 Re) was not factored into the hourly averages interspersed into OMNI, nor is it for OMNI 2. However, the ISEE 3, Wind and ACE spacecraft are frequently or always about an hour upstream of the magnetosphere. As their data are to be interspersed with data from much-closer-to-Earth spacecraft (e.g., IMP 8), it is appropriate to time-shift the hour-upstream data at higher resolution and to compute hourly averages "at Earth" for inclusion in OMNI and OMNI 2. Such shifting has been done for the field and plasma data of these three spacecraft, as described herein.

How to shift: Several factors determine optimal shifts: the geometry of the Earth- spacecraft separation vector; the Earth's orbital motion about the sun between observations upstream and at Earth; the geometry (shape, orientation) of the solar wind variation phase front; the solar wind flow direction; and local propagation of the phase front relative to the mean solar wind. (In the above, "Earth" can be replaced by "second spacecraft" for time shifts made for two-spacecraft comparisons.) Weimer et al (2002) have shown optimal shifts differ significantly from interval to interval, and it is best to analyze the available field and plasma data for each interval to determine its optimal shift. However, for the purpose of shifting many years of upstream data for OMNI 2, we seek a statistically optimal approach. Relative to the above factors, we assume the variation phase fronts are planar, of arbitrarily large extent normal to the Sun-Earth line and normal to the ecliptic, and that they merely convect outward with a solar wind flow assumed radial. It remains to specify the angle between the Sun-Earth line and the intersection between the phase front and the ecliptic plane.

It is useful to introduce the concept of impact parameter (IP) as the distance by which a plasma element, flowing radially from the sun with speed V and observed by one spacecraft misses being seen by a downstream spacecraft (or Earth). Simple geometrical considerations show that for bodies indexed by i and j and located at (Xi, Yi, Zi) and (Xj, Yj, Zj),

IPij = SQRT {[(Yi-Yj)+(Xi-Xj)*Ve/V]**2 + (Zi-Zj)**2}

Time-series plots of IPij for various combinations of Earth, IMP 8, ISEE 3, Wind and 
ACE are available (given that Ve = 30 km/s and assuming V = 428 km/s) at and at
IP values were used as filters in doing data set pair regressions, as will be 
discussed later.

For solar wind variation phase fronts normal to the ecliptic (no Zi-Zj dependence),  
further geometric considerations say that the time delay equation for one spacecraft
and Earth is

Delta-t = (X/V) * {[1 + (Y*W)/X]/[1 - Ve*W/V]},


Delta-t is the time shift in seconds,
X and Y are GSE X and Y components of the spacecraft position vector, in km,
V is the observed solar wind speed in km/s (assumed radial),
Ve is the speed of the Earth's orbital motion (30 km/s),
The W parameter is related to the assumed orientation of the phase front relative to 
the Earth-sun line.  The orientations to be considered correspond to cases of 
convection, corotation, and "half-way-in-between."

In first shifting ISEE 3 data for inclusion in OMNI many years ago, we showed that shifting by simple convection (assuming planes that were normal to the ecliptic plane and to the earth-sun line) and by "corotation" (assuming not normality to the Earth- sun line but alignment with the ideal IMF Parker spiral angle) were equally good statistically, and that both were significantly better than having no time shift. We somewhat arbitrarily chose to use the corotation shift. This is extensively discussed at

Subsequent to that early analysis, further study (Richardson and Paularena, 1998) revealed that best statistical correlation was obtained by assuming phase fronts were part way between the convection and corotation orientations considered in adding ISEE 3 data to OMNI. So in preparing ISEE 3, Wind and ACE data for OMNI 2, we have made the assumption that solar wind variation phase fronts are planar, normal to the ecliptic plane and intersecting the ecliptic plane along a line exactly half way between the ideal Parker-spiral and the normal to the Sun-Earth line (i.e., to the GSE Y axis).

For the three cases considered, we have for the W parameter introduced above:

Convection:	W=0
Corotation:	W=V/428
"Half-way":	W=tan [0.5 * atan (V/428)]
where 428 (km/s) is the speed in the (-) GSE Y direction at Earth of an ideal IMF spiral line due to solar rotation.

The upstream orbits: Let us review the ISEE 3, Wind and ACE orbits briefly. From shortly after its 8/12/78 launch until August, 1982, when it was directed towards the Earth's deep magnetotail, ISEE 3 was in an L1 libration point orbit with X in the range ~200-260 Re, Y in the range ~ +/- 100 Re, and Z in the range ~ -15 Re to + 20 Re. At its extremes (X ~ 220 Re, Y ~ +/- 100 Re, Z ~ 0), the impact parameter (IP, see above) values for ISEE 3 relative to Earth were ~ 83 Re and ~117 Re, where the asymmetry results from the Earth's motion towards -Y during the ~hour that the solar wind moves from Xisee to Xearth. Note that the time shifts for ISEE 3 could range between ~25 min for high flow speed (700 km/s) and Ygse = -100 Re and ~ 80 min for low speed (350 km/s) and Ygse = +100 km/s.

Wind has been in a variable orbit since its 11/01/94 launch. Through 1998, Wind executed a series of ~30 geocentric orbits with near-noon apogees of distances ranging between ~50 Re and ~250 Re and periods ranging between ~20 days and ~150 days. During these years, the Y component of the Wind position vector was typically in the range +/- 40 Re and almost never exceeded the range +/- 60 Re. For 1999 through the first half of 2000, Wind had three orbits reaching X values of 210, 180 and 100 Re, but otherwise many Wind apogees were of lower altitude and well away from the noon meridian. Starting in mid-2000, Wind was put into an orbit reaching extreme values of +/- 250-260 Re in the dawn-dusk meridian. After some time in this orbit, Wind was placed in an L1 orbit. Figure 4 shows the Wind-Earth impact parameter for 1994-2003.

ACE has been in a regular L1 orbit since shortly after its 08/25/97 launch, with X in the range ~218-248 Re, Y in the +/-40 Re range and Z in the +/- 24 Re range. The ACE project was assessing orbit adjustments in 2003.

Doing the time shifts: We have shifted 1-5 min ISEE 3, Wind and ACE IMF and plasma data using the above time-shift equation, using known locations of those spacecraft and using observed solar wind flow speeds in the data sets being shifted. We then created averages over all fine scale values whose shifted time tags placed them within a given hour "at Earth." Thus all the values with shifted time tags between 00:00 and 01:00 were averaged to give the first OMNI 2 average for a day.

The high resolution data sets that were fed into the time shift algorithm were:

ISEE 3 2-min merged IMF/plasma data at
Wind/SWE 92-s plasma data at
Wind 1-m IMF data at
ACE 4-min merged IMF/plasma data at

For ISEE 3, a given field-plasma-merged record was given a shifted time tag only if it had a flow speed value to use in the time shift equation above. If the record had no flow speed value, then its IMF data, if present, were not shifted nor otherwise carried forward for inclusion in OMNI 2. However, for both ACE and Wind, IMF data were shifted using a flow speed interpolated to the IMF record time tag with input from the closest-before and closest-after good flow speed values, regardless of the duration between the two input points used. (The treatment of ACE data in this regard was changed from being ISEE-3-like to being Wind-like in February, 2006.) Since plasma gaps may have been over many hours, users of shifted IMF data may want to assess the goodness of their time tags by examining whether there are concurrent plasma data and, if not, how long a plasma gap (during which flow speeds might have varied significantly) there was.

The above ISEE 3 and ACE merged data sets were created at NSSDC from data obtained from ISEE Principal Investigators and from the ACE Science Center.

A special case should be noted. After the February, 1980, failure of the ISEE 3 ion plasma instrument, only LANL-provided ISEE 3 hourly-averaged electron-based parameters are available. For OMNI, years ago, we time-shifted these using the corotation delay algorithm, then built Earth-time "hourly averages" as weighted averages of any hourly averages falling in part into the Earth-hour of interest. See for further detail. For OMNI 2, we have taken the even simpler approach of shifting each ISEE 3 electron- based density and flow speed in the LANL-provided hourly data set by one hour. No new "half-way" time shift of these electron-based parameters was done.

The hourly averages determined from the shifted field and plasma ISEE 3, Wind and ACE data are available, along with concurrent but unshifted IMP 8 data, at

Data Set Intercomparisons

[2008: All comparisons of Geotail data with ACE, Wind, or IMP 8 data are addressed at (HRO is High ResolutionOMNI.) Likewise all comparisons of Wind/SWE key parameter data with Wind/SWE non-linear-fits data are addressed there. Finally, the data set comparisons and cross normalizations section of that reference has results for years later than those addressed herein. Such recent-year cross normalizations have been applied to both HRO and to hourly OMNI 2.]

