ERNE observations of energetic particles associated with Earth-directed coronal mass ejections in April and May, 1997

A Anttila,T Sahla

doi:10.1007/s005850000276

Abstract

Two Earth-directed coronal mass ejections (CMEs), which were most effective in energetic (~1â50 MeV) particle acceleration during the first 18 months since the Solar and Heliospheric Observatory (SOHO) launch, occurred on April 7 and May 12, 1997. In the analysis of these events we have deconvoluted the injection spectrum of energetic protons by using the method described by Anttila et al. In order to apply the method developed earlier for data of a rotating satellite (Geostationary Operational Environmental Satellites, GOES), we first had to develop a method to calculate the omnidirectional energetic particle intensities from the observations of Energetic and Relativistic Nuclei and Electrons (ERNE), which is an energetic particle detector onboard the three-axis stabilized SOHO spacecraft. The omnidirectional intensities are calculated by fitting an exponential pitch angle distribution from directional information of energetic protons observed by ERNE. The results of the analysis show that, compared to a much faster and more intensive CMEs observed during the previous solar maximum, the acceleration efficiency decreases fast when the shock propagates outward from the Sun. The particles injected at distances <0.5 AU from the Sun dominate the particle flux during the whole period, when the shock propagates to the site of the spacecraft. The main portion of particles injected by the shock during its propagation further outward from the Sun are trapped around the shock, and are seen as an intensity increase at the time of the shock passage.Key words: Interplanetary physics (interplanetary shocks) â Solar physics, astrophysics and astronomy (energetic particles; flares and mass ejections)

Highlights

coronal mass ejections (CMEs) are vast outbursts of solar material, during which typically 109±1010 tons of plasma is ejected into interplanetary (IP) space with speeds ranging from less than 50 to more than 2000 km/s (e.g., Gosling, 1997)
Our model does not work perfectly for every moment of time, and the intensity pro®les (Figs. 3, 5) showuctuations, especially in low energies. These anomalies can partially result from the fact that the anisotropy distribution is of a more complex form than assumed in Eq 1
Because our model uses constant mean free path (MFP), it cannot ®t these kind of changes in IMF conditions, but the ®tted curves are ®ts for averaged conditions during the whole event

Summary

Introduction

CMEs are vast outbursts of solar material, during which typically 109±1010 tons of plasma is ejected into interplanetary (IP) space with speeds ranging from less than 50 to more than 2000 km/s (e.g., Gosling, 1997). If a CME propagates suciently faster than the ambient solar wind, a shock is formed in front of it. During the ®rst half of the year 1997, several Earth-directed CMEs, which were capable to accelerate particles up to MeV energies, occurred The events on April 7 and May 12 were the most intensive ones in energetic particle acceleration, being capable of accelerating particles up to energies above 50 MeV (Torsti, et al, 1998)

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Conclusion