On the identification of time interval threshold in the twin‐CME scenario

Abstract

Recently it has been suggested that the “twin‐CME” scenario may be a very effective mechanism in causing extreme solar energetic particle (SEP) events and, in particular, ground level enhancement (GLE) events. Ding et al. (2013) performed a statistical examination of the twin‐CME scenario with a total of 126 fast and wide western coronal mass ejections (CMEs). They found that CMEs having a preceding CME with a speed > 300 km/s within 9 h from the same active region have larger probability of leading to large SEP events than CMEs that do not have preceding CMEs. The choice of 9 h being the time lag τ between the preceding CME and the main CME was based on some crude estimates of the decay time of the turbulence downstream of the shock driven by the preceding CME. In this work, we examine this choice. For the 126 fast wide CMEs examined in, we vary the time lag τ from 1 h to 24 h with an increment of 1 h. By considering three quantities whose values depend on the choice of this time lag τ, we show that the choice of 13 h for τ is more appropriate. Our study confirms our earlier result that twin CMEs are more likely to lead to large SEP events than single fast CMEs. The results shown here are of great relevance to space weather studies.