There are random and systematic differences between hourly averages of pairs of like parameters obtained by two spacecraft. Among the reasons for the random differences may be (1) the two averages have differently time-located gaps in the averages, (2) spatial gradients in parameters being measured combined with offsets of the spacecraft locations relative to the flow direction, (3) incorrect (or no) time shifts used for one or another data set prior to hourly-average construction (see prior section), (4) etc. Among the reasons for systematic differences are differing processing approaches (e.g., taking fits vs. moments for deriving flow parameters from distribution functions), subtle calibration factors not adequately accounted for in data processing, etc.

As OMNI 2 involves the interspersal of IMF data and of plasma data from each of several spacecraft, it is desirable to understand and to compensate for the differences between pairs of sources. It is not feasible to decrease random differences between pairs of sources (except via identification and exclusion of "bad data" as discussed earlier), but it is valuable to understand their magnitude in order to understand the "accuracy" of the OMNI 2 data as representative of the nearby solar wind. It is feasible to find and compensate for systematic differences between pairs of data sets.

This section addresses differences found between pairs of IMF data sets and of plasma parameter data sets, while the next section addresses the compensation of these differences through normalizations of some plasma parameters to a fiducial plasma data set.

In preparing OMNI 2, we have done detailed new comparisons involving IMP 8, Wind and ACE IMF and plasma data, and we report the results in detail as these are the main sources of the past 20 years of data. We have also done new comparisons of plasma data from IMP 8, ISEE 3 and IMP 6 and 7 which we shall report in somewhat less detail. Recall that IMP 8 carries two independent plasma experiments, from LANL and from MIT, and we also did detailed comparisons of these.

Magnetic field comparisons: Using the scatter plot and regression fit interface at, we have done several runs of the form Bi,j = a + b * Bi,k, where i designates a field component (GSE X, Y, Z) or field magnitude ( <|B|>, for which i=M below), where j, k designate a spacecraft pair and where a and b are intercept and slope of a linear regression fit determined by minimizing sums of squares of perpendicular distances between (Bi,j, Bi,k) data points and the best fit line. Use of the "perpdist" regression approach rather than the "delta-Y" regression approach is more appropriate to cases where the uncertainties or errors in the Y and X variables are comparable. In all the runs, we required at least half an hour's coverage in each hourly average by requiring at least 120 fine scale (15-s) IMP points, 30 1-min Wind points and/or 10 4-min ACE points. We also looked for time dependence in the IMP-Wind regression. Units of all B values below are nanoTeslas.
[As of November, 2011, these results had not been updated to reflect the reprocessing of IMP 8 or Wind magnetic field data. However, the data set intercomparison software at the url's identified above presently address the reprocessed data, such that interested parties can run on the latest data.]

For 6388 points between November 21, 1994 and December 31, 1996, we found

B (Wind, X) =  .018 (+/- .008) + 1.021 (+/- .004) * B (IMP, X)     
B (Wind, Y) = -.002 (+/- .011) + 1.017 (+/- .004) * B (IMP, Y)
B (Wind, Z) =  .014 (+/- .013) + 1.016 (+/- .007) * B (IMP, Z)
B (Wind, M) = -.008 (+/- .016) + 1.016 (+/- .003) * B (IMP, M)

For 7558 points between January 1, 1997 and July 27, 1999, we found

B (Wind, X) =  .045 (+/- .010) + 1.016 (+/- .003) * B (IMP, X)
B (Wind, Y) = -.012 (+/- .010) + 1.017 (+/- .003) * B (IMP, Y)
B (Wind, Z) =  .045 (+/- .012) + 1.023 (+/- .004) * B (IMP, Z)
B (Wind, M) = -.026 (+/- .015) + 1.020 (+/- .002) * B (IMP, M)

For 10,090 points between February 5, 1998 and July 29, 1999, we found

B (Wind, X) =  .080 (+/- .007) + 0.985 (+/- .002) * B (ACE, X)
B (Wind, Y) = -.010 (+/- .008) + 0.997 (+/- .002) * B (ACE, Y)
B (Wind, Z) =  .053 (+/- .009) + 0.993 (+/- .003) * B (ACE, Z)
B (Wind, M) =  .020 (+/- .012) + 0.988 (+/- .001) * B (ACE, M)

For 6864 points between February 5, 1998 and June 6, 2000, we found

B (IMP, X) =  .051 (+/- .011) + 0.972 (+/- .003) * B (ACE, X)
B (IMP, Y) =  .019 (+/- .012) + 0.983 (+/- .003) * B (ACE, Y)
B (IMP, Z) = -.020 (+/- .014) + 0.980 (+/- .004) * B (ACE, Z)
B (IMP, M) =  .062 (+/- .011) + 0.969 (+/- .001) * B (ACE, M)

In the range +/- 10 nT, where most IMF values lie, the difference between an observed value and any of the values computed from the above equations is almost always within 0.2 nT of zero. 0.2 nT is much less that the natural spread of data points about the best fit regression lines (see representative scatter plots at Figure 5 and Figure 6), so we shall not perform any cross-normalizations of magnetic field data.

Prior cross comparisons of multi-source magnetic field data were given in OMNIWeb- accessible documentation. It was determined that the only IMF normalization needed was for the sun-pointing (spin-axis aligned) IMF component of the Prognoz 10 IMF vector. This was done for OMNI, and will be continued unchanged in OMNI 2.

Plasma parameter comparisons: We use the same approach to comparing multi-source plasma flow speed, density and temperature values as was used for IMF parameters. In this case we have four multi-source data sets: 1971-1978 LANL IMP6/IMP7/IMP8 data; 1973-2001 IMP8 MIT/LANL data; 1978-1982 IMP8/MIT-IMP8/LANL-ISEE3 data; and 1995-2001 IMP8/MIT-IMP8/LANL-Wind/SWE/fits-Wind/SWE/moments-ACE/SWEPAM data. The relevant interfaces for both scatter plots and regression fits and for overlying intensity-time profiles are at

We have not systematically regressed flow direction angles nor the newly included Na/Np ratios with an eye towards normalizing these parameters to a fiducial data set. (Users may make comparisons of such parameters from the interface.) [Note added November, 2004: Systematic differences exist between ACE/SWEPAM and Wind/SWE Na/Np ratios. For instance, annual best fit regression lines say that at Na/Np(ACE) = 0.05, Na/Np (Wind) is .056, .060, .055, .053 for the years 1998, 1999, 2000, 2001. As noted later, the years 1995-1998 used primarily Wind plasma data while the years 1999 and later used mainly ACE plasma data. ~10% of the decrease in OMNI's annual averaged Na/Np values in going from 1998 to 1999 is due to the unremoved systematic difference in the ACE and Wind data. Perhaps a future OMNI version will normalize this Na/Np difference away.]

While we have done both linear (the "_s2" interfaces) and logarithmic (the "_s3" interfaces) regressions ("logarithmic regression" is shorthand for "linear regressions of logs of parameters") for density and temperature, we have normalized densities and temperatures using the results of the logarithmic regressions because densities and temperatures tend to be more log-normally distributed than linearly distributed.

Our interfaces also allow one to filter the data used in a given regression run by the numbers of fine scale points in the hourly averages and by the "impact parameter" IP for the pair of spacecraft contributing data to the run. The IP is the transverse separation of the spacecraft pair. IP's are available for IMP8-ISEE3, IMP8-Wind, IMP8-ACE and Wind-ACE from and from Virtually all IP-relevant runs were done with IP<60 Re to minimize basing comparisons and subsequent cross- normalizations on plasma parameters from different plasma regimes. The filtering on numbers of fine scale points enabled us to minimize the effects of hours wherein hourly averages may be based on limited and different coverages during an hour.

This P1=a+b*P2 approach to normalizing is different than that of Petrinic and Russell (1993) who fit ratios of 2-source densities (e.g., Ni/Nj, where i and j index two sources) to functions of V and T.

The a, b regression coefficients in the P1, P2 equation above were taken to be independent of V, T and time in the preparation of OMNI. However, for OMNI 2, we more systematically sought time dependence in regression coefficients. We also sought V dependence by doing separate regressions in all cases for slow flows (V<350 km/s), modest flows (350<V<450 km/s), and fast flows (V>450 km/s).

We also note that solar wind densities are expected to decrease as 1/R**2 on average, with R the heliocentric distance. The L1 libration point at 200+ Re from Earth is close to 0.99 AU from the sun. This means the density of a plasma element measured at L1 should be decreased by ~ 2% when it reaches the Earth's magnetosphere at ~1.00 AU. We have not used this fact in our analysis, but we note that the density differences we find in comparing sources are typically greater than 2%.

