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 
    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 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 
    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)

Confidence Level Overview
  =========================
    These data have been highly processed to obtain the final dataset.
    Both the magnetometer data and the AACS data required to transform
    the mag data into geophysical coordinates are of lower than average
    quality. The dataset should not be used in wave studies. Much of the
    processing has been done in the frequency domain and the spectra
    clearly show the effects. Overall, the field values are probably
    correct to 0.01 nT and the AACS angles to better than a degree. The
    segment of data that were recorded to tape (16:53 ->17:22) do not
    carry as strong a warning.