************************************************************************ * * * Documentation to GIOTTO'S MAG-Experiment Data for the Encounter * * with Comet P/Grigg-Skjellerup on July 10, 1992 * * * * University of Cologne * * Institute of Geophysics and Meteorology * * Albertus - Magnus - Platz * * D - 50923 Koeln * * * ************************************************************************ [ Modified 22 August 1995, by A.C.Raugh, to reflect processing done at the Small Bodies Node prior to ingest. Added sections will be enclosed in brackets ([]). Note that no changes were made to any of the binary files during ingest. ] 1. General Data Structure ========================= The MAG data files were written in binary format using a Sun (IPC) work- station. The data therefore are given in Sun's IEEE data representation. Each record in the files contains 20 Bytes of data (see below). For each hour of the day(s) with data a new file is given. The sampling rate of the MAG experiment during the time covered with data was 640 Vectors per 22.66666667 seconds, resulting in 28.235 vectors per second or 101,647 Vectors (i.e. records) per hour, which leads to a file size of 2032940 bytes each. The data files cover a total of 48 hours in the following time intervals day in 1992 hours Total hours ====================================== 185 17 - 19 3 187 15 - 19 5 189 5 - 9 5 191 17 - 23 7 192 0 - 23 24 193 0 - 3 4 ===================================== TOTAL: 48 == The total space required for the high resolution MAG - data is therefore about 93 MBytes. Adding our average-data (covering the time interval 191/17 to 193/4) requires another 5 MByte of space. [ The ASCII equivalents of the average data require an additional 12.5 MBytes.] 2. File Structure ================= The file names start with "g8" (g means Grigg/Skjellerup, 8 is our internal correction level of the data) and end with "1.dat". The numbers in between denote the day of the year and the hour of the day, of which the data are contained in the file. Each record contains time information and the measured MAG values in the following format. [ The file names were changed to conform to PDS requirements. The new names for the binary table files are formulated as "magDDDHH.dat", where DDD is the sequential day number (0-366) and HH is the hour of the first observation in the file. ] VARIABLE TYPE MEANING ================================================================== CK CHARACTER*2 = 'v ' (label denoting vector record) IDAY INTEGER*2 day since 1992-01-01 IFRAC INTEGER*4 fraction of day in 10**(-4) sec IXO INTEGER*2 X-Component in 0.1 nT IYO INTEGER*2 Y-Component in 0.1 nT IZO INTEGER*2 Z-Component 0.1 nT IDX INTEGER*2 X-Component Difference in 0.1 nT IDZ INTEGER*2 Z-Component Difference in 0.1 nT IPH INTEGER*2 Phase Angle CK is a character string containing 'v ' (note the space). It denotes a MAG vector. If ck does not equal 'v ', the record should be ignored. 2.1 Time Information ==================== IDAY is measured from 1992-01-01, day of encounter is 192. IFRAC gives the time of the corresponding MAG Vector in units of 10**(-4) seconds since the beginning of the day IDAY. Example: 12:00 UT corresponds to IFRAC = 432,000,000. For an arbitrary vector, the time T in seconds from January 1st, 1992, 00:00 UT is therefore given by T = 8.64D4*DBLE(IDAY) + 1.0D-4*DBLE(IFRAC) NOTE: The time given on our data files (and hence T) is GROUND RECEIVE Time (GRT). It corresponds to a slightly modified OCOE ground receive time. From T, Space Craft Event Time (SCET) is obtained by subtracting (mainly) the light travel time of 11 min 52.9542 sec, i. e. T_SCET = T - 712.9542 CA using this time was at : ==> 192 / 15:18:43 SCET/UT (+- 14 sec) <== 2.2 Magnetic-Field-Vector Information ===================================== IXO, IYO and IZO give the magnetic field components in units of 0.1 nT at the corresponding time T (or T_SCET) as measured by the main (i. e. the outboard) magnetometer. IDX and IDZ are X- and Z-Components of the difference vector between outboard and inboard magnetometer. As