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COHOWeb Service Overview

COHOWeb provides access to heliospheric magnetic field, plasma and spacecraft position data for each of many spacecraft identified below. The primary COHOWeb interface was reorganized in October, 2004, to enable easier access to several recently created functionalities as well as to long-available functionalities. There are six columns on the primary page, corresponding to six sets of functionalities (all but the sixth refer to hourly resolution data):

a. This option enables graphical display or screen listing of any subset of the physical parameters in the COHOWeb-standard data records, at hourly or daily resolution and for any user-specified time interval. This is the original COHOWeb functionality.

b. This option enables a listing of any subset of physical parameters in the COHOWeb input records, for any user-specified time interval, as limited to hours when the value(s) of user-chosen parameter(s) fall within user-specified minimum and maximum values. This last constraint is called "filtering."

c. This option lets users create distributions, medians,averages and standard deviations for any one parameter of the COHOWeb input records for user- specified time intervals and with filtering agains any parameter(s). This page has a link to a further discussion of the use of this option.

d. This option lets users create scatter plots and linear regression fits for any pair of parameters in the COHOWeb input records, again with user selection of time span and filtering conditions.

e. This option provides ftp accessibility to annual ASCII files of COHOWeb-input records.

f. This option provides ftp accessibility to magnetic field, plasma and other data, at time resolutions higher than hourly, from the COHOWeb spacecraft.

COHOWeb Data Overview

COHOWeb data development starts with hourly-resolution data files provided to NSSDC by Principal Investigators on the relevant spacecraft missions. These files were typically, but not necessarily, in ASCII format. Some contained magnetic field data and others plasma data. Most had spacecraft position data. There were multiple coordinate systems used to specify magnetic field, flow velocity and spacecraft position vectors.

At NSSDC a new set of spacecraft-specific hourly ASCII records ("COHOWeb-input records" in the terminology of Section 1 above) was created wherein magnetic field, plasma and spacecraft position data were merged. Each such record contained a common set of parameters, including magnetic field Cartesian components in RTN coordinates and spacecraft position data in Heliographic Inertial (HGI) coordinates. These coordinate systems are defined below. Coordinate transformations, when needed, were performed by NSSDC. Some spacecraft-specific records had a few additional spacecraft-specific parameters.

Finally, a set of CDF-formatted files were created wherein the data records had only the common set of parameters across all the source spacecraft. (CDF is Common Data Format.)

In order that interplanetary data in the near-Earth vicinity be conveniently and concurrently accessible and usable with the deep space data of COHOWeb, we have added to COHOWeb a version of the multi-source (mostly ACE, Wind and IMP-8 for recent years) OMNI data set that has the same parameters, coordinate systems and CDF formatting as the deep-space CDF-formatted data. This is referred to as OMNI_M below. The "native" OMNI data are available from OMNIWeb.

The CDF-formatted files underlie the first functionality (graphical display, etc.) identified in Section 1 above, while the field-plasma-merged ASCII files underlie the functionalities identified as b, c, d and e above. These ASCII files, and their format statements, are ftp-accessible from

Identification of Available Data

Data currently available from COHOWeb are given as follows:
              Investigator Time Span (YYY/MM/DD)     Investigator  Time Span (YYY/MM/DD)      

*Helios 1      Mariani      1974/12/10 - 1981/06/14    Schwenn    1974/12 - 1980/12/30
*Helios 2      Mariani      1976/01/16 - 1980/03/05    Schwenn    1976/01/16 - 1980/03/05 
Mariner 2                                             Neugebauer  1962/08/30 - 1962/11/16
New Horizons                                          Elliott     2008/10/10 - 2016/06/22
OMNI_M        (Several)    1963/11/27 - 2017/12/25    (Several)   1963/11/27 - 2017/12/31
Pioneer 6     Ness         1965/12/16 - 1967/09/15    Bridge      1965/12/15 - 1971/05/18
Pioneer 7     Ness         1966/08/17 - 1967/10/29    Bridge      1966/08/15 - 1968/11/15
*Pioneer 10    E. Smith     1972/03/04 - 1975/11/17    Gazis      1972/04/19 - 1995/09/05
*Pioneer 11    E. Smith     1973/04/06 - 1992/08/02    Gazis      1973/04/21 - 1992/05/31
Pioneer Venus  Russell      1978/12/05 - 1988/08/08    Gazis      1978/12/05 - 1992/10/09
*STEREO-A      Luhmann      2007/01/01 - 2017/03/31    Galvin     2007/02/15 - 2017/08/31 
*STEREO-B      Luhmann      2007/01/01 - 2014/09/28    Galvin     2007/03/01 - 2014/10/31
*Ulysses       Balogh/Smith 1990/10/25 - 2009/06/30    Phillips   1990/11/18 - 2009/06/30
*Voyager 1     Ness         1977/09/07 - 2017/04/18    Belcher    1977/09/07 - 1980/11/23
*Voyager 2     Ness         1977/08/24 - 2014/12/31    Belcher    1977/08/24 - 2017/12/18
(Started at November, 2014) We have added proton fluxes for some spacecraft, they marked as(*) in the table above.
To get time span for Proton fluxes from different instruments click HERE

