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1: 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.
2: 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 ftp://nssdcftp.nasa.gsfc.gov/spacecraft_data/.spacecraft_series_name. (if relevant)/.spacecraft_name./merged.
3: Identification of Available Data
Data currently available from COHOWeb are given as follows:
SPACECRAFT MAGNETIC FIELD DATA PLASMA DATA
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
OMNI_M (Several) 1963/11/27 - 2009/01/11 (Several) 1963/11/27 - 2009/01/11
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
Ulysses Balogh/Smith 1990/10/25 - 2008/02/29 Phillips 1990/11/18 - 2009/02/07
Voyager 1 Ness 1977/09/07 - 2006/12/31 Belcher 1977/09/07 - 1980/11/23
Voyager 2 Ness 1977/08/24 - 2007/12/31 Belcher 1977/08/24 - 2009/02/03
Notes on Voyager 1 and 2 Magnetometer Data After 1989: (4/13/2007): 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). 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 that are perpendicular to the roll axis (which is nearly parallel to the radius vector to the Sun) at intervals of ¯3 months. There is no roll calibration for the third 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. 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 original dual magnetometer design (Ness et al., 1971; Behannon et al. 1977). 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 that are perpendicular to the roll axis (which is nearly parallel to the radius vector to the Sun) at intervals of 3 months. There is no roll calibration for the third magnetic axis. The sampling rate after the year 2000 is 0.48 sec (24 sec) for the primary (secondary) magnetometer. 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]. The 1 uncertainty for Voyager 2 is typically +/-0.05 nT (but it can be larger), and the uncertainty for Voyager 1 is somewhat smaller. Voyager 1, 2004 and later. The data for Voyager 1, after 2003 were processed with different methods than that described in the references above. Zero tables containing corrections to the observations were generated manually and more frequently than previously. The, zero tables were calculated every 48 sec for these data (except for the 2005 data, for which the zero tables were calculated every hour). We estimate that the procedure gives a 1 uncertainty of the measured F1 and each of the components of approximately ~0.015 nT for this particular interval. The uncertainties for any given hour can be significantly different than +/- 0.015 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. Voyager 2, 2005 and later. The data for Voyager 2, after 2004 were processed with a method similar to that for Voyager 1, 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 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.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 ftp://nssdcftp.gsfc.nasa.gov/spacecraft_data/ulysses/plasma/swoops/ion/
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.
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.
4: 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.
5: 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 ftp://nssdcftp.gsfc.nasa.gov/spacecraft_data/.spacecraft_series_name.(if relevant)/.spacecraft_name.. These directories also include field and plasma data at other than hourly resolution, and energetic particle data.The COHOWeb system and its data were first described in "Internet Access to NASA's OMNI and COHO Data Bases for Interplanetary Missions," by J. F. Cooper, J. H. King, G. J. Mathews, R. E. McGuire, N. E. Papitashvili, R. Parthasarathy, and S. S. Towheed, in Proceedings of the 24th Cosmic Ray Conference, Rome, Vol. 4, p. 1295-1298, Aug. 28 - Sept. 8, 1995.
6: Contact and Citation Information
For questions or comments about the COHOWeb service or data, please contact Dr. Natalia Papitashvili (natasha@mail630.gsfc.nasa.gov), Code 672, Goddard Space Flight Center, Greenbelt, MD, 20771.For questions or comments about other related data, please contact Dr. John F. Cooper (John.F.Cooper@nasa.gov), 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 |
- Curator(s): Dr. Natalia Papitashvili, James Gass
-
NASA Official: Dr. Robert McGuire
(301)286-7794, Robert.E.McGuire@nasa.gov