Helios 1 and 2 observations of particle decreases, ejecta, and magnetic clouds

Abstract

We have used >60 MeV/amu particle data from Helios 1 and 2 to demonstrate the close association between ejecta (as defined, for example, by regions of depressed solar wind proton temperature) and short-term (<3 days duration) particle decreases. Of 84 short-term decreases of >4%, we find that 88% were associated with an ejecta, and 70% of these were also associated with a shock. It is clear that the presence of a particle decrease is a robust signature for identifying ejecta in the ecliptic, at least within 1 AU. Conversely, ejecta are evidently important in the production of short-term cosmic ray depressions. The absence of a sample of well-defined ejecta without an associated particle decrease suggests that extended regions of open field geometry are rare inside ejecta, at least on the ∼0.005 AU scale sizes probed by these particles. Sixty-three percent of the ejecta decreases were associated with smooth magnetic field rotations characteristic of magnetic clouds. Our results suggest that there is no fundamental difference in the particle response to ejecta with or without magnetic cloud signatures. We find that some ejecta observed at multiple spacecraft have a magnetic cloud signature at one spacecraft but not at another. The most likely explanation is that magnetic clouds are a substructure of ejecta and the field structure observed depends on where the ejecta is intercepted. We also find that ejecta probably typically extend much less in longitude than the 100° inferred from single-spacecraft studies. Copyright 1997 by the American Geophysical Union.