Comparison of ≳40 keV Electron Events at High and Low Heliolatitudes: Ulysses/HI-SCALE and ACE/EPAM

Abstract

We have performed a joint survey of anisotropie ≳40 keV electron events from August 1997 to September 2000 using the matched detectors on the Ulysses (ULS)/HI-SCALE and the ACE/EPAM instruments. A computer algorithm selected events with strong, statistically significant pitch-angle anisotropies. Electron pitch-angle distributions at ACE (~1 AU) are often ‘beams’ that are strongly collimated along the local interplanetary magnetic field (IMF). These flare-associated impulsive injections can display rapid rise times (~15 min) and slower decays, or more irregular intensity histories. At ULS, the electron intensities are lower and the time histories smoother, but strong anisotropies are still observable, indicating direct, nearly field-aligned propagation outward from the Sun. We focus on four event periods, selected from the survey, during times when the angle between the footpoints of the IMF lines intersecting ACE and ULS is small. These events span three full years and cover a wide range of distances and heliographic latitudes. We found one reasonably good association between impulsive electron events at ACE and ULS, and two events with small field-aligned gradients.