Specific notes about some spacecraft

Notes (3/27/2014) on Voyager 1 and 2 Magnetometer Data After 1989:

 At the time of experiment proposal, the mission (then called "Mariner-Jupiter/Saturn") 
was designed to investigate Jupiter, Saturn and the interplanetary 
medium out to Saturn at 10 AU, where the interplanetary magnetic field strength is 0.6 nT.
 These objectives determined the required accuracy of the measurements, which together
 with the nature of the spacecraft determined the design of the instrument. 
The spacecraft magnetic field at the outboard magnetic field sensor, referred to 
as the primary unit, was expected to be 0.2 nT and highly variable, consistent with
 current estimates. Hence, the dual magnetometer design (Ness et al., 1971; Behannon et al. 1977). 

The magnetic field instrument on V1 (12, 13) has two identical triaxial sensors mounted on a 13 m 
boom. The output of each magnetic field sensor has a digitization step size of 0.004 nT, and the
primary sensor noise is ~0.003 nT RMS. The spacecraft is rolled 5-7 times about an axis
(the Z- axis) pointing at Earth (within 1° of the Sun beyond 60 AU) approximately every 
three 2 or 3 months, allowing calibrations of the sensors measuring magnetic fields in the 
payload X-direction and Y-direction and corrections for the corresponding components of 
the spacecraft magnetic field at the time of a roll. Calibrations and corrections of the 
payload Z-component of the magnetic field cannot be made with the information provided by a 
roll. There are data gaps of ˜ 8 - 16 hours each day, depending upon the schedule for tracking
by JPL’s Deep Space Network coverage.

At distances > 40 AU, the heliospheric magnetic fields are generally much weaker than 0.4 nT;
the average magnetic field strength near 40 AU and 85 AU is ~0.15 nT and ~0.05 nT, respectively.
The use of roll calibrations lasting ~6 hours permits determination of the effective zero 
levels for the two independent magnetic axes (X and Y) that are perpendicular to the roll axis Z
(which is nearly parallel to the radius vector to the Sun) at intervals of 2 - 3 months. 
There is no roll calibration for the Z magnetic axis. Comparison of the two derived magnetic 
vectors from the two magnetometers permits validation of the primary magnetometer data with 
an accuracy of 0.02 nT - 0.05 nT. A discussion of the uncertainties that must be considered 
when using these data is given in the Appendix of Burlaga et al. [1994] and in Appendix A of 
Burlaga et al. [2002].
Voyager 1 and 2 prior to 2004.
At the time of experiment proposal, it was expected that the required accuracy of the measurements
would be +/- 0.1 nT, determined by the combined noise of the sensors and the unresolved spacecraft
field by the dual magnetometer method. In RTN coordinates, the 1 s uncertainty for BR and BN 
determined from successive rolls of Voyager 2 is typically +/-0.05 nT (but it can be larger),
and the uncertainty of BR and BN determined from successive rolls of Voyager 1 is somewhat smaller.
The uncertainty in BR cannot be determined from the rolls. It is consistently found that BR can
 be estimated to within ± 0.05 nT using the condition that <BR> = 0, where the average is over
the interval from 26 days before the roll to 26 days after the roll. 

Voyager 1, 2004 - 2010. 
The data for Voyager 1, after 2003  were processed with different methods
that are described below. Zero tables containing corrections to the observations were generated 
manually (with the assistance of a computer).A zero table for each of these six sensors was 
calculated every 48 sec (except for the 2005 data, for which the zero tables were calculated 
every hour). We estimate that the procedure gives a 1 s uncertainty of the measured BR, BT, 
and F1 of approximately ~0.02 nT for this particular interval. The uncertainties for any given
hour can be significantly different than +/- 0.02 nT. The uncertainties in F1, BR, BT, and BN
can differ from one another, but there is no practical way to determine these uncertainties more
precisely at present. The uncertainties in BN were still calculated using the condition that 
<BR> = 0, where the average is over the interval from 26 days before the roll to 26 days
after the roll. 
Voyager 2, 2005 - 2011. 
The data for Voyager 2, after 2004 were processed with a method similar to that for Voyager 1 
from 2005 - 2010, with some differences to handle the special noise signals on Voyager 2 discussed
in the references below.  The zero tables were calculated every 48 sec for these data. We estimate
that the procedure gives a 1 s uncertainty of the measured F1 and each of the components of 
approximately ~0.03 nT. The uncertainties for any given hour can be significantly different 
than +/- 0.03 nT, and the uncertainties in F1, BR, BT, and BN can differ from one another, 
but there is no practical way to determine these uncertainties more precisely at present. 
Again, the uncertainties in BN were still calculated using the condition that <BR> = 0,
where the average is over the interval from 26 days before the roll to 26 days after the roll.

