The 1998 May 2-3 Magnetic Cloud: An Interplanetary "Highway" for Solar Energetic Particles Observed with SOHO /ERNE

Abstract

Using the Energetic and Relativistic Nuclei and Electron (ERNE) instrument on the Solar and Heliospheric Observatory (SOHO), we have measured the 17-22 MeV proton flux anisotropy during the 1998 May 2-3 solar energetic particle (SEP) event, when SOHO was inside a magnetic cloud associated with a previous coronal mass ejection (CME). The ability of the SOHO/ERNE to measure particle fluxes with high angular resolution makes it a unique instrument for observing SEPs under weak scattering conditions. During most of the first 4 hr of the SEP event that started shortly after 14 UT on 1998 May 2, the proton intensity parallel to the magnetic field was ~1000 times higher than in the perpendicular direction. The ERNE observations indicate that the magnetic flux-rope structure of the CME provides a "highway" for transport of solar energetic protons with a parallel mean free path of at least 10 AU. The particle telescope ERNE/High Energy Detector was able to measure the proton angular distribution, which was narrow to an extent that it could not be resolved with other instruments. Within 30-45 minutes after the onset of the proton event, a counterstreaming flux was also identified. The ERNE data also revealed a narrow magnetic tube in which protons were much less anisotropic than in the rest of the rope. We briefly discuss the implications of these findings for the origin and structure of the CME.