Wind/SWE vs. ACE/SWEPAM(OLD version, we do not use these results scince Feb. 20013)
It is likely that comparisons among currently active Wind/SWE and ACE/SWEPAM plasma data sources will be of most current interest as we write this note in 2003. Accordingly, we have generated an extra detailed discussion which, to minimize interruption to the flow of this note, we place as Appendix 1 to this note. Summarizing the results of the analyses reported and discussed in Appendix 1, we have:

V swe = -2.135 (+/- .387) + 1.010 (+/-.001) * V ace

V < 350 km/s:	 (log N) swe = .006 + (log N) ace
350 < V < 450:	 (log N) swe =-.091 + 1.036 * (log N)ace
V > 450 km/s:    (log N) swe =-.082 + (log N) ace

V < 350 km/s:	 (log T) swe = -1.680 + 1.348 * (log T) ace
350 < V < 450	 (log T) swe = -0.287 + 1.067 * (log T) ace
V > 450 km/s	 (log T) swe = -0.417 + 1.091 * (log T) ace
The speed equation is close enough to Y = X to warrant no normalization. The density equations say that N swe/N ace = 1.01 at V < 350 km/s, is 0.83 at V > 450 km/s and ranges between 0.83 at N = 2 to 0.91 at N = 20 in the 350-450 km/s flow speed range. The temperature equations say that T swe is typically larger than T ace by 5-25%.

IMP 8 (MIT & LANL) vs. Wind/SWE: For the Wind era, we have done many comparisons of IMP 8 MIT and LANL plasma parameters to each other and to Wind/SWE data. These are reported in Appendix 2.

We repeat here just the qualitative IMP/MIT-to-Wind and IMP/LANL-to-Wind results:

In general V swe > V imp. The difference V swe - V imp increases from 1.0 to 10.5 km/s ("imp" = imp/mit) or from 4.6 to 7.6 km/s ("imp" = imp/lanl) as V imp increases from 300 km/s to 800 km/s.

SWE densities are less than IMP densities by varying extents up to 15-20%, except that for slow flows (V < 350 km/s), SWE densities exceeded IMP/LANL densities by ~ 10%.

SWE temperatures are comparable to IMP/LANL temperatures except for slow flows for which Swe temperatures are less than LANL temperatures by 5% (at T = 25,000 deg K) to 30% (at T = 400,000 deg K). SWE temperatures are greater than IMP/MIT temperatures by up to a few tens of percent, except for relatively infrequent high temperature, slow speed cases.

IMP8/MIT vs. IMP8/LANL: IMP 8 plasma data span a 28-year, 1973-2001 interval. We have looked for time variation in the regressions between the two data sets, and have found it useful and appropriate to do density and temperature regressions in four separate time intervals: 1973-1979, 1980-1981, 1982-1994 and 1995-2001. We note that the 1980-1981 interval was highlighted in the analysis of Petrinic and Russell (1993) as having anomalously low Nlanl/Nmit ratios.

Appendix 3 gives the detailed results of the IMP/MIT - IMP/LANL comparisons for the four indicated time periods. The key points are that: flow speeds are virtually always within 3 km/s of each other; the (N lanl)/(N mit) ratio is anomalously low in the years 1980-1981; post-1981 (N lanl)/(N mit) ratios are a few percent higher than those for the 1970's. The results also show that the ratio increases with speed (consistent with earlier findings of Lazarus and Paularena, 1998) at given density, and decreases with density at given speed.

For the IMP8/LANL-IMP8/MIT case, it is important to allocate temporal variability to one or both sources. This is especially true because to normalize early data (e.g., IMP 6 or 7) to a later fiducial Wind/SWE data set, IMP 8 data must be use as the linkage. We shall assume all time variability in IMP8/LANL-IMP8/MIT regressions is due to variations in the LANL data. However, despite recent interactions with the LANL and MIT plasma teams, we are aware of no hard evidence to support or dispute this choice.

ISEE 3 vs. IMP 8 Comparisons: ISEE 3 was a very significant near-Earth solar wind monitor from shortly after its 1978/08/12 launch until late 1982 when it was moved from its L1 orbit to probe the deep geomagnetic tail.

The LANL plasma experiment on ISEE 3 separately measured ions and electrons. Unfortunately the ion instrument failed on February 19, 1980. We include in OMNI 2 (as we did for OMNI) ion-based density, flow speed, temperature and flow azimuth information through 1980/02/19, but only electron-based density and flow speed thereafter.

Appendix 4 details the results of comparisons of IMP 8 data with ISEE 3 ion-based and electron-based parameters. We show there that ISEE ion-based flow speeds agree fairly well with IMP speeds, the largest difference in the 300-800 km/s range being ~ 6 km/s at 300 km/s for ISEE 3 vs. IMP/MIT. The agreement of electron-based flow speeds with IMP is not as good. V isee is less than V imp by 5-15 km/s (IMP/LANL) or 7-10 km/s (IMP/MIT) over the 300-800 km/s range.

ISEE ion-based densities are less than IMP/MIT densities , except for slow flows (V < 350 km/s), with the ratio of (N isee)/(N imp) falling significantly with flow speed. On the other hand, ISEE electron-based densities are consistently greater than IMP/MIT densities. The (N isee)/(N imp) ratio increases from a few percent at N = 2/cc to ~ 25% at N = 20/cc, independent of flow speed.

ISEE ion-based temperatures are higher than IMP/MIT temperatures, by several tens of percent at low temperature to only ~10% at high temperature. We do not compare ISEE electron-based temperatures with IMP ion-based temperatures, nor do we include any ISEE electron-based temperatures (or flow angles) in OMNI 2.

LANL IMP 6, 7, 8 comparisons: There were LANL plasma instruments on IMP 6 (1971-1974), IMP 7 (1972-1978) and IMP 8 (1973-2001), all of which produced good data. As noted earlier, we have abandoned the earlier LANL-provided hourly/3hourly resolution merged IMP 6-7-8 data set that contained only ion densities, flow speeds and temperatures, in favor of computing new hourly averaged spacecraft-specific data sets from the ~2-min LANL-provided IMP 6, IMP 7 and IMP 8 data sets containing a greater range of parameters. LANL had originally built these 2-min data sets to contain only solar wind data, so we were able to build LANL hour averages for hours with bow shock crossings.

Appendix 5 contains the results of comparisons across spacecraft pairs. The main points of the Appendix are that IMP 7 and IMP 8 flow speeds and temperatures agree extremely well, while IMP 7 densities are on the order of 20% higher than the IMP 8 densities. IMP 6 and IMP 8 densities agree with each other to within 10%, as do IMP 6 and IMP 8 temperatures. IMP 6 and IMP 8 flow speeds agree near the middle of the 300-800 km/s speed range, but disagree near the ends of that range by about 12 km/s.

Data parameter normalizations

[2008: Cross-normalizations for years more recent than those addressed below, and for Wind/SWE/KP data relative to Wind/SWE nonlinear-fits data are found in]

Now, we must use all the two-spacecraft flow speed, density and temperature regressions of the previous section to normalize data appropriately. This is so that the multi- source OMNI 2 data set will have least impact of switching from using data from source x in hour n to using data from source y in hour n+1. Also, in this process, we would like to produce a data set whose parameters are most likely to be absolutely true and not useful merely for studies of relative changes and correlations.

In the foregoing section, we showed that, with rare exceptions, flow speed regression lines are typically within a few km/s of the Y = X line over the 300-800 km/s range, so we have not normalized any flow speed data.

On the other hand, density and temperature data frequently diverge from Y = X by more than a few percent, so we shall normalize all densities and temperatures for the 1971-2003 period that we have examined in detail. We shall use the time dependence of the 28-year IMP8/MIT - IMP8/LANL regression, but otherwise there will be no time dependence in the normalization of any given spacecraft. To be more explicit, we assume all time dependence in the IMP 8 LANL/MIT regression is due to IMP/LANL, so the IMP8/MIT normalization will itself be time invariant.

We shall also use the flow speed bins <350 km/s, 350-450 km/s and >450 km/s in performing normalizations as in the previous comparisons.

The key question is which data set to use as the fiducial data set. After interactions with the MIT and LANL plasma teams, we have decided to use the Wind/SWE data set as the fiducial data set. MIT's Dr. Justin Kasper wrote an extensive Ph.D. dissertation on SWE data that included comparisons of (a) SWE proton and alpha particle densities derived from nonlinear bimaxwellian fits to (b) electron densities computed from the Wind/Waves/TNR radio instrument. Folding in theory-based contributions of electrons from Z>2 ions, he concluded that the SWE ion densities are good to within 2%. The relevant sections of his dissertation are available from

There is no equivalent compelling reason for choosing any specific temperature data set to which to normalize all other temperature data sets. So, given the choice of Wind/SWE as providing the fiducial density data set, we also choose Wind/SWE as providing the temperature data set to which we will normalize all others.

Another key assumption is that there has been no significant time variation in IMP8/MIT density and temperature not previously found and compensated for by the MIT team. This enables us to normalize IMP 8 and ACE/SWEPAM data to Wind/SWE data and then to normalize all other data sources (not contemporaneous with Wind/SWE) by chaining the regressions of each such data set to IMP 8 with the IMP8-Wind/SWE regressions.

