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

Two Earth-directed coronal mass ejections (CMEs), which were most effective in energetic (similar to1-50 MeV) particle acceleration during the first 1 8 months since the Solar and Heliospheric Observatory (SOHO) launch, occurr ed on April 7 and May 12, 1997. In the analysis of these events we have dec onvoluted the injection spectrum of energetic protons by using the method d escribed by Anttila et al. In order to apply the method developed earlier f or data of a rotating satellite (Geostationary Operational Environmental Sa tellites, GOES), we first had to develop a method to calculate the omnidire ctional energetic particle intensities from the observations of Energetic a nd Relativistic Nuclei and Electrons (ERNE), which is an energetic particle detector onboard the three-axis stabilized SOHO spacecraft. The omnidirect ional intensities are calculated by fitting an exponential pitch angle dist ribution 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 acceleratio n efficiency decreases fast when the shock propagates outward from the Sun. The particles injected at distances < 0.5 AU from the Sun dominate the par ticle flux during the whole period, when the shock propagates to the site o f the spacecraft. The main portion of particles injected by the shock durin g 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.