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

Abstract

Abstract. Two Earth-directed coronal mass ejections (CMEs), which were most e ective in energetic ($1± 50 MeV) particle acceleration during the ®rst 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 ®rst 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 ®tting 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 e ciency decreases fast when the shock propagates outward from the Sun. The particles injected at distances <0.5 AU from the Sun dominate the particle¯ux 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.

The relation between cyclotron heating and energetic particles on open coronal field lines

“…Further acceleration and/or trapping of particles may still, of course, occur further away from the Sun. A more detailed analysis of these events by Anttila & Sahla (2000) revealed that protons were injected from the CME-driven shocks up to r > ∼ 10 R during these events with spectra that had exponential roll over at energies above 15 MeV, in agreement with our calculated mean free path.…”

“…Comparing the calculation to observations of small gradual particle events by Torsti et al (1998) and Anttila & Sahla (2000), we find that a preferred region of acceleration is the shell of small mean free path near the solar surface. Further diffusive acceleration by a CME-driven shock on open coronal field lines is shown to be capable of producing/maintaining power law proton spectra extending up to tens of MeVs at altitudes r < ∼ 10 R , even without waves of self-generated origin.…”

On the generation of Alfvén waves by solar energetic particles

“…1). According to Anttila & Sahla (2000), most part of the injection occurs when the source (the CME) is above probable heights of the Alfvén speed maximum, so only a fraction of this spectrum is available to generate waves in the low coronal regions near the Alfvén speed maximum. Thus, we conclude that test particle approximation is justified for these events at energies E > 1 MeV.…”

“…Let us investigate whether typical small gradual events, like the 7 April and 12 May 1997, events analyzed by Anttila & Sahla (2000), are below the wave growth threshold. The time-integrated injection spectrum in both events can be given approximately by E d 2 N/(dE dΩ ) ≈ 2 × 10 32 (MeV/E) 0.8 exp(−E/15 MeV) (see Fig.…”

“…where d 2 N/(dE dΩ ) is the energy spectrum of protons per unit solid angle at the solar surface (in coordinate space) injected into the flux tube, which can be deconvolved from observations of SEP intensities and anisotropies (e.g., Torsti et al 1996;Anttila et al 1998;Anttila & Sahla 2000;Laitinen et al 2000). Plugging these expressions in Eq.…”