Non‐relativistic solar electron events during December 1990: Results from Ulysses

Abstract

Several groups of impulsive flare accelerated non‐relativistic solar electron events occurred in November and December 1990. In a total of 18 separate events, most show the characteristic dispersion in arrival times at the Ulysses spacecraft corresponding to travel time for the various particle energy groups. After maximum development, the electron intensities generally decay smoothly with characteristic times of 10 to 15 hours. In a few cases there are marked departures from this “classic” behavior. On 15 December 1990, these departures are associated with a structure in the interplanetary magnetic field having a width of ∼4 × 10 6 km at 1.37 A.U. The particle intensities inside this structure varied greatly with spatial position. Some of the changes occurred over distances as small as 3 × 10 4 km. We believe that this interplanetary structure had its origin in the complex magnetic fields that overlay the solar active region in which the electron accelerating flares occurred, and that it is these fields near the Sun that determine access of the flare accelerated electrons to interplanetary field lines. A solar electron event on 11 December 1990 is similar in many respects to the one on 15 December. Neither of these events is directly related to an interplanetary shock wave.