Solar particle composition: Measurements in the March 1991 event at 2.5AU
Forsyth, R.; Tappin, S. J.; Lanzerotti, L. J.; Gold, R. E.; Maclennan, C. G.; Hawkins, S. E., III
United States, United Kingdom
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 HI-SCALE 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 ∼0.5-1.0 MeV/nucl. to values of∼0.2 for energies ∼8-16 MeV/nucl.