Data Set Information
DATA_SET_NAME GALILEO ORBITER A MAG SUMM GASPRA SUMMARY V1.0
DATA_SET_ID GO-A-MAG-4-SUMM-GASPRA-SUMMARY-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Dataset Overview ================ This dataset contains data acquired by the Galileo Magnetometer during the Gaspra flyby on Oct 29. 1991. The browse dataset has been created by averaging samples from the 7.68 kB Low Rate Science (LRS) tape record mode to 20 s second sampling and merging the data with the 8 RIM averages of the optimal averager to provide a continuous dataset for the encounter day. Limited space on the tape recorder forced the magnetometer team to limit their high time resolution observations to a few short intervals in the pre-encounter solar wind, a main record period near closest approach. The optimal averager section of the instrument (please see the instrument description) was configured to acquired 8 RIM (~8 minute) averages to fill in the gaps between the recorded intervals. All of these data have been fully processed to remove instrument response function characteristics and interference from magnetic sources aboard the spacecraft. The magnetometer data are provided in heliographic (RTN) coordinates. Trajectory data have been provided as a separate archive product. Primary Dataset Reference: Kivelson, M.G., L.F. Bargatze, K.K. Khurana, D.J. Southwood, R.J. Walker, P.J. Coleman, 'Magnetic Field Signatures Near Galileo's Closest Approach to Gaspra', Science, Vol. 261, p331-334, 16 July, 1993. [KIVELSONETAL1993] Wang, Z., and Kivelson, M.G., 'Asteroid interaction with solar wind', J. Geophys. Res., 101, 24479, 1996. [WANG&KIVELSON1996] Primary Instrument Reference: Kivelson, M.G., K.K. Khurana, J.D. Means, C.T. Russell, and R.C. Snare, 'The Galileo magnetic field investigation', Space Science Reviews, 60, 1-4, 357, 1992. [KIVELSONETAL1992] Data ==== ------------------------------------------------------------------ Table 1. Data record structure, RTN Coordinates ------------------------------------------------------------------ Column Description ------------------------------------------------------------------ time S/C event time (UT) given in PDS time format YYYY-MM-DDThh:mm:ss.sssZ Br Magnetic field radial component in RTN coordinates Bt Magnetic field tangential component in RTN coordinates Bn Magnetic field normal component in RTN coordinates Bmag Average magnetic field magnitude Missing data value = 99999.999 Fortran Format of the data file: (1X, A24, 4F11.3) Data Acquisition ---------------- The data for this dataset were all acquired in by the outboard magnetometer sensors in the flip left mode in the low field mode (ULLR). This mode has a digitization step size of 0.0625 nanoTesla. However, these data are acquired at 30 vectors/second and then recursively filtered in the instrument. The high rate data that are recorded to tape have a sample rate of 4.5 vectors/second. If there is sufficient variation in the 6-7 input samples that make up a single output sample, then the effective digitization step size becomes much smaller. The long averages provided in the browse dataset should not show any quantization effects. Data Sampling ------------- The high rate recorded data are not evenly sampled within a minor frame. However, these data have been averaged using an averaging routine to produce evenly sampled data. The averaging process takes all available data during a 20 second period centered on the output time tag. Averages are not overlapping. The time tag gives the spacecraft event time (SCET) of the center of the averaging interval in universal time (UT). The optimal averager section of the instrument 'averages' data by applying a recursive filter to onboard processed and despun data and then decimating the data at the appropriate sample times. This process is explained in detail in the 'cruise' dataset that has been included with this archive product. The final processed product that has been incorporated into this dataset is sampled about every 8 minutes where the time tag marks the center of the averaging interval in SCET / UT. Coordinate System ================= The data are provided in heliographic RTN (radial-tangential-normal) coordinates. The radial direction is taken along the instantaneous Sun->S/C line, positive away from the sun. The tangential direction is found by taking the cross product of the sun spin axis with the radial (T = Omega x R) direction. Finally, the normal direction is the cross product of R and T (N = R x T). The magnetic field perturbation associated with the Gaspra flyby [KIVELSONETAL1993] is most easily understood in a coordinate system that is organized by the interplanetary magnetic field (IMF). The IMF coordinate system used by [KIVELSONETAL1993] to analyze the Gaspra flyby data takes data from the GaSE coordinate system and rotates about the Asteroid-Sun line (X) such that the average upstream IMF direction (between 22:30 and 22:33 UT) lies in the X-Y plane. This requires a righthanded rotation of 32.44 degrees about the GaSE X-axis to generate By_imf and Bz_imf from By_GaSe and Bz_GaSE. An IMF coordinate system is only valid for a short interval near the time interval that defines the IMF direction. [KIVELSONETAL1993] uses this coordinate system only in the analysis of data acquired between 22:15 and 23:05 UT on the day of encounter (10/29/91). Ancillary Data ============== A subset of the Galileo interplanetary cruise magnetometer dataset (GO-SS-MAG-4-SUMM-CRUISE-RTN-V1.0) has been supplied as an ancillary data product with this archive. The cruise data are provided to place the encounter data in context with large scale structures in the solar wind and IMF. These data are provided in RTN coordinates which is a standard coordinate system for solar wind data analysis. The time interval provided (9/10/91 - 11/24/91) spans roughly 3 solar rotations centered on the asteroid flyby. These data show that the Gaspra encounter occurred in an 'away' sector a day or so before a large field compression associated with a corotating interaction region. The data are stored as an ASCII table in the file 'CRUISE.TAB'. These data ------------------------------------------------------------------ Table 2. Data record structure, RTN coordinates cruise data ------------------------------------------------------------------ Column Description ------------------------------------------------------------------ time S/C event time (UT) given in PDS time format sc_clk S/C clock counter given in the form rim:mod91:mod10:mod8 Br Magnetic field radial component Bt Magnetic field tangential component Bn Magnetic field normal component Bmag |B| Magnitude of B R Radial distance of the spacecraft from the Sun LAT Solar latitude of the spacecraft LON Solar east longitude of the spacecraft avg_con Onboard averaging interval for the magnetometer data delta Magnetic inclination angle: delta=arcsin(Bn/Bmag) lambda Magnetic azimuth angle: lambda = atan2(Bt/Br) * 1 RIM = 60.667 seconds (spacecraft major frame)
DATA_SET_RELEASE_DATE 2004-05-01T00:00:00.000Z
START_TIME 1991-10-29T06:03:20.834Z
STOP_TIME 1991-10-30T02:51:03.452Z
MISSION_NAME GALILEO
MISSION_START_DATE 1977-10-01T12:00:00.000Z
MISSION_STOP_DATE 2003-09-21T12:00:00.000Z
TARGET_NAME GASPRA
TARGET_TYPE ASTEROID
INSTRUMENT_HOST_ID GO
INSTRUMENT_NAME DUAL TECHNIQUE MAGNETOMETER
TRIAXIAL FLUXGATE MAGNETOMETER
MAGNETOMETER
FLUXGATE MAGNETOMETER
INSTRUMENT_ID MAG
INSTRUMENT_TYPE MAGNETOMETER
Magnetometer
NODE_NAME planetary plasma interactions
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Confidence Level Overview ========================= The sources of error which are the most significant are those associated with magnetic sources aboard the spacecraft, especially those with temporal or spacecraft orientation variations. The next greatest contributor of error is the data from the AACS which affects our knowledge of the spacecraft orientation and hence rotates the magnetic field vector. Lastly, telemetry or software errors which produce 'spikes' or bit errors in the data are error sources. In this dataset the magnetic sources associated with the spacecraft were fairly constant and magnetic field values are accurate to at least 0.01 nT. The spacecraft orientation has been well determined and is known to better the 0.1 degree.
CITATION_DESCRIPTION Kivelson, M.G., Khurana, K.K., Russell, C.T., Walker, R.J., Joy, S.P.,Green, J., GALILEO ORBITER A MAG SUMM GASPRA SUMMARY V1.0, GO-A-MAG-4-SUMM-GASPRA-SUMMARY-V1.0, NASA Planetary Data System, 2004
ABSTRACT_TEXT Galileo Orbiter Magnetometer (MAG) calibrated averaged data from the Gaspra flyby in RTN coordinates. These data cover the interval 1991-10-29 06:03 to 1991-10-30 02:51.
PRODUCER_FULL_NAME MARGARET G. KIVELSON
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