Association Rate of Major Sep Events As a Function of CME Speed and Source Longitude

Abstract:

We report on the fraction of fast and wide coronal mass ejections (FW CMEs; speed>900 km/s; width>60°) that produce solar energetic particle (SEP) events. Source regions of the FW CMEs were identified using SOHO/EIT, STEREO/EUVI, and SDO/AIA. Using STEREO EUV observations, we are able to determine the source locations behind the west limb with a high degree of accuracy. The >10 MeV proton flux at Earth was examined using GOES/SEM. The >10 MeV proton flux at the STEREO spacecraft was estimated using STEREO/HET, which detects energetic protons in 11 channels from 13.6 to 100 MeV. We extrapolated the proton flux in the 10 - 150 MeV range with a power-law fit and derived the corresponding >10 MeV proton flux. For each FW CME, we have three proton flux values for three different relative source longitudes with respect to the observer. The SEP association rate is determined by dividing the number of SEP-producing CMEs by the total number of events in each 20° bin of the relative source longitude. We found that the SEP association rate peaked at the W60°–W80° bin and the magnetically well-connected longitudes range from W20°–W100°. We also found that the rate distribution is skewed: the eastern wing of the rate distribution drops slowly compared to the western wing. This indicates that the eastern CMEs (longitude < 0°) have a better chance to reach the magnetic field line connected to Earth than the far-behind-the-west-limb (longitude > 120°) CMEs.