**NSSDCA ID:** PSFP-00401

**Availability:** Archived at NSSDC, accessible from elsewhere

**Time span:** 1991-10-29 to 1991-10-30

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

Data Set Overview ================= This dataset contains data acquired by the Galileo Magnetometer from the Gaspra encounter. The data are at the full instrument resolution for the 7.68 kB Low Rate Science (LRS) real time telemetry mode. These data have been fully processed to remove instrument response function characteristics and interference from magnetic sources aboard the spacecraft. The data are provided in physical units (nanoTesla) in 4 coordinate systems. One set of data files contain data in Inertial Rotor Coordinates (IRC = despun spacecraft). These files include many of the data processing parameters from the AACS system as well as the sensor zero levels. The other set of files contain magnetic field data in Gaspra-centric Solar Ecliptic (GaSE), Earth Mean Equatorial equinox 1950 (EME-50), and finally in Heliographic (RTN) coordinates. Trajectory data are provided as a separate dataset for each of the geophysical coordinate systems. Primary Dataset References: 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, Spacecraft Coordinates (IRC) -----------------------------------------------------------------Column Description -----------------------------------------------------------------time S/C event time (UT) given in seconds since 1966 rotattt Rotor twist angle (EME-50) rotattd Rotor attitude declination (EME-50) rotattr Rotor attitude right ascension (EME-50) spinangl Rotor spin angle - inertial S/C coordinates spindelt Rotor spin motion delta <radians/minor frame> screlcon Rotor-Platform relative cone angle screlclk Rotor-Platform relative clock angle Bx_sc Magnetic field X component in S/C (IRC) coordinates By_sc Magnetic field Y component in S/C (IRC) coordinates Bz_sc Magnetic field Z component in S/C (IRC) coordinates Bmag |B| Magnitude of B -----------------------------------------------------------------Table 2. Data record structure, Geophysical Coordinates (GaSE, EME-50, and RTN) -----------------------------------------------------------------Column Description -----------------------------------------------------------------time S/C event time (UT) given in PDS time format YYYY-MM-DDThh:mm:ss.sssZ Bx_GaSE X component of B in GaSE coordinates (towards Sun) <nT> By_GaSE Y component of B in GaSE coordinates (towards dusk) <nT> Bz_GaSE Z component of B in GaSE coordinates (|| to ecliptic normal) <nT> Bx_eme50 Magnetic field X component in EME-50 coordinates <nT> By_eme50 Magnetic field Y component in EME-50 coordinates <nT> Bz_eme50 Magnetic field Z component in EME-50 coordinates <nT> 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 |B| Magnitude of B <nT> Data Acquisition ---------------The data were acquired by the Galileo Magnetometer in the normal Low Rate Science (LRS) manner except that the data were recorded to tape and played back during the Earth 2 encounter. The data were acquired by the outboard magnetometer in the flip left position and the low range (high gain) mode. The Galileo magnetometer has 8 possible LRS acquisition configurations (modes). There are two sensor triads mounted 7 and 11 meters from the rotor spin axis (inboard and outboard) along the boom. Each of the sensor triads has two gain states (high and low). In addition, the sensor triads can be 'flipped' to move the spacecraft spin-axis aligned sensor into the spin plane and visa versa. Please see the instrument description for full details on the instrument, sensors, and geometries. The combinations of sensor, gain state, and flip direction form modes. -------------------------------------------------------------------Table 3. Mode Characteristics -------------------------------------------------------------------Mode Name Acronym range quantization -------------------------------------------------------------------Inboard, left, high range* ILHR +/-16384 nT 8.0 nT Inboard, right, high range* IRHR +/-16384 nT 8.0 nT Inboard, left, low range* ILLR +/- 512 nT 0.25 nT Inboard, right, low range* IRLR +/- 512 nT 0.25 nT Outboard, left, high range* ULHR +/- 512 nT 0.25 nT Outboard, right, high range* URHR +/- 512 nT 0.25 nT Outboard, left, low range* ULLR +/- 32 nT 0.008 nT Outboard, right, low range* URLR +/- 32 nT 0.008 nT * range is the opposite of gain Data Sampling ------------The Galileo magnetometer samples the magnetic field 30 times per second. In order to reduce the data rate to the MAG LRS rate, the instrument performs an onboard averaging process. Recursive Filter: B(t) = 1/4 Bs(t) + 3/4 B(t-1) B = output field Bs = input field measured by the sensor t = sample time The high rate samples are recursively filtered and then resampled by the instrument at 4.5 vectors per second using a 7,7,6 decimation pattern. The pattern is generated by doubling the spacecraft clock modulo 10 counter and then applying the decimation scheme. This gives 3 vectors every spacecraft minor frame (about 2/3 second) which are sampled