Data set Overview : On MARCH 6, 1986, AT 7:20:06 UT, the Vega spacecraft passed the nucleus of comet Halley at a distance of 8,890 km with a relative velocity of 78 km/s. The magnetic field experiments MISCHA carried four fluxgate sensors, with three sensors mounted on a boom at the end of the solar panels and the fourth sensor mounted one meter closer. During the cruise phase, the TRASSA-1 mode of the instrument (1 vector/2.5 min) was used. The sensors were switched to the TRASSA-2 mode (1 vector/min) during the flyby which started roughly two days before the encounter. From 3 h before closest approach (CA) until 1 hour after CA, the HS-mode (1 vector/6s) and the DT-mode (1 vector/100ms) was used [DELVAETAL1991].   Parameters : The MISCHA experiment delivered a time series of measurements of the magnetic field. Four quantities were collected: three rectangular components (Bx, By, Bz) at one position and a fourth component, Bu, in the direction of Bx, but at a position closer to the spacecraft. The field components measured by the sensor are composed of both the interplanetary contribution and the field from the spacecraft called the offset.   Processing : Data are first converted from the Rohdata files to Flat Files with an ASCII .hed file (/DOCUMENT/PROCESS/DATACONV/flatspec); this is ''original'' data. A further processing is done to provide offsets and corrections for the spacecraft spin; two methods are used for this step which are outlined in the calibration paper (DELVAETAL1990) which is provided as a series of GIF images located in the 'DOCUMENT/CALIB' directory. The main correction is to account for the magnetic field of the spacecraft and the offsets introduced by the spin of the spacecraft around one primary axis.  Offsets were also determined through a correlation technique or through a set of linear equations. The actual method used is given through a number after the actual offset value:  example: 22 dec 84 13 34 22 -> begin of time interval 23 dec 84 08 17 00 -> end '' '' '' 5.5 0.0 0.5 9.5 -> offset values Ox,Oy,Oz,Ou 0 -> method (0-spin,1-Russell,2-Hedgecocke) 0. -> flag to be set (0.: NO problems)   Data : The data are provided in five forms: original, master, survey, FBT1SE, and FBT2SE.  Original file  The flat file of original data has components of the magnetic field, as measured by the MISCHA experiment, in the coordinate system of the spacecraft, termed PSeudo SOlar (PSSO). In addition, a Bt and BUX (Bu- Bx) is provided as well as a clock time with epoch started on Jan 1, 1965; a quality flag indicates missing data.  MASTER file  The MASTER file of the magnetic field data, measured by the MISCHA - experiment on board of the Vega spacecraft, contains the following types of data: the magnetic field data, the position of the space- craft in heliocentric (solar) ecliptic coordinates and in heliographic coordinates from the prime meridian, information about the orientation of the spacecraft, magnetic field data after calibration and rotation to a coordinate system centered at the spacecraft with x - axis to the sun and (x,y) - plane in its orbital plane ( i.e. equal to the ecliptic). In addition, quality flags give information about missing or bad data.  SURVEY file  The SURVEY file of the magnetic field data, measured by the MISCHA - experiment on board of the Vega spacecraft, contains the following types of data: the position of the spacecraft in heliocentric (solar) ecliptic coordinates (H.SE), magnetic field data in a coordinate system centered at the spacecraft with x - axis to the sun and (x,y) - plane in its orbital plane (i.e. almost equal to the ecliptic). In addition, quality flags give information about missing or bad data.  FBT1SE file  The S*FBT1SE file of the magnetic field data, measured by the MISCHA - experiment on board of the Vega spacecraft, contains the following types of data: the position of the spacecraft in heliocentric (solar) ecliptic coordinates (H.SE), magnetic field data in a coordinate system centered at the spacecraft with x - axis to the sun and (x,y) - plane in its orbital plane (i.e. almost equal to the ecliptic plane), z approx. to ecliptic north pole. In addition, quality flags give information about missing or bad data.   The magn. field data in these files are collected from FOUR different modes: - for some time intervals (around closest approach), data are available with high time resolution (DT or 1 vector/(0.1 sec)); - for other intervals, data are available in HS mode : 1 vector/(6 sec); - for other intervals, data are available in T2 mode : 1 vector/(60 sec); - for other intervals, data are available in T1 mode : 1 vector/(150 sec).  