Comparison of the 26 May 2012 SEP Event with the 3 November 2011 SEP Event

Abstract:

We compare the solar and interplanetary events associated with two large solar energetic particle (SEP) events on 26 May 2012 and 3 November 2011. Both SEP events were detected at three longitudinally widely separated locations by STEREO A and B spacecraft (more than 100 deg away from Earth) and the Wind and SOHO spacecraft near Earth. In Earth view, the November 2011 eruption occurred far behind the east limb at N09E154, whereas the May 2012 eruption occurred closer to the west limb at N15W121, suggesting that SEPs accelerated during the 2012 event might have easier access to Earth. Even though the 2012 event was more intense in the GOES >10 MeV proton channel (peak intensity 14 pfu) than the 2011 event (peak intensity 4 pfu), we find that the latter event was more intense at higher energies (> 40 MeV). Also, the initial rise at lower energies was slightly faster for the 2011 event as measured by SOHO/ERNE. In addition, the CME associated with the May 2012 event was faster with an estimated space speed of ~2029 km/s than that in the November 2011 event (1188 km/s). STEREO/EUVI images of the associated post-eruption arcades (PEAs) indicate that their orientations were different: the PEA of the May 2012 event had a high inclination (north-south), while the inclination of the PEA of the 2011 event was more moderate. Differences in the flux rope orientation may also have effect on the longitudinal extent of the SEP events. These observations suggest that the dependence of solar proton intensities on the observer's longitudinal distance from the solar source is more complex than traditionally assumed.