unevenly. The first vector in a minor frame is sampled approximately 0.200 seconds after the last vector in the preceding minor frame. The other two samples are taken approximately 0.233 seconds apart. The time tag associated with a sample is the decimation time. Coordinate Systems ================== The Galileo magnetometer data are being archived in 4 coordinate systems. The first coordinate system is referred to as inertial rotor coordinates (IRC). This coordinate system has the Z axis along the rotor spin axis, positive away from the antenna and the X and Y axes lies in the rotor spin plane. In a crude sense, when the spacecraft is far from Earth, +X points south, normal to the ecliptic plane, positive Y lies in the ecliptic plane in the sense of Jupiter's orbital motion and positive Z is in the anti-earth direction. The spacecraft antenna (negative Z direction) is kept earthward pointing to about +/- 10 deg accuracy. Gaspracentric Solar Ecliptic (GaSE) is a Gaspra centered coordinate system defined by the primary vector along the instantaneous Gaspra->Sun (GSun) line and the Earth's ecliptic north pole (ENP) as the secondary vector. In this coordinate system: X is the GSun unit vector taken to be positive towards the Sun. Y is the formed by the unitized cross product ENP x GSun Z completes the right handed set (Z = X x Y) such that the X-Z plane contains the ecliptic north pole. The Earth Mean Equatorial equinox of 1950 (EME-50) coordinate system is an inertial reference system. The primary vector in this system points from the Earth towards Aries at the reference epoch. The secondary vector is along the Earth's rotational axis positive towards it's north pole. In this coordinate system: X is a unit vector in the direction of Aries at the equinox of 1950 Y is a unit vector in the direction of Z x X Z is a unit vector in the direction of the Earth's equatorial north pole at the equinox of 1950. The EME-50 coordinate system is directly supported by SPICE as the 'FK4' inertial reference frame. The heliographic (RTN) coordinate system centered at the Sun. The primary vector in this system points from the center of the Sun to the spacecraft (R). The secondary vector in this system is the Sun's north rotational axis (Omega). R is along R, positive away from the Sun. T is along the cross product Omega x R N completes the right handed set (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] use 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'. -----------------------------------------------------------------Table 4. 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) Data Processing =============== These data have been processed from the PDS dataset: 'GO-E/V/A-MAG-2-RDR-RAWDATA-HIRES-V1.0' The 'raw data' product was created from the EDR dataset by removing the data processing done by the instrument in space. The raw data dataset contains the raw instrument samples which have been recursively filtered and decimated as described above. The processed data in IRC coordinates were rotated into geophysical coordinates using the SPICE libraries and kernels provided by NAIF. In order to generate the IRC processed data, the following procedure was followed: 1) Sensor zero level corrections were subtracted from the raw data, 2) Data were converted to nanoTesla, 3) A coupling matrix which orthogonalizes the data and corrects for gains was applied to the data (calibration applied), 4) Magnetic sources associated with the spacecraft were subtracted from the data, 5) Data were 'despun' into inertial rotor coordinates, 1) Zero level determination: The zero levels of the two spin plane sensors were determined by taking averages over a large number (about 50) of integral spin cycles. The zero level of the spin axis aligned sensor was determined by a variety of means. First, since the spin axis aligned sensor can be flipped into the spin plane, the value of the zero level determined in the spin plane can be used in the other geometry. This works well if there are no spacecraft fields and the zero level is stable. If there are spacecraft fields present which remain constant over relatively long time periods (many hours), then another method of zero level determination is used. The spacecraft spin axis is along the Z direction, the data in the X and Y directions have already had zero level corrections applied. Bm(z) = B(z) + O(z) |Bm|^2 = B(x)^2 + B(y)^2 + Bm(z)^2 = B(x)^2 + B(y)^2 + B(z)^2 + O(z)^2 + 2B(z)O(z) = |B|^2 + O(z)^2 + 2B(z)O(z) = |B|^2 + O(z)^2 + 2O(z)[Bm(z) - O(z)] = |B|^2 - O(z)^2 + 2O(z)Bm(z) m = measured value - no subscript = true value Now if |B| remains constant over a short interval and O(z) remains constant over a much longer interval, we can take averages and reduce this equation to: |Bm|^2 - <|Bm|^2> = 2O(z)[Bm(z) - <Bm(z)>] <> indicates average value Data can be processed using short averages of |B| until many points are accumulated and then fit with a line in a least squares sense. The slope of this line is twice the required offset. The scatter in the data give an indication of the error in the assumption the |B| and O(z) have remained constant. Intervals with large rms errors are not retained. The zero levels removed from the data are given in the IRC data file. 2) Conversion to nanoTesla simply requires dividing the instrument data numbers by a constant scale factor. For the inboard high range (low gain) mode the scale factor is 2. For the inboard low range and outboard high range, the scale factor is 64. The outboard low range data has a scale factor of 1024. 