Particle acceleration above the pick-up energy at comet 1P/Halley

摘要

Spacecraft measurements made at three comets (P1/Halley, 21P/Giacobini Zinner and 26P/Grigg-Skjellerup) indicated that, in each of these environments, cometary ions were accelerated significantly above their highest available pickup energies. Energy spectra obtained aboard the Giotto spacecraft by the energetic particle analyzer (EPONA) at 1P/Halley and at 26P/Grigg-Skjellerup, supplemented by data from the onboard Implanted Ion Sensor (IIS) indicate that the mean free path corresponding to random scattering by Alfven waves was of the order of 30 ion gyroradii upstream of the bow shock. This result implies that, at this location, energy diffusion was weak. It is shown using Giotto data that, close to the inbound and outbound shock surfaces of 1P/Halley and 26P/Grigg-Skjellerup the most likely candidates for accelerating ions above their pickup energies was compressive shock acceleration supplemented by transit time damping.