Voyager 1 (2010 - and later) and Voyager 2 (2011 - and later)
As Voyager 1 and Voyager 2 move deeper into the heliosheath, one must be increasingly concerned about the validity of the assumption used to calibrate the Z component of the magnetic field, namely <BR> = 0, where the average is over the interval from 26 days before the roll to 26 days after the roll. This assumption is a good approximation from approximately 10 AU to distances of 20 or 30 AU in the heliosheath, but it is not justified in the interstellar medium or in the outermost regions of the heliosheath. Consequently, for Voyager 1 data after 2009, and Voyager 2 data after 2011, we have introduced a new method of calibrating the Z component and calculating BR. This new method of calibrating the Z- component was suggested by Mario Acuña, as described by D. Berdichevsky (2009). Basically, the idea is that one might be able to use data from “magcals” that are obtained approximately every 30 days by the magnetometers, to calibrate the magnetometers. This idea has been refined, implemented and tested by Berdichevsky over the course of approximately 2 years. Because the quality of the corrections derived from the magcals depends on the individual magcals, corresponding uncertainty in BR lies within the range ± 0.02 nT to ± 0.10 nT). We take ± (0.06 ± 0.04) nT as a nominal value. The sampling rate of the triaxial fluxgate magnetometers in the heliosheath and beyond is equal to 2.08 samples s-1, and the repetition rate of the telemetry is 0.0208 Hz. The standard output of the processing system for each spacecraft is a set of 48 second averages the inboard and outboard magnetometers. The data are processed using the procedure introduced in 2004 and 2005 for the Voyager 1 and Voyager 2, respectively, except that the new method for calibrating the Z component has been added. The 1-s uncertainties associated of BR and BT are still ± 0.02 nT for Voyager 1 and ± 0.03 nT for Voyager 2, respectively. The uncertainties are dominated by systematic errors. In general, the systematic errors are highly variable. We make a correction for the systematic errors every 48 s. The systematic errors are generally larger than the uncertainties that can be described by statistical methods, which must also be considered. The extreme outliers are removed and each 48 s average of BR, BT, and BN is examined by eye to remove magrolls, other spacecraft maneuvers or events, magcals, and various types of noise (including noise from the sensors, and electronics associated, signals from the spacecraft, and noise from the spacecraft telemetry system and the ground tracking stations. The noise associated with the magnetometers on V2 is greater than that associated with the magnetometers on V1. Another important source of noise, unique to V2, was produced as an unintended consequence of a spacecraft command, nominally unrelated to the magnetometer, which rotated two of the sensors on the outboard magnetometer through 58°(complicating the mag-roll calibrations) and heated the sensor triad well beyond the design limits of the instrument. The heat damaged the instrument, resulting in an additional significant noise signal, which cannot be separated uniquely from the other sources of noise. Thus, extracting the heliosheath magnetic field from the various types of noise in the V2 data is a very challenging and lengthy task. There remain significant uncertainties in the results that must be considered in the interpretations of these data. In addition to these sources of noise, there are limitations associated with the telemetry coverage by ground stations which gives data gaps of ~12–16 hr every day. There are smaller data gaps associated with the removal of magrolls, magcals, and various types of noise. For the V2 data in 2010, there was no data for the interval from DOY 112 through 142, owing to a spacecraft malfunction. The hourly averages are derived from the final “clean” 48 s component averages. References Behannon, K.W., M.H. Acuna, L.F. Burlaga, R.P. Lepping, N.F. Ness, and F.M. Neubauer, Magnetic-Field Experiment for Voyager-1 and Voyager-2, Space Science Reviews, 21 (3), 235-257, 1977. Burlaga, L.F., Merged interaction regions and large-scale magnetic field fluctuations during 1991 - Voyager-2 observations, J. Geophys. Res., 99 (A10), 19341-19350, 1994. Burlaga, L.F., N.F. Ness, Y.-M. Wang, and N.R. Sheeley Jr., Heliospheric magnetic field strength and polarity from 1 to 81 AU during the ascending phase of solar cycle 23, J. Geophys. Res., 107 (A11), 1410, 2002. Ness, N., K.W. Behannon, R. Lepping, and K.H. Schatten, J. Geophys. Res., , 76, 3564, 1971.
Notes on Voyager 2 Plasma Data After 1989:
Vtheta, Vphi, RTN is often NOT reliable after 1989

Notes about Pioneer 6 and 7 and Mariner 2 data
The Pioneer 6 and 7 and Mariner 2 data were provided in 2000 by Dr. Marcia Neugebauer and Joyce Wolf after they created them in COHOWeb-ASCII format in order to be able to use them, along with later heliospheric data acquired from COHOWeb, in the analysis reported in Neugebauer et al., The solar magnetic field and the solar wind: Existence of preferred longitudes, J. Geophys. Res., 105, 2315, 2000.

