The 2012 July 23 Backside Event: An Extreme Energetic Particle Event?
Abstract
The backside coronal mass ejection (CME) of 2012 July 23 has received considerable attention because many of its characteristics place it among the historical extreme solar events. For example, the shock transit time from Sun to 1 au was 18.5 hours, similar to the two 2003 Halloween events on October 28 and 29. The CME speed well exceeded 2000 km/s as observed from three views: the STEREO-Ahead, STEREO-Behind, and SOHO. The CME erupted from S17W141 and was heading roughly towards STEREO-A. The solar energetic particle (SEP) event had a peak value of 5000 pfu in the >10 MeV channel. The energetic storm particle (ESP) event was an order of magnitude larger, placing it among the most intense events in the space era. In this paper, we examine whether the CME in this event could have accelerated particles to GeV energies. The STEREO particle detectors do not have energy channels higher than 100 MeV, so we determine the SEP spectrum in the range 10-100 MeV and compare it with that of other ground level enhancement (GLE) events. We find that the spectrum of the 2012 July 23 event is very hard (power law index 1.66), harder than that of all GLE events in cycles 23 and 24. Only the most intense GLE of cycle 23 (2005 January 20), had a 10-100 MeV spectral index of 1.66. The two GLE events of cycle 24 had spectral indices of 2.14 (2012 May 17) and 2.18 (2014 January 6). Thus we conclude that the 2012 July 23 event is likely to be an extreme event in terms of the energetic particles it accelerated. We also discuss additional characteristics of the event, which support this conclusion.
- Publication:
-
Solar Heliospheric and INterplanetary Environment (SHINE 2016)
- Pub Date:
- July 2016
- Bibcode:
- 2016shin.confE.173G