Energetic particle composition variations during the March 1991 events measured with the Ulysses EPAC instrument

Abstract

We report on energetic particle measurements obtained with the EPAC instrument on board the Ulysses spacecraft during March 1991, when a series of important flares occurred at the Sun. The EPAC instrument measures electrons (0.1–0.38 MeV), protons (0.3–1.5 MeV) and heavy ions (0.2–6.0 MeV/N). We discuss the time interval March 22 through March 29 (DOY 81 through 88) and divide this interval in three periods with different ion compositions. At a quasi‐perpendicular shock on March 23, shock drift acceleration of protons, helium and electrons was observed. Thirteen hours after this shock the energetic ion composition changed dramatically by almost two orders of magnitude, signalling the arrival of a coronal mass ejection (CME) or driver gas. This driver gas was still present at the spacecraft when a second quasi‐perpendicular shock passed the spacecraft. The ratio Fe/O increased from 0.6 to 1.0 indicative of a connection to solar particles for about 6 hours after the second shock. The second shock did not accelerate ions as well and electrons not at all. Six hours after this shock the same oxygen and iron composition was observed as before, indicating that the second shock did not alter the energetic ion composition. A third ion composition was observed before the driver gas disappeared which was significantly different from those observed before the first and between the two shocks.