Factors Affecting the Occurrence of Large Solar Energetic Particle Events

Abstract:

In order to understand the paucity of high-energy solar energetic particle (SEP) events in solar cycle 24, we examined all major eruptions (soft X-ray flare size ≥M5.0) on the front side of the Sun during the period from December 1, 2008 to January 31, 2014. There were 59 such eruptions that were associated with CMEs. When a flux rope was fitted to the white-light CMEs observed by SOHO and STEREO it was found that the CME sources were on the disk only for 55 eruptions. There were 16 large SEP events (proton intensity ≥10 pfu in the >10 MeV channel) detected by GOES and 4 by STEREO-B in association with these eruptions. When the CMEs were grouped according to their speeds (<1500 km/s and ≥ 1500 km/s) it was found that only three of the <1500 km/s CMEs (or 11%) were associated with large SEP events compared to 17 or (61%) of the ≥ 1500 km/s CMEs. This result confirms the importance of CME speed for SEP association. In fact there were ten other large SEP events with flare size <M5.0, but associated with fast CMEs, suggesting that the flare size does not determine SEP association. In order to narrow down the properties of CMEs that produce GLE events, we divided the SEP-associated CMEs into a different speed range: ≥2000 km/s and 1500-1999 km/s. We also required that the CMEs originated from the longitudinal range of W20 to W90 (traditional GLE longitudes). There were sixteen such events, 6 with speed in the range 1500-1999 km/s and 10 with ≥2000 km/s. When we further applied the criterion that the latitudinal distance of the CME sources from the ecliptic must be within ±13^o (Gopalswamy et al. 2013 ApJL 765, L30), we found that there were only four CMEs that met this criterion. One of the four was the GLE event of 2012 May 17 whose CME speed was only slightly less than 2000 km/s. The 2011 August 9 CME was ejected into a tenuous medium, which means the shock was likely weak due to higher ambient Alfven speed, even though the CME speed was well above 2000 km/s. The speed of the 2011 June 7 CME (1680 km/s) was well below the typical speed of GLE CMEs. The last one, the 2013 May 22 event was an interacting CME event, but the speed of the primary CME was only 1880 km/s. Thus we conclude that the CME speed, the ecliptic distance of the CME source, and the ambient conditions are all important in deciding whether an SEP event would have GLE particles.

Work supported by NASA’s Living with a Star Program.