***** File IMSHERS.DOC EXPLANATION OF THE IMS/HERS DATA FORMATS This document describes the format of the Giotto Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) data submitted to the International Halley Watch Data Archive. A description of the experiment has been published in Balsiger et al., ESA-SP 1077, pp 129-148, European Space Agency, March, 1986 (republished in J. Phys. E: Sci. Instrum., 20, 759, 1987.) For proton and alpha-particle data, the ASCII files can be read with the following FORTRAN statements: OPEN(1,FILE='filename',STATUS='OLD',READONLY) READ(1,2)IDAY,IUT,DECHR,VX,VY,VZ,DEN,TLOG 2 FORMAT(1X,I3,I8,F8.3,3F7.0,F7.1,F7.2,5X)!for protons 2 FORMAT(1X,I3,I8,F8.3,3F7.0,F7.2,F7.2,5X)!for "alphas" On the tape as provided to the International Halley Watch, this file and three other files were supplied with 60 byte records and a block size of 600 bytes. The last data file (M/Q>12) was subsequently broken into 10 pieces as listed in Appendix A. For heavier ions, the following FORTRAN statements should be used: OPEN(1,FILE='filename',STATUS='OLD',READONLY) READ(1,2)IDAY,IUT,ISEC,DECHR,VX,VY,VZ,DEN,TLOG 2 FORMAT(1X,I3,I8,I6,F8.3,3F7.0,F6.1,F7.2) Where the parameters have the following meanings: IDAY = Day of year (Day 72 = March 13, 1986) IUT = Spacecraft event time in hhmmss at the center of the measurement interval ISEC = Length of averaging interval in seconds (Heavy ions only) DECHR = IUT converted to decimal hours VX = x component of velocity, in km/s, in HSE coordinates VY = y component of velocity, in km/s, in HSE coordinates VZ = z component of velocity, in km/s, in HSE coordinates DEN = density, in particles/cm**3 TLOG = Log (base 10) of temperature (K) HSE coordinates are Halley-relative Solar Ecliptic coordinates Vx is the velocity component toward the Sun Vz is the velocity component toward the north ecliptic pole Vy forms a right-handed system The temperature is defined by 3nkT = m(v - )**2 It must be emphasized that the IMS/HERS instrument could not detect ions with spacecraft-relative velocity vectors within 15 degrees of the ram direction. The method of data reduction assumed spherical shell distributions, and corrected for the portion of a spherical shell outside the field of view of the experiment. However, if a spherical shell fell entirely within the blind area, no correction could be made. These unmeasured cold ions are an important or the major component of the ion density after ~2345UT, March 13. The following files are included in the data set. PROTONS.IHW ALPHAS.IHW HEAVY.IHW For protons, the data start at 1830 UT on March 13 and extend to the time of instrument failure at 0003 UT on March 14. For alphas, the data also start at 1830 UT on March 13 but end at 2330 UT, March 13, after which it became difficult to separate He++ ions from the H2+ (see Fuselier et al., Geophysical Research Letters, Vol. 15, pages 549- 552,1988). For heavier ions, we give data only when the fluxes were large enough to permit calculation of density and temperature. Until 2348 UT on March 13, proton spectra were obtained every 4th spacecraft spin (i.e., one measurement every 16 seconds) and the data are given at this time resolution, with some gaps corresponding to missing or erroneous data. After 2348 UT, the data mode changed such that 8 spins (32 s) were required to obtain a proton spectrum. The proton parameters were calculated using a moment summing technique. The estimated densities include corrections for protons outside the field of view of the experiment; the corrections were based on the assumption that the proton distribution was isotropic. Alpha spectra were also obtained every 4th spacecraft spin and four consecutive spectra have been summed before calculation of the physical parameters. The time resolution of the "alpha" data is therefore 64 s. The estimates of the alpha parameters were obtained by fitting to an isotropic distribution, and the estimated densities include corrections for alphas outside the field of view of the experiment. The heavier ion spectra are at nonconstant intervals because it was possible to obtain estimates more frequently when fluxes were higher. Data are given only for those intervals and those species for which there were at least 8 counts in the appropriate part of phase space. Note that the components of velocity are independent of M/Q for any given interval. This results from the method of calculation. For each interval, the water-group ions (M/Q = 16-18) were used to determine the velocity that gave the best (least-squares) fit of the ion distributions to an isotropic distribution. For each time interval, the best-fit water-group vector velocity was then used to transform the measured velocity distribution for each ion species into a moving coordinate system in which the density and temperature were calculated, again assuming isotropy. The estimated densities include corrections for ions outside the field of view of the experiment. To convert from time to cometocentric distance: Distance (km) = (24.05 - dechr) * 3600 * 68.4 For further information, contact one of the following people at: Mail Stop 169-506, Jet Propulsion Laboratory, Pasadena, CA 91109, USA: Bruce E. Goldstein, Telephone 818-354-7366 Raymond Goldstein, 818-354-0241 Marcia Neugebauer, 818-354-2005 ******************************************************************* Appendix A Note: a corrected file of "HEAVY" densities was sent on April 3, 1991. These data have been reformatted into separate files. A complete list of files, including protons and alphas, follows: Filename Content IMSHER01 protons IMSHER02 alphas IMSHER03 M/Q = 12 IMSHER04 13 IMSHER05 14 IMSHER06 15 IMSHER07 16 IMSHER08 17 IMSHER09 18 IMSHER10 28 IMSHER11 29 IMSHER12 32 *******************************************************************