Multispacecraft analysis of energetic ion flux dropouts

We present an analysis of dispersionless flux dropouts in solar energetic particles (SEPs) associated with impulsive solar flares observed by ACE/ULEIS and Wind/STEP. These flux dropouts are thought to be caused by an observer encountering magnetic field lines that are populated with particles or unpopulated, depending on their connection to the source. Since ACE and Wind typically have spatial separations less than or of order a correlation scale of the turbulent component of the interplanetary magnetic field (L_c ∼ 10⁶ km), they are ideal for studying the effects of the turbulent magnetic field on impulsive SEP events, especially those that exhibit intensity dropouts. By examining the timing of the dropout events seen by each spacecraft and accounting for the convection of frozen-in magnetic fields by the solar wind, we have found that both spacecraft generally see the same event features when they are separated by less than L_c and different features when they are separated by more than L_c. This is consistent with the propagation of impulsive SEPs along random walking magnetic lines of force. We find that dropouts occur in approximately half of the impulsive SEP events in our sample, indicating that this is a fairly common phenomenon. Moreover, we find that there is not a clear correspondence between the timing of these events and changes in the local interplanetary magnetic field more often than random coincidence. This supports the idea that intensity dropouts are mostly caused by field line mixing rather than by localized magnetic structures.