The normalization parameters will be specified as pairs (a b) where a and b are the intercepts and slopes in the equations

(logN)norm = a + b * (logN)obsvd
(logT)norm = a + b * (logT)obsvd

In most of the lines below, the three (a, b) pairs are for the flow speed bins < 350 km/s, 350-450 km/s and >450 km/s.

In the other lines below, having 4 parameters in each of three parentheses, we show the ratios of normalized-to-observed density values at 4 points (N = 2, 5, 10, 20 /cc) Again the three parentheses correspond to the three speed bins

The normalization parameters for ACE/SWEPAM(OLD version, we do not use these results scince Feb. 20013) are

N:  (.006  1.000)     (-.091  1.036)   (-.082  1.000)
T:  (-1.680  1.348)   (-.287  1.067)   (-.417  1.091)

N   (1.01 1.01 1.01 1.01)  (.83 .86 .88 .90)  (.83 .83 .83 .83)

The normalization parameters for IMP8/MIT are

N:  (.020 .941)  (.033 .919)  (.019 .907)
T:  (.864 .839)  (.491 .920)  (.702 .890)

N   (1.01 .95 .91 .88)  (1.02 .95 .90 .85)  (.98 .90 .84 .79)

[Note added June, 2005:  The analysis of Merka et al,
Pl. Sp. Sci., 53, p 79, includes an independent
comparison of Wind/SWE and IMP8/MIT plasma densities.
Grouping data for all flow speeds, considering 1995-2000,
and assuming a fit of the type
N(Wind)/N(IMP8) = c + 1/(a + b*N(IMP)), they find
N(Wind)/N(IMP8) = 0.73 + 1/(1.86 + 0.29*N(IMP))
At N(IMP8) = 2, 5, 10 and 20 cm**-3, this ratio is
1.14, 1.03, 0.94 and 0.86, respectively.  These ratios
are directly comparable to the N(norm)/N(obsvd) values of
the preceding line.  They are clearly larger than our ratios
above, except at N ~ 20 cm**-3.  Use of the Merka et al
equation for normalization of IMP8 plasma data for OMNI 2
would yield higher IMP8-based pressures in OMNI 2.
These might be significant for the pre-1995 era, when OMNI
coverage is dominated by IMP8 data.  We choose
not to comment at this time on the likely correctness of their
equation relative to ours, but simply offer it to OMNI users
for their awareness.]

The normalization parameters for IMP8/LANL are

for densities
1973-1979    (.111 .943)    (.064 .951)    (-.011 .958)
1980-1981    (.140 1.024)   (.092 1.010)   (-.028 1.048)
1982-1994    (.064 .965)    (-.020 1.000)  (-.085 1.006)
1995-2001    (.040 1.007)   (-.023 1.013)  (-.093 1.022)

1973-1979   (1.24 1.18 1.13 1.09)  (1.12 1.07 1.04 1.00)  (.95 .91 .89 .86)
1980-1981   (1.40 1.43 1.46 1.48)  (1.24 1.25 1.26 1.27)  (.97 1.01 1.05 1.08)
1982-1994   (1.13 1.10 1.07 1.04)  (.95 .95 .95 .95)      (.83 .83 .83 .84)
1995-2001   (1.10 1.11 1.11 1.12)  (.96 .97 .98 .99)      (.82 .84 .85 .86)

for temperatures
1973-1979    (.621 .879)  (.290 .958)    (.271 .969)
1980-1981    (.840 .811)  (-.482 1.091)  (.207 .964)
1982-1994    (.497 .894)  (+.044 .997)   (.130 .982)
1995-2001    (.441 .895)  (-.044 1.008)  (-.165 1.036)

The normalization parameters for ISEE3 protons are:

N:   (.059 .855)   (.097 .911)   (.076 .972)
T:   (.348 .926)   (.007 1.000)  (-.053 1.016)

N    (1.04 .91 .82 .74)  (1.18 1.08 1.02 .96)  (1.17 1.14 1.12 1.10)

The normalization parameters for ISEE3 electrons are

N:  (.019 .888)  (.052 .842)   (.004 .859)

N   (.97 .87 .81 .75)  (.94 .81 .73 .65)  (.92 .80 .73 .66)

The normalization equations for IMP6/LANL are

N:  (.000 1.075)  (.037 1.007)  (.018 .952)
T:  (.509 .894)   (.308 .950)   (.560 .910)

N  (1.05 1.13 1.19 1.25)  (1.09 1.10 1.11 1.11)  (1.01 .96 .93 .90)

The normalization equations for IMP7/LANL are

N:  (-.050 .983)  (-.053 .967)  (-.099 .968)
T:  (.641 .872)   (.359 .942)   (.322 .957)

N  (.88 .87 .86 .85)  (.87 .85 .83 .81)  (.79 .77 .74 .72)
To avoid discontinuities at 350 and 450 km/s in the normalization process, we have defined bands at 340-360 km/s and 440-460 km/s. Any normalizations of densities or temperatures for hours with speeds in these bands use weighted averages of the two sets of normalization parameters appropriate to either side of the band. For instance for a flow speed of 345 km/s, an averaged pair of regression parameters would be determined in which the <350 km/s regression parameters would be weighted by 75% and the 350-450 km/s regression parameters would be weighted by 25%.

Since the original OMNI data set was normalized to the LANL IMP 7 data set, and since the OMNI 2 data set is being normalized to the Wind/SWE data set, the last row of numbers [(N norm)/(N obsvd)] should be close to (N swe)/(N imp7-lanl) which should in turn be close to (N omni 2)/(N omni). That is, the renormalization of density data yields a decrease in OMNI 2 densities, relative to OMNI densities, of about 20%. Figure 7 in fact shows 27-day-averaged IMP7-normalized OMNI densities and temperatures and Wind-normalized OMNI 2 densities and temperatures. The ~20% density decrease is quite evident.

Because there is virtually no overlap between the OMNI and OMNI 2 data prior to the 1971 IMP 6 launch and after that launch, it is impossible to directly cross-normalize the data for those two eras. We have not attempted any new normalization of the pre-1971 OMNI data, but have simply written the pre-1971 data into OMNI 2.

Spacecraft prioritization for OMNI 2 inclusion

The final point to cover is the rules for selecting which source to use data from, when data from multiple sources are available for a given hour. In OMNI, we chose data from the closer-to-Earth spacecraft when the choice was between an hour-upstream spacecraft and a minutes-away spacecraft. This meant that we chose data from IMP 8 rather than data from ISEE 3, Wind or ACE when both were available. This was in part to support solar wind-magnetosphere coupling studies with the more reliable data from the closer spacecraft, that is, the data in which users could have more confidence that the spacecraft-observed data were representative of the field and plasma impinging on the magnetosphere. However, as all current event-oriented studies use higher resolution solar wind data, and as longer-term statistical studies probably do as well with L1 data as with closer-to-Earth data, we have chosen to prioritize for OMNI 2 the selection of more time-continuous upstream data over closer-to-Earth gappier data (i.e., IMP 8 data) when both are available. The smaller number of transitions between sources realized by this prioritization , even given our attempts at uniformizing OMNI 2 through normalizations, may be statistically advantageous.

We choose Wind data when available in the 1994-1998 interval and ACE data thereafter. We use Wind data as the backup to ACE after 1998 and IMP 8 as the backup to those two for 1994-2001. (Recall that IMP 8 IMF data will only extend to June 10, 2000.) Note that we frequently add Wind data for most recent times, as these become available somewhat earlier than do ACE data, and then replace Wind data with ACE data when the latter become available. Likewise in the ISEE 3 era, we choose ISEE 3 in preference to IMP 8.

In June, 2007, we made the following change. Because ACE/SWEPAM densities are sometimes unreliable at times of slow solar wind flows, our input ACE data set, obtained from the ACE Science Center, sometimes had flow velocity values but no densities values. This led to gaps in OMNI 2 density and related parameters that could be filled with Wind values. In fact, we searched for 1999-2007 OMNI 2 records having ACE flow velocities but not densities, and replaced all data in such records with Wind/SWE parameter values if available.

For the frequent choice between IMP8/MIT and IMP8/LANL data, we choose the IMP8/MIT data in part because it has flow latitude data which the LANL data do not have. However for hours when data from both sources are available, we add alpha-to- proton ratios to the OMNI records from the LANL data. There are far more IMP8/LANL data in OMNI 2 than there were in OMNI.

Plasma data from ISEE 1 and from ISEE 3 (for the period when only electron-based parameters were available) were chosen only when no other data were available.

Geotail data were added to OMNI 2 only for hours for which data from IMP 8, Wind and ACE were all unavailable.

We would greatly appreciate any comments on this text that would make it more usable over the long term.

