PAMELA's Measurements of Magnetospheric Effects on High Energy Solar Particles

TL;DR

This paper presents PAMELA's high-energy SEP measurements during the 2012 May 17 event, revealing distinct transport behaviors and anisotropies, and offers insights into local scattering effects in Earth's magnetosheath.

Contribution

First comprehensive measurements of SEP transport effects in Earth's magnetosheath using PAMELA data, distinguishing between low- and high-energy particle behaviors.

Findings

Low-energy SEPs show significant scattering and broad pitch angle distribution.

High-energy SEPs are beamed and less affected by transport effects.

Transport effects are likely occurring locally near Earth.

Abstract

The nature of particle acceleration at the Sun, whether through flare reconnection processes or through shocks driven by coronal mass ejections (CMEs), is still under scrutiny despite decades of research. The measured properties of solar energetic particles (SEPs) have long been modeled in different particle-acceleration scenarios. The challenge has been to disentangle to the effects of transport from those of acceleration. The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) instrument, enables unique observations of SEPs including composition and the angular distribution of the particles about the magnetic field, i.e. pitch angle distribution, over a broad energy range (>80 MeV) -- bridging a critical gap between space-based measurements and ground-based. We present high-energy SEP data from PAMELA acquired during the 2012 May 17 SEP event. These data exhibit differential anisotropies and thus transport features over the instrument rigidity range. SEP protons exhibit two distinct pitch angle distributions; a low-energy population that extends to 90{\deg} and a population that is beamed at high energies (>1 GeV), consistent with neutron monitor measurements. To explain a low-energy SEP population that exhibits significant scattering or redistribution accompanied by a high-energy population that reaches the Earth relatively unaffected by dispersive transport effects, we postulate that the scattering or redistribution takes place locally. We believe these are the first comprehensive measurements of the effects of solar energetic particle transport in the Earth's magnetosheath.