3) Calibration matrix applied: The determination of a calibration matrix is too complex to describe here. The method employed has been well described in [KEPKOETAL1996]. 4) After the data were initially processed (calibrated and despun), it was clear that there were still coherent noise sources remaining in the data. Dynamic spectra of the magnetometer data revealed coherent energy at high order (2nd, 3rd, 4th) harmonics of the spin period. High order harmonics of the spin period can be generated by spinning about a fixed dipole source such as a source on the despun platform. The source of the high order harmonics was modeled using 1-D (spacecraft clock angle) Fourier transforms of high pass filtered data. This allows us to resolve the source in terms of the relative spin phase of the scan platform. Model fields associated with this source (approximately 0.15 nT at the inboard sensors in the lowest harmonic) have been subtracted from the data. 5) Despinning: Data are despun and checked in inertial rotor coordinates before transforming to geophysical coordinates. Any errors in the processing will be most readily apparent in inertial rotor coordinates. The nominal transformation to IRC from SRC is (Bx) ( cos(theta) -sin(theta) 0 ) (Bxs) (By) = ( sin(theta) cos(theta) 0 ) (Bys) (Bz) ( 0 0 1 ) (Bzs) Where s denotes spinning coordinates and theta is the rotor spin angle. Frequency dependent phase delays associated with the analog anti-aliasing filter and the digital recursive filter have been removed during the despinning of the data. The dominant frequency in the spinning data is at the spacecraft spin frequency. The phase angle delay associated from all known sources is computed at the spin frequency and removed from the data during despinning. Analog Filter: Digital Filter (Nyquist Freq Fn = 15Hz) 1543 1/3 ------------------ --------------------s^2 + 55.5s + 1543 4/3 - exp(-PI*i*f/Fn) s = 2*PI*i*f Imaginary = 55.5s Imaginary = -sin(PI*f/Fn) Real = 1543 + s^2 Real = 4/3 - cos(PI*f/FREQ_N) f = frequency delay = tan^-1(Im/Re) In addition, there is an electrical delay associated with the A/D conversion of about .037 milliseconds. This delay is converted to an angle using the instantaneous spin frequency. These 3 sources of delay are then summed in to the quantity 'phase' and then the despinning matrix becomes: (Bx) ( cos(theta - phase) -sin(theta - phase) 0 ) (Bxs) (By) = ( sin(theta - phase) cos(theta - phase) 0 ) (Bys) (Bz) ( 0 0 1 ) (Bzs) 6) Data was transformed to geophysical coordinates: Data are transformed from inertial rotor coordinates to the Earth Mean Equatorial (equinox 1950) coordinate system. This system is directly supported by the SPICE software provided by the Navigation and Ancillary Information Facility (NAIF) at JPL as inertial coordinate system 'FK4'. The angles required for this transformation come directly from the Galileo Attitude and Articulation Control System (AACS) data. The transformation matrix for this rotation is: -- -|(cosTsinDcosR - sinTsinR) (-sinDsinTcosR - cosTsinR) cosDcosR| | | |(cosTsinDsinR + sinTcosR) (-sinDsinTsinR + cosTcosR) cosDsinR| | | |-cosDcosT sinTcosD sinD | -- -where R = Rotor-Right Ascension D = Rotor-Declination T = Rotor-Twist - Rotor-Spin-angle (despun data) Once in an inertial coordinate system, SPICE software provides subroutines which allow a user to construct coordinate system transformation matrices for any ephemeris time. These matrices were constructed from the SPICE kernels by directly or indirectly extracting the primary and secondary vectors defining the coordinate system and then following the procedure outlined in the coordinate systems section of this document. The spacecraft/planet (SPK), leap second (TS), and planetary constants (PCK) kernels required for these transformations have been archived in the PDS by NAIF. These SPICE kernels are available on the CD_ROM which contains the magnetometer data. The SPICE toolkit (software) can be obtained from the NAIF node of the PDS for many different platforms and operating systems.

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

https://pds-ppi.igpp.ucla.edu/data/GO-A-MAG-3-RDR-GASPRA-HIGH-RES-V1.0/

- GO-A-MAG-3-RDR-GASPRA-HIGH-RES-V1.0

- Planetary Science: Fields and Particles

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

Name | Role | Original Affiliation | |
---|---|---|---|

Dr. Margaret Galland Kivelson | Data Provider | University of California, Los Angeles | mkevelson@igpp.ucla.edu |

Dr. Margaret Galland Kivelson | General Contact | University of California, Los Angeles | mkevelson@igpp.ucla.edu |