Joe King and Natalia Papitashvili
August 11, 2003


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Appendix 1. Comparisons between Wind/SWE and ACE SWEPAM parameters

Introduction: This Appendix is to report results of comparisons of Wind/SWE plasma parameters with ACE/SWEPAM parameters.

[2008: As of 2008, the Wind/SWE non-linear fits-based proton parameters (cf. below) had not been determined past late November, 2004. Accordingly we have used in OMNI 2 a cleaned version of the Wind/SWE key parameter data set for later times. See the documentation of thisdata set in the High Resolution OMNI documentation file at]

We use hourly averages created at NSSDC from time-shifted 92-s SWE parameters provided by MIT some months ago, and we use hourly averages NSSDC-computed from similarly time-shifted LANL/ASC-provided 64-s SWEPAM parameters newly computed by LANL in mid-2003. As a reminder, the tools we use are at, .../pla_iwa_s2.html and .../pla_iwa_s3.html. Sample scatter plots (all-V results for V, N and T) corresponding to these results are browsable as Figure 8, Figure 9 and Figure 10.

Flow Speed

We get for flow speed regression 11,640 hours and:

V swe = -2.135 (+/-0.387) + 1.010 (+/-.001) * V ace

This says V swe - V ace = 1 km/s at 300 km/s and = 6 km/s at 800 km/s.  We take this to 
be close enough to Y = X to not require flow speed normalization.

Plasma (proton) densities:  

For densities, we obtain:

All V:	(log N)swe = -.107 (+/-.004) + 1.062 (+/-.005) * (log N)ace, 10,894 hrs 
V<350:  (log N)swe =  .007 (+/-.012) +  .999 (+/-.013) * (log N)ace,   1340 hrs
350-450 (log N)swe = -.091 (+/-.005) + 1.036 (+/-.006) * (log N)ace,   5759 hrs
V>450:  (log N)swe = -.085 (+/-.006) + 1.002 (+/-.008) * (log N)ace,   3890 hrs

It is nteresting that at low speed, we have effectively N swe = N ace.  LANL 
deliberately excluded densities, but retained flow speeds, for many low-speed hours 
in the latest version of its 64-s data set on which the ACE plasma data in OMNI 2 
are based.

Density ratios corresponding to these equations are 

N ace		2	5	10	20


All V		.816	.864	.902	.941
V<350		1.016	1.015	1.014	1.013
350-450		.831	.859	.881	.903
V>450		.823	.825	.826	.827
This says that at low speed, the SWE and SWEPAM densities are virtually the same while at high speed, N swe is only ~82% of N ace. At intermediate speed we get a transitional regression, akin at low N to the high speed result and half way between the high and low speed results at high N. Using only the speed-independent regression equation would lead to inappropriately decreasing N ace to match N swe at low speeds and would lead to insufficiently decreasing N ace to match N swe at high speeds (at all but the lowest N). So we shall use the V-dependent regression results in our OMNI-2-driven normalizations as we have for other data being prepared for OMNI 2.

We next look for evidence of time dependence in the Wind/SWE-ACE/SWEPAM density normalization by obtaining year-specific results integrated over full flow speed ranges. We find:

1998:	(log N)swe = -.122 (+/-.006) + 1.070 (+/-.007) * (log N)ace,    4621 hrs
1999:	(log N)swe = -.107 (+/-.006) + 1.067 (+/-.007) * (log N)ace,    3550 hrs
2000:	(log N)swe = -.094 (+/-.009) + 1.055 (+/-.012) * (log N)ace,    1880 hrs
2001:	(log N)swe = -.109 (+/-.013) + 1.098 (+/-.018) * (log N)ace,     843 hrs

It is risky to infer results from similarities of coefficients, especially from all-V 
results which we showed above could be misleading.  So we next look at Nswe/Nace ratios 
corresponding to these equations:

Nace			2	5	10	20


1998			.792	.845 	.887 	.931
1999			.819	.871	.912 	.955
2000			.837	.880 	.914 	.950
2001			.833	.919 	.975	1.044

At the lowest N, these ratios increase for the first 3 years, and the ratio for the 4th 
year matches that for the 3rd year.  At the intermediate and higher N values, the ratio 
takes jumps from 1998 to 1999 and from 2000 to 2001, but the ratios for 1999 and 2000 
are similar to each other.

Let's next look for time dependence in the two higher V bins where there are enough 
points to give reasonable statistics. 

350 < V < 450 km/s

1998	(log N)swe = -.112 (+/-.009) + 1.048 (+/-.010) * (log N)ace,   2516 hrs
1999	(log N)swe = -.093 (+/-.009) + 1.048 (+/-.011) * (log N)ace,   1924 hrs
2000	(log N)swe = -.092 (+/-.017) + 1.052 (+/-.020) * (log N)ace,    897 hrs
2001	(log N)swe = -.079 (+/-.020) + 1.073 (+/-.024) * (log N)ace,    422 hrs

V > 450

1998	(log N)swe = -.107 (+/-.010) + 1.023 (+/-.014) * (log N)ace,    1301 hrs
1999	(log N)swe = -.077 (+/-.009) + 0.989 (+/-.013) * (log N)ace,    1350 hrs
2000	(log N)swe = -.070 (+/-.011) + 0.988 (+/-.015) * (log N)ace,     877 hrs
2001	(log N)swe = -.090 (+/-.021) + 1.021 (+/-.030) * (log N)ace,     362 hrs

and the corresponding Nswe/Nace ratios at Nace = 2, 5, 10 and 20:

350 < V < 450

1998	.799	.835	.863	.892
1999	.835	.872	.902	.932
2000	.839	.880	.912	.945
2001	.877	.938	.986	1.037

V > 450

1998	.794	.811	.824	.837
1999	.831	.823	.817	.810
2000	.844	.835	.828	.821
2001	.825	.841	.853	.866

Consider first the less-spread-out V>450 Nswe/Nace ratios.  Only the N=5 column shows 
a monotonic increase with time.  Two years have regression equation slopes >1 and two 
<1.  The range of ratios is limited to .794-.866, with most values in the range .80-.85.  
Recall that the 1998-2001 combined result for V>450 was 
(log N)swe = -.085 (+/-.006) + 1.002 (+/-.008) * (log N)ace.
We believe it would be reasonable to normalize SWEPAM densities to SWE densities in the 
V>450 range, for all time, using the equation (log N)norm = -.082 + (log N)obsvd.  This 
would give ACE Nnorm/Nobsvd ratios of .828 at all time and density values.  It would 
introduce discrepancies of up to ~3% relative to what we'd get with time-dependent 
normalizations at some parts of the relevant span of N values.

Now consider the 350 < V < 450 km/s range.  Here we see the same behavior in the ratios as 
we saw in the all-V analysis earlier, that is, the ratios at each N jump from 1998 to 1999 
and from 2000 to 2001, but are near each other for 1999-2000.  An inclination here is to 
use one equation for 1999-2000

(log N)swe = -.092 (+/-.008) + 1.048 (+/-.010) * (log N)ace,   2821 hrs,
[ratios (2, 5, 10, 20) = .836 .870 .904 .934]

and to use the above two equations for 1998 and 2001.

However, let's look at scenarios for suppressing the time dependence.  Let's gather 
the various 350-450-Vbin ratios for various times:

1998-2001	.833	.862	.885	.909		5759 hrs
1998		.799	.835	.863	.892		2516 hrs
1999-2000	.836	.870	.904	.934		2821 hrs
2001		.877	.938	.986	1.037		422 hrs

If we used the 1998-2001 equation, we'd have "errors" relative to using separate 1998 and 1999-2000 equations of mostly <2.7% but up to 3.4% for N=2, 1998. There are not many hours with N~2, and a 3.4% error in 2.0 gives 2.07 which is insignificantly different than 2.0.

Using the 1998-2001 equation for 2001, rather than the 2001-specific equation, gives N values "too low" by 4.4%, 7.6%, 10.1% and 12.8% at N = 2, 5, 10, 20. These are getting to be large numbers. However, the equation for 350-450, 2001 is based on only 422 hours (because Wind was mostly >100 Re to the side of the Earth from late August, 2000). We have plotted the 1998-2001 best fit line on the 2001 scatter plot. The 1998- 2001 line lies in the main distribution of points, but visually removed from the center of the points.

This is an important judgement as to how to normalize the post-2000 ACE data, as these will be OMNI's dominant data for 2000 and beyond. It will be important to get more Wind/SWE data for periods when Wind has finished its far-side-of-Earth orbits to see if the SWE-SWEPAM comparison looks like the 422-point-based 2001 result or is more typical of what we see for 1999-2000. The Wind/SWE team at MIT has made "key parameter" data available to mid-2003, but not yet any definitive 2002-3 data; only definitive SWE data are used in OMNI 2.

