Intense solar near‐relativistic electron events at 0.3 AU

Abstract

AbstractWe study the two most intense electron events observed by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during the rising phase of solar cycle 24. Both events occurred during intense periods of solar activity when sequences of coronal mass ejections (CMEs) emanated from the Sun. The first event occurred on 4 June 2011, when two successive CMEs, spaced by 15 h, were released from the same active region. A possible interpretation for this event is that the first CME, located beyond MESSENGER's heliocentric distance (R = 0.33 AU) at the time of the second CME, acted as a magnetic barrier for solar energetic particles (SEPs) accelerated during the second CME. Elevated electron intensities at MESSENGER resulted from particle confinement between the two CME‐driven shocks. This scenario is consistent with that proposed by Kallenrode and Cliver [2001a] to explain intense SEP events. The second event, observed by MESSENGER at R = 0.31 AU on 7 March 2012, belonged to a series of events generated by a single active region. A fast (>1500 km s−1) CME occurred 45 h before the onset of the main electron event. That implies that the prior CME‐driven shock was at such a distance that particle reflection from behind that shock might have not been sufficient to account for the initial onset of sunward‐directed electrons at MESSENGER, and additional mirroring and/or scattering processes closer to MESSENGER were necessary. Elevated intensities in this event are consistent with strong injection of SEPs from close to the Sun and particle reflection at some point in interplanetary space.