Solar particle composition: Measurements in the March 1991 event at 2.5AU

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.