We shall use for OMNI 2 the 1998-2001-based Nace-to-Nswe regression equation above for normalizing all 1998-2003 ACE/SWEPAM densities to SWE densities for hours with Vace in the 350-450 range. In doing so, we are introducing ~<3% discrepancies relative to what we'd get with time-span-specific normalizations for 1998-2000, and somewhat larger discrepancies in 2001 where the Nswe-Nace regression is based on relatively few points. We anticipate revisiting post-2000 SWE-ACE normalizations in the future, when large numbers of data pairs are available for Wind-ACE impact parameter < 60 Re.

Now let's go back to the V<350 case, where we only had 1340 hours for all of 1998- 2001. These hours distribute as follows for 1998-2001 respectively: 851, 307, 119, 63.

The Nswe/Nace ratios for these years, for Nace = 2,5,10,20 are

1998	1.004	.998	.993	.988
1999	.929	.979	1.019	1.060
2000	.943	1.065	1.167	1.279
2001	1.219	1.189	1.167	1.145

Recall we had for V<350

1998-2001	1.016	1.015	1.014	1.013

If we use the 1998-2001 equation, we'd be changing results in 1998, where most of the 
points are, by <2.5% and likewise by <3.5% in 1999 at N=5,10 where again most of the 
points are.  Since the later years and, for 1999, the N=2 and 20 limits, have only few 
hours with ACE and Wind/SWE data, we should use the 1998-2001 V<350 equation and 
plan on revisiting a different normalization for ACE/SWEPAM when later SWE data 
become available.  For simplicity, we change the 1998-2001 equation from

(log N)swe = .007 + .999 * (log N)ace


(log N)swe = .006 + (log N)ace

which gives a constant Nswe/Nace [or (Nnorm/Nobsvd)ace] of 1.014

In summary, we will compute normalized ACE/SWEPAM densities, time independently, 

V<350:		(log N)norm =  .006 + (log N)ace
350<V<450	(log N)norm = -.091 + 1.036 * (log N)ace
V>450		(log N)norm = -.082 + (log N)ace
These say that we increase observed ACE densities by 1.014 at V<350, decrease them by 0.828 at V>450 and decrease them in the 350<V<450 range by N-dependent amounts, ranging from 0.833 at N=2 to 0.909 at N=20.

Effect of fits vs. moments determination of densities

We expect fits-based proton densities to be, on average, somewhat less than moments- based densities. This presumably contributes significantly to the extent to which moments-based Nace values need to be reduced at V>350 to match fits-based Nswe values. Let's try to quantify this by comparing SWE fits-based densities with SWE moments-based densities (both of which were provided by Justin Kasper.MIT and both of which are addressible with the ftpbrowser tools identified earlier in this note.

With many thousands of points in each run, and with uncertainties in the slopes and intercepts in the .004-.007 range, we get

V<350, 1998-2001:	(log N)fit = -.080 + 1.051 * (log N)mom
350-450, 1998-1999:	(log N)fit = -.062 + 1.035 * (log N)mom
350-450, 2000-2001:	(log N)fit = -.063 + 1.034 * (log N)mom
V>450, 1998-1999:	(log N)fit = -.051 + 1.019 * (log N)mom
V>450, 2000-2001:	(log N)fit = -.046 + 1.015 * (log N)mom

The two higher-speed bins were split into two 2-year spans because the analysis 
software only handles 12,000 points per run.  However, it is clear that in each V bin 
there is no significant time dependence.  In the following, we shall work with the 1st, 
2nd and 4th of these equations.  They give for Nfit/Nmom ratios at Nmom = 2, 5, 10, 20:

V < 350		.86	.90	.94	.97
350 < V < 450	.89	.92	.94	.96
V > 450		.90	.92	.93	.94

The corresponding Nswe-fit/Nace ratios were

V < 350		1.014	1.014	1.014	1.014
350 < V < 450	.833	.862	.885	.909
V > 450		.828	.828	.828	.828

There is a significantly larger difference, in each speed bin, between Nswe-fit and 
Nace than between Nswe-fit and Nswe-mom.  This "extra difference" may be instrumental 
and/or the different ways the LANL and MIT groups took moments and/or ???


Using the same data and tools as for densities above, we find


All V:  (log T)swe = -0.508 (+/-.049) + 1.110 (+/-.011)*(log T)ace,   10974 hrs
V<350:	(log T)swe = -1.680 (+/-.291) + 1.348 (+/-.063)*(log T)ace,   1353 hrs
350-450	(log T)swe = -0.287 (+/-.081) + 1.067 (+/-.016)*(log T)ace,   5785 hrs
V>450	(log T)swe = -0.417 (+/-.104) + 1.091 (+/-.020)*(log T)ace,   3932 hrs


1998	(log T)swe = -.204 (+/-.113) + 1.049 (+/-.024)*(log T)ace,   2517 hrs
1999	(log T)swe = -.402 (+/-.133) + 1.091 (+/-.029)*(log T)ace,   1914 hrs
2000	(log T)swe = -.339 (+/-.339) + 1.078 (+/-.055)*(log T)ace,   897 hrs
2001	(log T)swe = -.204 (+/-.310) + 1.054 (+/-.064)*(log T)ace,   422 hrs

We next show the ratios Tswe/Tace evaluated at Tace = 25, 63, 160 and 400 K deg 
corresponding to these equations:

.946	1.047	1.160	1.283
.709	.978	1.352	1.860
1.018	1.083	1.153	1.226
.962	1.047	1.139	1.238

1.026	1.074	1.125	1.176
.996	1.083	1.179	1.282
1.009	1.085	1.167	1.253
1.080	1.135	1.191	1.255

Tswe is typically greater than Tace by 5-25%

Time dependence in the 350-450 V bin is not systematic.  Tswe/Tace is steady for 
1998-2000 and then jumps into 2001, for the lower T values.  On the other hand, 
Tswe/Tace is steady for 1999-2001 for the higher T values, after having been lower 
in 1998.  Using the 1998-2001 350-450 V-bin equation rather than the year-specific 
350-450 V-bin equations introduces "errors" of less than ~6%.  We shall do likewise 
for the V<350 and V>450 bins.  

Thus, we shall use the following, time independently, for normalizing ACE temperatures 
for OMNI 2.

V<350:		(log T)norm = -1.680 + 1.348 * (log T)obsvd
350<v<450	(log T)norm = -0.287 + 1.067 * (log T)obsvd
V>450		(log T)norm = -0.417 + 1.091 * (log T)obsvd

The changes in T values upon normalizing are similar in our two higher speed bins; 
both increase from near zero at T=25,000 deg to ~25% at T=400,000 deg.  At low speed, 
however, the change upon normalizing is a much more dramatic function of T, ranging 
from -29% at T=25,000 deg to +86% at T=400,000 deg.

Alpha-to-proton density ratios ( OLD)

Both Wind/SWE and ACE/SWEPAM provide alpha-to-proton density ratios.  This 
parameter will be added to OMNI 2; it has not been in OMNI up to now.  We do not 
propose to cross-normalize these ratios.  However, we have done cross-regression runs 
and find, for 1998-2001,

All V		(Na/Np)swe = .003 (+/-.000) + 1.080 (+/-.005) * (Na/Np)ace,  4503 hrs
V<350		(Na/Np)swe = .003 (+/-.000) + 0.868 (+/-.011) * (Na/Np)ace,  701 hrs
350<V<450	(Na/Np)swe = .003 (+/-.000) + 1.029 (+/-.006) * (Na/Np)ace,  2700 hrs
V>450		(Na/Np)swe = .008 (+/-.000) + 1.173 (+/-.012) * (Na/Np)ace,  1151 hrs

These four equations give (Na/Np)swe values of .057, .046, .054 and .067 for (Na/Np)ace 
= .050.

In the corresponding scatter plots, we see significantly more scatter than for the V-V 
or logN-logN or logT-logT scatter plots.

Appendix 2. Comparisons of IMP 8 and Wind/SWE and ACE parameters

We report herein the results of comparisons of IMP8/MIT, IMP8/LANL and Wind/SWE 
data for 1995-2001.  For flow speeds, we find:

V lanl = -8.40 (+/- 0.49) + 1.016 (+/- .001) * V mit,  10758 hours
V swe  = -4.68 (+/- 0.59) + 1.019 (+/- .001) * V mit,  9660 hours
V swe  =  2.80 (+/- 0.36) + 1.006 (+/- .001) * V lanl, 5376 hours

For the first of these three, we required >9 fine scale points in both the LANL and MIT 
averages, for the second, >19 points in the IMP/MIT average and > 29 points in the SWE 
average and, for the third, > 14 points in the LANL average and > 29 points in the SWE 
average.  We also required that the IMP-Wind impact parameter should be less than 60 Re. 

