Scanning an Interplanetary Magnetic Cloud Using High‐Energy Protons

Abstract

We applied a numerical modeling of the solar energetic particle (SEP) event inside a magnetic cloud in the solar wind to analyze the 17-22 MeV proton flux anisotropy observed on 1998 May 2 with the Energetic and Relativistic Nuclei and Electron (ERNE) instrument on the Solar and Heliospheric Observatory (SOHO), when SOHO was inside a magnetic cloud associated with a previous coronal mass ejection (CME). The analysis revealed a strong intermittency of the SEP transport parameters when different magnetic tubes were convected past the spacecraft. The estimated mean free path value varies over 1 order of magnitude, from ~2 to ~20 AU. The SEP event has been modeled with a prolonged injection of particles from a new CME into the previous ejecta comprising a set of magnetic loops. Both the prompt, direct proton flux and the delayed, counterstreaming flux were observed in the beginning and maximum phase of the event, but then the counterstreaming flux waned. The lack of counterstreaming protons can be explained either by the very fast escape of high-energy protons from the magnetic cloud, before they could complete one bounce in a narrow loop, or by the proton injection predominantly into one leg of a wide loop. An imprint of the magnetic compression at the leading part of the CME can be also found in the proton flux anisotropy data of SOHO/ERNE. These findings illustrate how high-precision anisotropy measurements and a numerical modeling can provide a kind of probe for the CME structure.