Recently it has been suggested that the “ twin-CME ” scenario \citep Li.etal12 may be a very effective mechanism in causing extreme Solar Energetic Particle ( SEP ) events and in particular Ground Level Enhancement ( GLE ) events . \citet Ding.etal13 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 hours 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 hours being the time lag \tau 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 \citep Ding.etal13 , we vary the time lag \tau from 1 hour to 24 hours with an increment of 1 hour .
By considering three quantities whose values depend on the choice of this time lag \tau , we show that the choice of 13 hours for \tau 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 .