These equations are reasonably consistent, given that "chaining" the first two yields an 
inferred Wind/SWE - IMP/LANL relation of V swe = 3.74 + 1.003 * V lanl, which is 
very close to the "directly observed" V swe - V lanl relation.  

As flow speeds increase from 300 km/s to 800 km/s, Vy - Vx vary across the range:

 y	x	Vy - Vx 	Vy - Vx
		@ Vx=300	@ Vx=800

lanl	mit	-3.6		+4.4
swe	mit	+1.0		+10.5
swe	lanl	+4.6		+7.6

Summarizing, the IMP/MIT and IMP/LANL flow speeds are within ~ 4 km/s of each 
other over the speed range of most solar wind flows, while the Wind/SWE flow speed 
tends to be greater either IMP speed, especially at higher speeds.   For completeness, 
we note from Appendix 1 that V swe - V ace increases from 0.9 to 5.9 km/s as V ace 
increases from 300 to 800 km/s. 

Consider next plasma densities.  Here we do separate comparisons/regressions in the flow 
speed bins <350 km/s, 350-450 km/s (abbreviated 35-45 below) and >450 km/s.  With 
column headings consistent with (log N)y = a (+/- del-a) + b (+/- del-b) * (log N)x, with 
ny and nx being the minimum numbers of fine scale points required in the density 
averages from sources y and x and with #hrs being the number of hourly averages folded 
into the regression run, we summarize the results as

 Y	x	V	ny	nx	#hrs	a	b	del-a	del-b

Lanl	mit	<350	4	5	3648	.001	.922	.008	.008
Lanl	mit	35-45	4	4	6926	.064	.904	.006	.006
Lanl	mit	>450	4	5	3745	.117	.882	.007	.009

Swe	mit	<350	10	5	3344	.020	.941	.009	.008
Swe	mit	35-45	10	5	6224	.033	.919	.006	.007
Swe	mit	>450	10	5	4099	.019	.907	.007	.010

Swe	lanl	<350	10	4	2727	.040	1.007	.008	.009
Swe	lanl	35-45	10	4	5186	-.023	1.013	.006	.007
Swe	lanl	>450	10	4	3043	-.093	1.022	.008	.010

These regression equations yield Ny/Nx density ratios, in the same sequence as the 9 
lines above, as

Nx =	2	5	10	20

	.95	.88	.84	.79
	1.08	.99	.93	.87
	1.21	1.08	1.00	.92

	1.01	.95	.91	.88
	1.02	.95	.90	.85
	.98	.90	.84	.79

	1.10	1.11	1.11	1.12
	.96	.97	.98	.99
	.82	.84	.85	.86

These equations say that on IMP 8, LANL densities exceed by MIT densities by up to 
20% for fast, dilute flows whereas the opposite is true for slow, dense flows.  Also, 
SWE densities are less than IMP densities by varying extents up to 15-20% except that 
for slow flows, SWE densities exceed IMP/LANL densities by ~10%.  For completeness, 
recall from Appendix 1 that SWE and ACE/SWEPAM densities were comparable for slow 
flows, but that SWE densities were less than ACE densities by 10-18 % for moderate 
and fast flows (i.e., V > 350 km/s).

Finally, we address the temperature comparisons among IMP/MIT, IMP/LANL and 
Wind/SWE.  Using the same format as for densities above, and recognizing that 
log-temperatures (not temperatures themselves) are being regressed, we find:

 Y	x	V	ny	nx	#hrs	a	b	del-a	del-b

lanl	mit	<350	4	5	3649	.594	.909	.067	.016
lanl	mit	35-45	4	5	6927	.542	.910	.059	.012
lanl	mit	<450	4	5	3751	.774	.870	.097	.019

swe	mit	<350	10	5	3344	.864	.839	.063	.014
swe	mit	35-45	10	5	6224	.491	.920	.064	.013	
swe	mit	>450	10	5	4101	.702	.890	.090	.018

swe	lanl	<350	10	4	2727	.441	.895	.050	.011
swe	lanl	35-45	10	4	5187	-.044	1.008	.041	.009
swe	lanl	>450	10	4	3086	-.165	1.036	.074	.014

These yield Ty/Tx temperature rations, in the same sequence as above (with "K" below 
referring to thousand, not deg Kelvin), as 

Tx=	25K	63K	160K	400K	

	1.56	1.44	1.32	1.21
	1.40	1.29	1.18	1.09
	1.59	1.41	1.25	1.11

	1.43	1.23	1.06	0.92
	1.38	1.28	1.19	1.10
	1.65	1.49	1.35	1.22

	0.95	0.87	0.78	0.71
	0.98	0.99	0.99	1.00
	0.98	1.02	1.05	1.09

These say that IMP/MIT temperatures are systematically less than IMP/LANL and SWE 
temperatures, while the latter two sources have temperature values quite close to 
each other at V > 350 km/s.

Appendix 3. Comparisons of IMP8/MIT and IMP8/LANL parameters

We have done separate regression runs for the two IMP 8 plasma data sets for the time 
intervals 1973-1979, 1980-1981, 1982-1994, 1995-2001.  With the requirement that each 
MIT average have at least 30 fine scale points, and each LANL average have at least 18 
points (except relaxing those numbers to 10 and 10 for 1980-1981, we find for flow 

1973-1979  V lanl = -5.69 (+/- 0.98) + 1.010 (+/- .002) * V mit,  2549 hours
1980-1981  V lanl = -6.82 (+/- 1.17) + 1.012 (+/- .003) * V mit,  1585 hours
1982-1994  V lanl = -5.61 (+/- 0.51) + 1.009 (+/- .001) * V mit,  7917 hours
1995-2001  V lanl = -7.31 (+/- 0.97) + 1.013 (+/- .002) * V mit,  2728 hours

These equations give V lanl - V mit differences  between ~ - 3 km/s at 300 km/s and 2-3 
km/s at 800 km/s.

Next, consider densities in each of the four time spans and in each of the three flow 
speed bins <350 km/s, 350-450 km/s and >450 km/s.  All the runs reported require at least 
four LANL and five MIT fine scale points in the relevant hours.  In all cases we report 
the results for (log N) lanl = a (+/- del-a) + b (+/- del-b) * (log N) mit:

Time span	speed	a	b	del-a	del-b	#hrs

1973-1979	<350	-.096	.998	.010	.009	2872
		35-45	-.033	.966	.005	.005	7154
		>450	.031	.947	.005	.007	6833

1980-1981	<350	-.117	.919	.016	.017	818
		35-45	-.058	.910	.016	.018	1066
		>450	.045	.865	.015	.018	651
1982-1994	<350	-.046	.975	.008	.007	4124
		35-45	.053	.919	.006	.006	7460	
		>450	.103	.902	.005	.006	8350

1995-2001	<350	.001	.922	.008	.008	3648
		35-45	.064	.904	.006	.006	6926
		>450	.117	.822	.007	.009	3745

These regression equations yield (N lanl)/(N mit) ratios as follows (in the same sequence 
as the above 12 rows):

N mit =	2	5	10	20

	.80	.80	.80	.80
	.91	.88	.86	.84
	1.04	.99	.95	.92

	.72	.67	.63	.60
	.82	.76	.71	.67
	1.01	.89	.81	.74

	.88	.86	.85	.84
	1.07	.99	.94	.88
	1.18	1.08	1.01	.95

	.95	.88	.84	.79
	1.08	.99	.93	.87	
	1.21	1.08	1.00	.92

These ratios show the anomalously low (N lanl)/(N mit) ratio in the 1980-1981 interval. 
Neither the MIT nor LANL team had a good explanation for this when queried in late 
2002.   The ratios show largely similar values for the 1982-1994 and 1995-2001 intervals, 
with these ratios being a few percent higher than the corresponding rations of the 1973-
1979 interval.  The results also show that the ratio increases with speed (consistent 
with earlier findings of Lazarus and Paularena, 1998) at given density, and decreases 
with density at given speed.

