A correlation of energetic particle flux anisotropies with magnetic field variations in- and outside the coma of comet Halley

Abstract

Anisotropies and energy spectra of cometary ions (mainly O +, OH +, H 2O +) derived from the EPA/EPONA measurements on board of Giotto during the encounter (12–14 March 1986) with comet Halley are used to study the acceleration processes taking place in the vicinity of the comet. Further evidence was found for the second order Fermi process. The intensity spikes observed outside the bowshock (outbound) can most likely be explained by the field line merging process at the frontside of the magnetic pileup region. The shock drift acceleration mechanism combined with the first order Fermi process seems to be responsible for the spike observed at 06:45 – 08:00 UT on 14 March 1986. The observed flux anisotropies are caused by the velocity transformation effect. Furthermore it can be concluded that the magnetic field lines were “open” on the inbound side (19:00 – 23:00 UT on 13 March), whereas a field line connection between cometary and interplanetary field lines must have existed on the outbound side until ≈ 07:00 UT on 14 March 1986.