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Solar Particle Composition: Measurements in the March 1991 Event at 2.5 AU

Authors: L. J. Lanzerotti and C. G. Maclennan, AT&T Bell Laboratories, Murray Hill, NJ; R. E. Gold and S. E. Hawkins III, Johns Hopkins University Applied Physics Laboratory, Laurel, MD; S. J. Tappin, Univ. of Birmingham, U.K.; and R. Forsyth, Imperial College, London, U.K.

Presented at: Sept. 1992 COSPAR Meeting, Washington, DC.

Abstract. The time evolution of the very large solar particle event occurring on days 82-90 (March 23-31), 1991, as measured at 2.5 AU by the instrumentation on the Ulysses spacecraft was quite complex. Measurements by the HISCALE instrument of the nuclear composition (emphasizing Z ≥ 6) of the interplanetary particles at a time resolution of two hours provides information on the different interplanetary regions which swept over the spacecraft. The Fe/O abundance ratio is found to differ slightly in the regions before and after two tangential discontinuities. The Fe/O abundance ratio is also found to depend strongly on the energy/nucleon of the particle, with values of ~0.7 for energies of for energies of ~0.5-1.0 MeV/nucl. to values of ~0.2 for energies ~8-16 MeV/nucl. 


Figure 1. The HISCALE experiment: Complete instrument assembly Figure 2. Schematic outline of detector telescope configurations in the 2 separate mechanical mounts. The composition data are acquired by the CA60 aperture. The detector coincidence conditions are DCB.
Figure 3. Instantaneous look direction of the four LEMS/LEFS telescopes on the 4π steradian sphere. One spacecraft rotation (~12 sec) takes each telescope through 360, and thus the entire sphere is covered each rotation.
Figure 4. Calibration data. Energy-loss matrix for the composition aperture for various incident ions. The solid lines outline the discrete delement groups detected (Table 1).
Figure 5. Time-intensity (2-hour average) counting rates for 3 Fe energy channels and an O and a Ne-Mg-Si (NMS group) energy channel for days 82-90, 1992. Representative error bars are shown. Figure 6. Ion flux composition matrix measured by the CA detector system during day 82, 1991, at the beginning of the solar event.
Figure 7. Ion flux composition matrix measured by the CA detector system during day 83, 1991. Figure 8. Ion flux composition matrix measured by the CA detector system during day 84, 1991.
Figure 9. Iion flux composition matrix measured by the CA detector system during day 85, 1991. Figure 10. Details of the onsets of the heavy ions on days 82-83, 1991. At the bottom are plotted the half-hour averages of the interplanetary magnetic field strength at Ulysses. The time of the first interplanetary shock (S) and onset of the driver gas (DG) are indicated.
Figure 11. Time dependence of the 2-hour average Fe rates on day 84, 1991, during the time of the second interplanetary shock (S) and the 2 tangential discontinuities (TD). Figure 12. Two-hour average heavy ion spectra and atomic abundances for several species, as well as atomic abundance ratios (Z>6) relative to O for 2 different time intervals in the event. The energy range for the ratios C/O, N/O, S/O, Fe/O is 5-16 MeV/nucl.; for G/O (NMS/O) the range is 1.0-1.0 MeV/nucl.
Figure 13. Energy dependence of the Fe/O abundances for three different MeV/nucl. channels. The times of shocks (S), tangential discontinuities (TD), and the driver gas (DG) are noted. Figure 14. Sketch of the interplanetary regions delineated by examination of the heavy ion composition as a function of time during the March 1991 event.

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Last modified February 28, 2006
T. Hunt-Ward