Comparison on the CME-shock Acceleration of Three Widespread SEPs during Solar Cycle 24

Using forward-modeling of three-dimensional (3D) flux rope plus oblate spheroid shock model, we analyze three solar energetic particle (SEP) events observed in three views by STEREO and SOHO spacecraft. The three SEP events occurred on August 14, 2010, November 03, 2011, and March 05, 2013. All three SEP events show widespread distribution in longitude (> 90 degree) but with different onset delays, rate of flux increase, and electron anisotropies. By fitting the 3D flux rope + shock model to white-light and EUV images from STEREO, SOHO and SDO, we are able to best determine the 3D shape of the CME shock wave. The forward-modeling technique constrained with multi-spacecraft observations allowed us to determine the locations where the shock fronts intersect the magnetic footpoints of three spacecraft (STA, B and ACE/Wind), and compute the shock speeds and expansion speeds at the intersecting points. Our fitting results show that the November 03, 2011 SEP has the fastest expansion speed among three events, which is consistent with the rapid flux rises at all three spacecraft and small onset delay between well-connected STA and poor-connected STB and ACE/Wind ( 30 min). On the other hand, the March 05, 2013 SEP has the largest shock leading-edge speed thus the largest electron intensity. However, due to its relatively weak expansion, the shock ceased expanding at 100 degree. The intensity observed by ACE/EPAM and Wind/3DP ( 140 degree away from the SEP solar source) rose very slowly with a large delay of 5 hr, which cannot be explained by the shock acceleration. For the Aug. 14, 2010 event, the SEP observed at STB and ACE/Wind are shown to be accelerated and injected by the shock and consistent with the large anisotropies observed at these two locations. The cross-field diffusion is the likely cause of the Aug. 14, 2010 SEP event observed by STA and the March 05, 2013 event observed by STB and ACE/Wind, which show almost no anisotropy. This investigation helps explore the relative importance of the various mechanisms of SEP acceleration.