**NSSDCA ID:** PSFP-00414

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

**Time span:** 1990-02-09 to 1990-02-10

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

Data Set Overview ================= This dataset contains data acquired by the Galileo Magnetometer during the Venus flyby. The data have been averaged down to twenty second samples from the 7.68 kB Low Rate Science (LRS) tape record mode. The flyby data were recorded and later played back during the early portion of the Earth 1 flyby. Limited space on the tape recorder required 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, and then some post encounter solar wind data. Large gaps in the high rate data have been filled in with 16 RIM (~ 16 minute) averages from the optimal averager section of the instrument. 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 Venus Solar Orbital (VSO) coordinates. Primary Dataset Reference: Kivelson, M.G., C.F. Kennel, R.L. McPherron, C.T. Russell, D.J. Southwood, R.J. Walker, C.M. Hammond, K.K. Khurana, R.J. Strangeway, and P.J. Coleman, 'Magnetic field studies of the solar wind interaction with Venus from the Galileo flyby: First Results', Science, 253, 5029, 1518, 1991. [KIVELSONETAL1991] 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, VSO Coordinates -----------------------------------------------------------------Column Description -----------------------------------------------------------------year Year = 1990 day Day of year (40 , 41) sec Second of day sc_clk S/C clock counter in the form rim:mod91:mod10:mod8 Bx_vso Magnetic field X component in VSO coordinates <nT> By_vso Magnetic field Y component in VSO coordinates <nT> Bz_vso Magnetic field Z component in VSO coordinates <nT> Bmag |B| Magnitude of B <nT> stBx_vso Standard deviation of the X component <nT> stBy_vso Standard deviation of the Y component <nT> stBz_vso Standard deviation of the Z component <nT> stBmag Standard deviation of the Magnitude of B <nT> npts Number of points in the average dqf Data quality flag Data Acquisition ---------------The data for this dataset were all acquired in by the outboard magnetometer sensors in the flip left mode in the high field mode (ULHR). This mode has a digitization step size of 0.25 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. 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. Each averaged value includes all available data from the prior and subsequent 10 seconds in the high rate data. Averages are not overlapping. The standard deviations and the number of samples in the average have been included with the final dataset. Coordinate Systems ================== The Galileo magnetometer Venus flyby data are being archived in Venus Solar Orbital (VSO) coordinates. The VSO X direction is taken along the Venus-Sun line, positive towards the Sun. The Z direction is parallel to the normal of the Venus orbital plane (Venusian ecliptic), positive northward, and Y completes the right-handed set (towards dusk). Data Processing =============== These data have been processed from the PDS dataset: 'GO-E/V/A-MAG-3-RDR-IRC-COORDS-HIRES-V1.0' In order to generate the IRC processed dataset, 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 6) Data were transformed into VSO coordinates 7) Data were averaged to 20 second sampling 8) Optimal averager data (in VSO coordinates) were merged in to fill gaps between record intervals. 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. A file which contains zero levels as a function of time has been provided as an ancillary product with this dataset. 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 as well as some subharmonic frequencies. 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. Subharmonic energy can be created by a dipole source which spins with magnetometer but changes orientation at a frequency which is near the spin frequency. The source of the high order harmonics was modeled using 2-D (clock and cone angle). Fourier transforms of high pass filtered data. This allows us to resolve the source in terms of the relative spin phase and look direction 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. A similar approach was taken for the isolation and removal of sources of subharmonic energy. Data were band pass filtered to isolate the source signature and then resolved into components as a function of the Energetic Particle Detector (EPD) motor position (look direction). EPD interference (at about 0.05nT) has been removed from the data on Dec 8, 1990. Both sets of interference coefficients were calculated using data from the inboard sensors. When the outboard sensors are in use, these values are extrapolated using the inverse power law appropriate for the source of each term. It should be noted here that both of these interference corrections are less then the quantization level for the inboard sensors. Data resolution coming out of the recursive filter can actually be better than that coming out of the A/D converter if there is sufficient noise at the single bit level. 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 1 millisecond. This delay is converted to an angle using the instantaneous spin frequency. These 3 sources of delay are then summed in to the quantity 'delay' 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) In order to create the processed VSO dataset the following procedure was used. Data are 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) 6) Once in an inertial coordinate system, SPICE software provides the subroutine which returns the transformation matrix to VSO (G_GSETRN) 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. 7) Standard non-overlapping 20 second averages and standard deviations were computed about the central time stamp. 8) Optimal averager data were taken continuously across during the cruise period near the Venus flyby. These data were corrected for offsets and phase delays associated with the averaging. The data were then transformed into VSO coordinated and merged with the 20 second averages, preserving only the data values during the record interval gaps.

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

https://pds-ppi.igpp.ucla.edu/data/GO-V-MAG-4-SUMM-VENUS-SUMMARY-V1.0/

- GO-V-MAG-4-SUMM-VENUS-SUMMARY-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 |

Mr. Steven P. Joy | General Contact | University of California, Los Angeles | sjoy@igpp.ucla.edu |