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Availability: Archived at NSSDC, accessible from elsewhere

Time span: 1993-08-28 to 1993-08-28


This description was generated automatically using input from the Planetary Data System.

Data Set Overview ================= This data set contains data acquired by the Galileo Magnetometer during the Ida flyby on Aug 28. 1993. The browse dataset has been created by averaging samples from the 4/3 sec optimal averager data to 20 s second sampling and merging the data with the 1 and 2 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 highest time resolution observations to a ~30 minute interval beginning after closest approach to Ida. In order to acquire the high time resolution mag data at Ida necessary to look for magnetic signatures similar to those observed at Gaspra, a new method of using the instrument optimal averager section was tested. In this method, the CDS sampled the MAG memory to retrieve data every 4/3 second. The data were returned to the ground via a CDS memory readout (MRO). The data were highly overfiltered by the instrument but many of the effects of over filtering were recovered in ground data processing by using knowledge of the filter response function. The optimal averager section of the instrument (please see the instrument description) was configured to acquire 1 and 2 RIM (RIM = 60.667 sec) averages to cover the rest of the encounter day. These data have been processed to remove most instrument and filter response function characteristics from the data. The magnetometer data are provided in heliographic (RTN) coordinates. Trajectory data have been provided as a separate archive product. Primary Dataset References: Kivelson, M.G., Z. Wang, S. Joy, K.K. Khurana, C. Polanskey, D.J. Southwood, and R.J. Walker, 'Solar Wind Interactions with Small Bodies: 2. What Can Galileo's Detection of Magnetic Rotations Tell Us About GASPRA and IDA', Advances in Space Research, 459, 1995. [KIVELSONETAL1995] 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 -----------------------------------------------------------------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 <nT> Bt Magnetic field tangential component in RTN coordinates <nT> Bn Magnetic field normal component in RTN coordinates <nT> Bmag Average magnetic field magnitude <nT> 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 data are next passed to an onboard processing algorithm which corrects the data for offsets, gains, and geometry. The data can now be sent to the tape recorder or passed to the optimal averager section of the instrument. Optimal averager data are decimated to 1 vector every minor frame such that only the first sample of he minor frame is retained. These data are then despun into Inertial Rotor Coordinates (IRC) and passed to another recursive filter operation. The filter used in the Ida high resolution (4/3 sec) data was: Bo(i) = (15/16) * Bo(i-1) + (1/16)*Bi(i) where Bo(i) is the output of the i-th sample and Bi(i) is the new input vector at the time of the i-th sample. The 1 and 2 RIM optimal averager data were more highly filtered (1/32 and 1/64 filters respectively). Data Sampling ------------The high rate optimal averager data have been resampled at 20 second resolution where the time stamp indicates the center of the average. Averages are non-overlapping. The low rate optimal averager data are received on the ground with an end of average time stamp. The timing of these data is then corrected to an earlier time approximately 1/3 sampling interval earlier than the end of average. The timing correction has been determined empirically from observations where both high rate and optimal averager data were simultaneously acquired. The time tag gives the spacecraft event time (SCET) in universal time (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 Ida flyby [KIVELSONETAL1995] is most easily understood in one of two principal axis coordinate systems. At Gaspra the data were rotated into what was called an IMF coordinate system by rotating the field about the asteroid-Sun line (X-axis, solar wind flow direction) such that the IMF was contained in the X-Y plane. This same type of coordinate system is useful at Ida. The average upstream field orientation between 16:40 and 16:45 in IdaSE coordinates defines the IMF direction. The rotation from IdaSE coordinates to IdaIMF coordinates is a 15.86 degree righthanded rotation about the IdaSE +X-axis. In this coordinate system, the solar wind flow direction is the principal axis (X) and the IMF direction is the secondary vector. [KIVELSONETAL1995] define a second principal axis coordinate system which places the X axis along the upstream IMF direction while maintaining the solar wind flow direction in the X-Y plane. This coordinate system can be obtained by a -62.52 degree righthanded rotation of the IdaIMF coordinate system data about the IdaIMF +Z-axis. These coordinate systems are only valid for a short interval near the time interval that defines the IMF direction. [KIVELSONETAL1995] use these coordinate systems only in the analysis of data acquired between 16:30 and 17:20 UT on the day of encounter (8/28/93). 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'. -----------------------------------------------------------------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 <nT> Bt Magnetic field tangential component <nT> Bn Magnetic field normal component <nT> Bmag |B| Magnitude of B <nT> R Radial distance of the spacecraft from the Sun <AU> LAT Solar latitude of the spacecraft <degrees> LON Solar east longitude of the spacecraft <degrees> avg_con Onboard averaging interval for the magnetometer data <RIM*> delta Magnetic inclination angle: delta=arcsin(Bn/Bmag) <radians> lambda Magnetic azimuth angle: lambda = atan2(Bt/Br) <radians> * 1 RIM = 60.667 seconds (spacecraft major frame)

These data are available on-line from the Planetary Data System (PDS) at:

Alternate Names



  • Planetary Science: Fields and Particles

Additional Information



Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II



NameRoleOriginal AffiliationE-mail
Dr. Margaret Galland KivelsonData ProviderUniversity of California, Los
Dr. Margaret Galland KivelsonGeneral ContactUniversity of California, Los
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