Data have been AVERAGED down to a common resolution of 150 sec. So the whole file has an EQUAL time resolution of 1 vector/(150 sec). The times have been averaged and are in the header, in the list of intervals marked with DT, HS, T2.  FBT2SE file  The S*FBT2SE file of the magnetic field data, measured by the MISCHA - experiment on board of the Vega spacecraft, contains the following types of data: the position of the spacecraft in heliocentric (solar) ecliptic coordinates (H.SE), magnetic field data in a coordinate system centered at the spacecraft with x - axis to the sun and (x,y) - plane in its orbital plane (i.e. almost equal to the ecliptic). In addition, quality flags give information about missing or bad data.  The magn. field data in these files are collected from THREE different modes: - for some time intervals (around closest approach), data are available with high time resolution (DT or 1 vector/(0.1 sec)); - for other intervals, data are available in HS mode : 1 vector/(6 sec); - for other intervals, data are available in T2 mode : 1 vector/(60 sec).  Data have been AVERAGED down to a common resolution of 60 sec. So the whole file has an EQUAL time resolution of 1 vector/(60 sec). The times have been averaged and are in the header, in the list of intervals marked with DT, HS.  Ancillary Data : During some short time periods , the spacecraft is spinning about one fixed axis (x-axis). As described in the processing reference, the state of the spacecraft spin are called spinning sessions as outlined in the table below (which is also provided in /DOCUMENT/OFFSET/B0OFFSET.ASC):  1. spinning sessions of vega1 ______________________________ from to flatfile date, hour day date, hour day 1 22-12-84 15:16:28 357 23-12-84 09:58:58 358 b01ct1 2 26-12-84 14:34:44 361 14-01-85 09:54:39 14 '' 3 18-01-85 09:02:08 18 21-01-85 08:14:39 21 '' 4 02-02-85 10:12:09 33 18-02-85 13:28:45 49 '' 5 09-03-85 09:32:42 68 14-03-85 07:26:27 73 ''   Coordinate Systems : The MISCHA data were initially submitted in multiple coordinate systems, but after discussions with the submitter, the following three are being reviewed.  PSSO - coordinate system: PSuedo SOlar coordinate system: spacecraft centered, x-axis only approximately directed to the sun (slight deviation possible), z-axis only approximately to ecliptic pole and (xy)- plane approximately the ecliptic in case of three-axis- stabilisation; in case of one-axis stabilisation of spacecraft no information about z-axis.  SE - coordinate system: Solar Ecliptic system: spacecraft-centered, x- axis to sun, (xy)-plane is orbital plane of spacecraft, i.e. nearly equal to the ecliptic plane, z-axis nearly to ecliptic pole.  H.SE - coordinate system: Heliocentric Solar Ecliptic system: (xy)-plane is the ecliptic, x-axis to vernal equinox 1950, z-axis to ecliptic pole   Software : An initial program, MISCHA, is used to convert the Rohrdata to flat files; the MASTERFILE program applies the offset corrections using a B0*offse.* file.  The files flattesthed.asc/.dat and example.asc can be used to control data conversion from the flat file data format to an other data format. The files contain the same data so it is easy to compare a conversion.  For more information about the flat file data format read the Flat File Specification document (flatspec.doc)   Media/Format : The standard distribution format for the data is CDROM.

Confidence Level Overview :  For SE - FLYBY data-files: 1. Additional offset correction was done very carefully with variable offsets, in addition to the constant ones already applied Constant offsets are for V1: (-10, -5, 40) Example for additional offset correction: for 1986-03-05 07:21:15:00: (Ox, Oy, Oz: -3, 4, 1 in PSSO -data) This leads to a substantial difference in B-total after offset- correction!  Transformation to SE from PSSO was done with variable matrices, according to the S/C position. However, the difference should not be too large for times close to the closest approach .  The Z(PSSO) sensor clearly saturates near C/A; there is no mention of the digitization/quantization that was performed.   Review :  The MISHCA data is part of the Small Bodies Node review in Dec 2002.   Data Coverage and Quality : During the cruise and flyby phases there are data gaps which are not explained. In addition, for Vega 2, one of the sensors was lost after encounter on Mar 9, 1986, so from that time forward, only Bx data was available.   Limitations : Since the SE-data have been better offset-corrected and better transformed, it is recommenced to use the SE data as much as possible; they should be more reliable.