Data Set Overview : This dataset contains data acquired by the Galileo Magnetometer during the Venus flyby. The data have are given at the full sample rate available 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 were covered by 16 RIM (~ 16 minute) averages from the optimal averager section of the instrument in another dataset. 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 both Inertial Rotor Coordinates (IRC : despun spacecraft) and Venus Solar Orbital (VSO) coordinates. The two different coordinate systems are provided in a single ASCII file. 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 ------------------------------------------------------------------ Column Units Description ------------------------------------------------------------------ Time seconds spacecraft event time (UT)in the format yyyy-mm-ddThh:mm:ss.sssZ Bx_vso nT X component in VSO coordinates By_vso nT Y component in VSO coordinates Bz_vso nT Z component in VSO coordinates Bx_sc nT X component in IRC coordinates By_sc nT Y component in IRC coordinates Bz_sc nT Z component in IRC coordinates Bt nT |B| Magnitude of B dqf n/a 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 Galileo magnetometer samples the magnetic field 30 times per second. These highest rate samples are recursively filtered and then resampled by the instrument at 4.5 vectors per second using a 7,7,6 decimation pattern. 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 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 2 coordinate systems. The first coordinate system is referred to as Inertial Rotor Coordinates (IRC) which are commonly called 'despun' spacecraft coordinates. 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. The second coordinate system is called Venus Solar Orbital (VSO) coordinates. For those who familiar with the Geocentric Solar Ecliptic (GSE) coordinates, this is the same basic system defined using Venus and is orbital plane about the Sun. 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-V/E/A-MAG-2-EDR-RAW-DATA-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 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) - ] <> 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.

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 regions where the magnetic sources associated with the spacecraft are fairly constant, magnetic interference is probably reduced by data processing to better than 0.05 nT at the inboard sensors. In these same regions, sensor zero levels (offsets) are known equally well. The data processing software does a fairly good job of removing all currently identified sources of magnetic interference. However, there are some time intervals when the zero levels of the spin plane sensors show large variations (1-5 nT) on short time scales (minutes to hours). After a while (hours usually) the offsets return to their nominal levels. The source of these magnetic fields has not yet been identified. The method of removing offsets from the spin plane sensors does remove these effects, but the method of determining the spin axis aligned sensor offsets does not. In regions where large variations are detected in the spin plane sensors it is reasonable to assume that similar variations are taking place in the spin axis aligned sensor. Rapid variations in the sensor zero levels were detected during the interval between Feb 10, 05:40 through 08:30. These data have been excluded from this dataset for that reason. Our data processing software creates a data quality flag (dqf) which is an assessment of AACS and telemetry error source contamination of a given data point. This number is binary integer where each bit indicates the presence or absence of some error source. The number 0 represents the absence of all error sources which are tested. The higher order bit (larger number) error sources are considered to be more significant error sources. Data are examined for gradients in the field which might be associated with telemetry bit errors, for regions of bad AACS angles, and for completely missing data. If the error is considered completely unrecoverable, the data values are replaced with a missing data flag. In the case of a flag in the rotor spin angle, the vector components may be flagged but the magnitude is still valid. Here is a list of all of the error checks and the bits they set in the dqf field. DQF_GOOD_DATA 0 Good data DQF_BX_GRAD_WARNING 2^0 Component gradient warning DQF_BY_GRAD_WARNING 2^1 Component gradient warning DQF_BZ_GRAD_WARNING 2^2 Component gradient warning DQF_INTERP_ROTATTR 2^3 Missing rotor RA interpolated DQF_INTERP_ROTATTD 2^4 Missing rotor DEC interpolated DQF_INTERP_SPINDELT 2^5 Missing rotor Spin Delta interpolated DQF_INTERP_SCRELCON 2^6 Missing Relative Cone angle interpolated DQF_INTERP_SCRELCLK 2^7 Missing Relative Clock angle interpolated DQF_INTERP_ROTATTT 2^8 Missing rotor Twist interpolated DQF_INTERP_SPINANGL 2^9 Missing rotor Spin interpolated DQF_ROTATTR_FLAG 2^10 Missing rotor RA flagged DQF_ROTATTD_FLAG 2^11 Missing rotor DEC flagged DQF_SPINDELT_FLAG 2^12 Missing rotor Spin Delta flagged DQF_SCRELCON_FLAG 2^13 Missing Relative Cone angle flagged DQF_SCRELCLK_FLAG 2^14 Missing Relative Clock angle flagged DQF_ROTATTT_FLAG 2^15 Missing rotor Twist flagged DQF_AACS_TELEMETRY_HIT_FLAG 2^16 Telemetry hit in AACS record DQF_MAG_TELEMETRY_HIT_FLAG 2^17 Telemetry hit in mag record DQF_SPINANGL_FLAG 2^18 Missing rotor Spin flagged DQF_BX_GRAD_ERROR 2^25 Component gradient error DQF_BY_GRAD_ERROR 2^26 Component gradient error DQF_BZ_GRAD_ERROR 2^27 Component gradient error DQF_BX_FLAG 2^28 Component flagged DQF_BY_FLAG 2^29 Component flagged DQF_BZ_FLAG 2^30 Component flagged Magnetic field gradient warning or error levels are set during the data processing according to expected variances depending on the region of space. In the solar wind, gradient warnings are usually issued at gradients of 10 nT/sec and errors at 15 nT/sec. In the magnetosheath, these values may be 50 percent larger. In the inner magnetosphere, these dqf flags may be completely turned off. Similarly, AACS angles are interpolated across gaps during the processing if the gap length is relatively short (less than 10 minutes typically). If the gaps in spacecraft attitude are long, the AACS angles are flagged and not interpolated. Errors associated with AACS angles have various effects on the data. The rotor right ascension and declination are crucial to the understanding of the spacecraft orientation. Fortunately, these angles are slowly varying and can be interpolated to better than 1 degree of accuracy for long (many hour) time periods except near major spacecraft maneuvers. The relative clock and cone angles are used to remove scan platform interference. In their absence, no interference is removed (+/- 0.15 nT error possible in each component). The rotor motion spin delta is used to determine the instantaneous spin frequency for the phase delay computation. In its absence, the last known phase delay is applied to the current data point. The rotor spin angle and twist angle must be present in order to despin the data. These angles are generally not interpolated for more than ten minutes because the rotor spin period drifts over time periods on this order.