Helium ion anisotropies in corotating interaction regions at 1 AU

Abstract

We investigated the first-order flow anisotropies in the solar wind frame of 0.06-0.95MeV nucleon-1 He ions during three corotating interaction region (CIR)-associated particle intensity enhancements observed at 1AU by Wind, examining CIRs with (1) a reverse shock, (2) a well-formed compression region, and (3) a weak compression region. We identified anti-sunward flows in the compression region downstream of the CIR trailing edge in the events with a reverse shock and well-formed compression that transitioned to sunward flows across and upstream of this boundary. These observations suggest that the trailing edge is organizing the He ion flows and is a local source for the particles in these events, this source being inside 1AU prior to the trailing edge arrival and beyond 1AU after its passage. The event with the weak compression region had predominantly sunward flows throughout, indicating that the source of the He ions was beyond 1AU. These observations provide compelling evidence that Heions at 1AU can be accelerated to suprathermal energies at the CIR trailing edge in events where the compression region is well formed. © 2012. The American Astronomical Society. All rights reserved..