the inboard magnetic field vectors are only given every tenth outboard vector, IDX and IDZ are set to -9999, if no vector difference is available. Note that -9999 always indicates a data gap in our data. IPH gives the value of an phase angle of the S/C at the time of measurement in degrees between 0 and 360 (for internal use only). The average data files contain a slightly different set of information. Their data structure is: VARIABLE TYPE MEANING ================================================================== CK CHARACTER*2 = 'v ' (label denoting vector record) IDAY INTEGER*2 day since 1992-01-01 IFRAC INTEGER*4 fraction of day in 10**(-4) sec at center of average IXO INTEGER*2 Average of X-Components in 0.1 nT IYO INTEGER*2 Average of Y-Components in 0.1 nT IZO INTEGER*2 Average of Z-Components in 0.1 nT IRMS INTEGER*2 RMS-value in 0.1 nT IMAG INTEGER*2 Average of the magnitude in 0.1 nT IDUMMY INTEGER*2 set identical to zero (dummy) The average file names start with "g81" and then contain the average time in seconds with leading zeros, e. g. file "g8100128.dat" contains sliding averages of the with an average period of 128 seconds. We provide average data files with the following average periods: 1 sec, 2 sec, 4 sec, 8 sec, 16 sec, 64 sec, 128 sec The times given in the files correspond to the center of the average interval. The time period covered by average data is 191/17:00:00 to 193/04:00:00. [ The ASCII versions of these files have names beginning with "magav", followed by three digits indicating the averaging interval. The CK and IDUMMY fields mentioned above, which contain fixed values, have been deleted from the ASCII tables. In addition, the IFRAC value has been converted to hours, minutes and seconds. The format of the ASCII files can be found in the accompanying PDS label. The binary versions of these files have names of the form "magNNN.dat", where NNN indicates the averaging interval. Their record format has not been altered. ] 2.3 Coordinate System ===================== The components of the magnetic field vectors are given in the CSE system (COMET CENTERED SOLAR ECLIPTIC SYSTEM): - The center of the coordinate system is comet P/GRIGG SKJELLERUP. - The x-axis is directed towards the sun. - The y-axis lies in the ecliptic plane and is directed opposite to the sense of orbital motion of the planets. - The z-axis completes the right-handed system. Three basical unit-vectors to create this coordinate system can be defined as follows: CSE_1 = - r_c / |r_c| CSE_2 = e_Z x CSE_1 / |e_Z x CSE_1| CSE_3 = CSE_1 x CSE_2 where r_c : position vector of the comet in a HELIOCENTRIC SOLAR ECLIPTIC SYSTEM e_Z=(0,0,1): unit vector pointing north in a HELIOCENTRIC SOLAR ECLIPTIC SYSTEM 2.4 Reading the Data ==================== To read the binary MAG data, either direct access reading provided by FORTRAN can be used, e. g. as follows: CHARACTER*2 CK INTEGER*2 IDAY, IVAL(6) INTEGER*4 IFRAC OPEN (12,FILE='g8192151.dat',STATUS='OLD',FORM='UNFORMATTED', > ACCESS='DIRECT',RECL=20) IREC = 1 10 READ(12,REC=IREC,ERR=999) CK, IDAY, IFRAC, (IVAL(J), J=1,6) IREC = IREC + 1 ... GO TO 10 999 CLOSE(12) ... or, stream input as it is provided in C, can be used, e. g.: #include void main() { char infile[12], ck[2]; int ifrac; short iday, ival[6]; FILE *in; sprintf(infile,"g8192151.dat"); in = fopen(infile,"rb"); while(!feof(in)) { fread( ck,1,2,in); fread(&iday,2,1,in); fread(&ifrac,4,1,in); fread(ival,2,6,in); } fclose(in); } In case of any questions concerning the data, please feel free to ask: Prof. F. M. Neubauer or Dr. H. Marschall Institute of Geophysics and Meteorology University of Cologne Albertus-Magnus-Platz D-50923 Koeln Fax : +49-221-470-5198 Phone: +49-221-470-2552 (F. M. Neubauer) or : +49-221-470-3387 (H. Marschall) E-Mail (Internet): neubauer@geo.Uni-Koeln.DE (F. M. Neubauer) or : hartmut@geo.Uni-Koeln.DE (H. Marschall) ============================ End of README =============================== Questions concerning the ingets process should be directed to the Small Bodies Node of the Planetary Data System.