Notes about Ulysses SWOOPS data
Two temperatures, T-large and T-small, were provided by the Ulysses SWOOPS team. See swoops_ion_users_guide_update_20030214.txt for a discussion of the difference between the two. Both T-large and T-small are contained in the Ulysses field-plasma-merged ASCII files described above. The values of T-small were used in the Ulysses CDF file.

Data format

The common set of words in each record, and their format when included in an ASCII output record are:
 WORD  DESCRIPTION                                  UNITS               FORMAT

 1  Time                                            (1)                 (1)
 2  S/C Heliocentric Distance                       AU                  F6.2
 3  S/C HelioGraphic Inertial (HGI) Latitude,       deg.                F6.1
 4  S/C Longitude, HGI                              Deg                 F6.1
 5  IMF BR in RTN(Radial-Tangential-Normal)         nT                  (2)
    coordinate system                                          
 6  BT in RTN                                       nT                  (2)
 7  BN in RTN                                       nT                  (2)
 8  B Field Magnitude                               nT                  (2)
    (average of fine scale magnitudes)    
 9  Proton Flow Speed                               Km/sec              F6.1
10  Proton Density                                  No/cc               (2)
11  Proton Temperature                              Deg K               F8.0
12  Proton Flow Elevation Angle/Latitude (RTN)      Deg                 F6.1
13  Proton Flow Azimuth Angle/Longitude (RTN)       Deg                 F6.1

(1) There are actually two time words included in the COHOWeb CDF 
records.  One is primarily for the CDF display software to use, and 
the other is more human-comprehensible and is the only time word 
downloaded with an ASCII file or files in most other available formats.  
Its format as included in downloaded records is: DD-MN-YYYY HH:00
(2) The format depends on which spacecraft; those further from the sun 
(where smaller field intensities and densities are encountered) have 
more precision.  For any data file created and downloaded, a companion 
format file is also provided.

Description of Coordinate Systems

Heliographic Inertial Coordinate System (HGI): The HGI coordinates are Sun-centered and inertially fixed with respect to an X-axis directed along the intersection line of the ecliptic and solar equatorial planes. The solar equator plane is inclined at 7.25 degrees from the ecliptic. This direction was towards ecliptic longitude of 74.367 degrees on 1 January 1900 at 1200 UT; because of precession of the celestial equator, this longitude increases by 1.4 degrees/century. The Z axis is directed perpendicular and northward from the solar equator, and the Y-axis completes the right-handed set. This system differs from the usual heliographic coordinates (e.g. Carrington longitudes) which are fixed in the frame of the rotating Sun.

RTN Coordinate System The RTN system is centered at a spacecraft or planet and oriented with respect to the line connecting the Sun and spacecraft or planet. The R (radial) axis is directed radially away from the Sun through the spacecraft or planet. The T (tangential) axis is the cross product of the Sun's spin vector (North directed) and the R axis, i.e. the T axis is parallel to the solar equatorial plane and is positive in the direction of planetary rotation around the Sun. The N (normal) axis completes the right handed set. The RTN system is preferable for analyzing solar wind and energetic particle data.

Access to Other COHOWeb-Related Data and Information

As mentioned earlier, COHOWeb's ASCII field-plasma-position-merged annual files, are themselves FTP-accessible from "merged" subdirectories under each COHOWeb spacecraft directory at These directories also include field and plasma data at other than hourly resolution, and energetic particle data.

Contact and Citation Information

For questions or comments about the COHOWeb service or data, please contact Dr. Natalia Papitashvili (, Code 672, Goddard Space Flight Center, Greenbelt, MD, 20771.

For questions or comments about other related data, please contact Dr. John F. Cooper (, Code 672, NASA Goddard Spaceflight Center, Greenbelt, MD 20771.

Overall definition and guidance for COHOWeb effort provided by Dr. Joseph H. King

Acknowledgements to the SPDF COHOWeb database or to ANON/FTP data files as the source of data used in publications is requested. Copies of preprints or reprints of publications sent to John Cooper (address above) would be appreciated for tracking purposes.

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