Next, we repeat the above for temperatures.  Again, we require 4 LANL and 5 MIT fine 
scale points in each hourly average included.  Again, we show parameters in the equation
(log T) lanl = a (+/- del-a) + b (+/- del-b) * (log T) mit

time span	speed	a	b	del-a	del-b	#hrs

1973-1979	<350	.276	.954	.067	.015	2709
		35-45	.210	.960	.046	.010	7249
		>450	.460	.915	.059	.012	6904

1980-1981	<350	.029	1.034	.191	.043	805
		35-45	.892	.843	.136	.029	1084
		>450	.518	.923	.190	.038	658

1982-1994	<350	.410	.938	.066	.016	4214
		35-45	.448	.923	.059	.012	7460
		>450	.582	.906	.070	.013	7133

1995-2001	<350	.594	.909	.067	.016	3649
		35-45	.542	.910	.059	.012	6927
		>450	.774	.870	.097	.019	3751

The corresponding (T lanl)/(T mit) ratios are 

T mit =	25K	63K	160K	400K	

	1.19	1.14	1.09	1.04
	1.08	1.04	1.00	0.97
	1.22	1.13	1.04	0.96

	1.51	1.56	1.61	1.66
	1.59	1.38	1.19	1.03
	1.51	1.41	1.31	1.22

	1.37	1.30	1.22	1.16
	1.29	1.20	1.12	1.04
	1.47	1.35	1.24	1.14

	1.56	1.44	1.32	1.21
	1.40	1.29	1.18	1.09	
	1.59	1.41	1.25	1.11

These ratios show that the moments-determined LANL temperatures are almost always 
greater that the fits-based MIT temperatures.  The ratios are smallest in the 1970's.  
The ratios typically decrease with temperature, except for the slow flow case 
(V <350 km/s) in the density-anomalous years of 1980-1981.  The ratios of the 
1982-1994 interval are intermediate between the 1980-1981 period and the 1995-2001 

Appendix 4. Comparisons of ISEE 3 and IMP 8 parameters

All runs below required that the IMP 8 - ISEE 3 impact parameter be 60 Re or less.  
"lanl" and "mit" below refer to IMP 8 data.  Consider first the 1978/08/16 - 1980/02/16 
interval of availability of ion-based plasma parameters from ISEE 3.  For these runs, we 
required at least 5 fine scale points in each IMP average and 10 fine scale points in each 
ISEE average.  For flow speeds we find

V isee = -6.468 (+/-1.001) + 1.013 (+/- .003) * V lanl,   946 hours
V isee = -10.667 (+/-.988) + 1.016 )+/- .003) * V mit,  1733 hours

These equations yield for V isee - V imp:

V imp =	300	800

Imp=lanl	-2.6	3.9
Imp=mit	-5.9	2.1

For ion-based densities and for IMP/MIT only, we have

V<350	(log N) isee = -.046 (+/- .025) + 1.100 (+/- .023) * (log N) mit, 352 hrs
353-450	(log N) isee = -.070 (+/- .017) + 1.009 (+/- .019) * (log N) mit, 842 hrs
V> 450	(log N) isee = -.059 (+/- .016) + 0.933 (+/- .020) * (log N) mit, 551 hrs

These yield (N isee)/(N imp) ratios of 

N imp =	2	5	10	20

V<350		.96	1.06	1.13	1.21
350-450	.86	.86	.87	.87
V>450		.83	.78	.75	.71

For all but slow flows (V < 350 km/s), ISEE densities have to be increased significantly 
to match IMP/MIT densities.

For ion-based temperatures, we find

V < 350	(log T) isee = .557 (+/- .173) + .906 (+/- .038) * (log T) mit, 352 hrs
350-451	(log T) isee = .484 (+/- .155) + .920 (+/- .033) * (log T) mit, 843 hrs
>450	(log T) isee = .743 (+/- .194) + .876 (+/- .039) * (log T) mit, 557 hrs 

These yield (T isee)/(T imp) ratios of

T imp =	25K	63K	160K	400K

V<350		1.39	1.28	1.17	1.07
350-450		1.35	1.26	1.17	1.09
>450		1.58	1.41	1.25	1.12

Independent of flow speed, T isee exceeds T mit by several tens of percent at low 
temperature, and by about 10 % at high temperature.

Now let's compare the electron-based ISEE densities and flow speeds with IMP ion-based 
values.  For these runs, we require at least 5 IMP points in each average, but it is 
meaningless to specify an equivalent number the electron-based parameters since they 
came to NSSDC originally as hourly averages with no indication of numbers of 
contributing fine scale values.

For flow speed, we find

V isee = 1.042 (+/- 1.534) + 0.980 (+/- .003) * V lanl,  1293 hours
V isee =  -5.786 (+/- 1.409) + .995 (+/- .003) * V mit,  2175 hours

These equations yield for V isee - V imp:

V imp =		300	800

Imp=lanl	-5.0	-15.0
Imp=mit		-7.3	-9.8

These delta-V's are large enough to warrant cross normalization, but this has not been 
done in OMNI 2.  Perhaps a next version of OMNI will do so.

For densities, we have

V<350	(log N) isee =  .001 (+/- .046) + 1.060 (+/- .043) * (log N) mit, 607 hrs
350-451	(log N) isee = -.022 (+/- .030) + 1.091 (+/- .032) * (log N) mit, 938 hrs
>450	(log N) isee =  .018 (+/- .025) + 1.056 (+/- .032) * (log N) mit, 649 hrs

and these equations yield the (N isee)/(N imp) ratios

N imp = 	2	5	10	20

V<350		1.04	1.10	1.15	1.20
350-450	1.07	1.16	1.23	1.32
>450		1.08	1.14	1.19	1.23

ISEE 3 electron based densities are greated than IMP/MIT ion-based densities by a few 
percent at low density and by ~25% at high densities, largely independent of flow speed.

Appendix 5: Comparisons of LANL data from IMPs 6, 7 and 8

These runs require at least 5 fine scale points in each average, and they are all run 
over the common IMP 6-7-8 interval of October, 1973, to the end of 1974.  We use 
subscripts i6, i7, i8 to designate IMP 6, IMP 7 and IMP 8.

For flow speeds we find

V i8 = -27.776 (+/- 1.359) + 1.050 (+/- .003) * V i6
V i8 =   2.183 (+/- 1.263) + 0.997 (+/- .002) * V i7

These yield V i8 - V ix differences of

V ix =		300	800

V i6		-12.8	12.3
V i7		1.3	-0.2

The IMP 7 and 8 flow speeds agree very well, the IMP 6 flow speed disagrees by ~12 
km/s at both ends of the 300-800 km/s range, although it matches the IMP 8 speed at V ~ 
550 km/s.

For densities we find

V<350	(log N) i8 = -.118 (+/- .166) + 1.140 (+/- .173) * (log N) i6, 80 hrs
350-450	(log N) i8 = -.028 (+/- .031) + 1.059 (+/- .036) * (log N) i6, 450 hrs
> 450	(log N) i8 =  .030 (+/- .015) + 0.994 (+/- .022) * (log N) i6, 867 hrs

V<350	(log N) i8 = -.171 (+/- .103) + 1.042 (+/- .087) * (log N) i7,  126 hrs
350-450	(log N) i8 = -.123 (+/- .035) + 1.017 (+/- .035) * (log N) i7,  400 hrs
>450	(log N) i8 = -.092 (+/- .021) + 1.010 (+/- .026) * (log N) i7,  816 hrs

These correspond to (N i8)/(N ix) ratios of

N ix =		2	5	10	20

ix = i6		.84	.95	1.05	1.16
		.98	1.03	1.07	1.12
		1.07	1.06	1.06	1.05

ix = i7		.83	.87	.89	.92
		.76	.77	.78	.79
		.81	.82	.83	.83

These equations and ratios say that the IMP 6 and IMP 8 densities typically agree to 
within 10%, while the IMP 7 density is on the order of 20% greater than IMP 8 densities.
In fact, it was this IMP 7 density "overage" that led the LANL plasma team to normalize 
its IMP 6 and IMP 8 densities upward in its creation of the merged hourly IMP 6-7-8 data 
set provided to NSSDC years ago and used by NSSDC in the 1970's as the fiducial data set 
for OMNI to which to normalize other densities.

Finally, let us consider temperatures:

V<350	(log T) i8 = -.127 (+/- .363) + 1.017 (+/- .077) * (log T) i6,  80 hrs
350-450	(log T) i8 =  .019 (+/- .178) +  .992 (+/- .035) * (log T) i6,  449 hrs
>450	(log T) i8 =  .298 (+/- .113) +  .939 (+/- .021) * (log T) i6,  871 hrs

V<350	(log T) i8 = .023 (+/- .232) + 0.992 (+/- .051) * (log T) i7,  126 hrs
350-450	(log T) i8 = .072 (+/- .123) + 0.983 (+/- .025) * (log T) i7,  400 hrs
>450	(log T) i8 = .053 (+/- .138) +  .988 (+/- .026) * (log T) i7,  816 hrs

These yield (T i8)/(T ix) ratios of 

T ix =		25K	63K	160K	400K

ix = i6		.89	.90	.92	.93
		.96	.96	.95	.94
		1.07	1.01	.96	.90

ix = i7		.97	.97	.96	.95
		.99	.98	.96	.95
		1.00	.99	.98	.97

These say that IMP 7 and IMP 8 temperatures agree extremely well, and that IMP 6 and 
IMP 8 temperatures agree to within 10% of each other.

If you have any questions/comments about OMNIWEB system, contact: Dr. Natalia Papitashvili Mail Code 672, NASA/Goddard Space Flight Center, Greenbelt, MD 20771

NASA Official: Dr. Robert McGuire, Head of the Space Physics Data Facility