Solar wind‐magnetosphere coupling leading to relativistic electron energization during high‐speed streams

Abstract

Enhancements in relativistic electron fluxes in the outer radiation belt often occur following magnetic storms and have been suggested to result from resonant interactions with enhanced whistler‐mode chorus emissions observed on the dawnside. Using observations during a period of persistent high‐speed, corotating, solar wind streams, we investigate the aspects of solar wind‐magnetosphere coupling that lead to these enhanced chorus emissions. We find that relativistic electron energization occurs in association with large‐amplitude Alfvén waves within the high‐speed streams. These waves last for multiday periods and cause multiday intervals having intermittent periods of significantly enhanced convection. The enhanced convection periods are followed by repetitive substorm onsets caused by the Alfvén wave related repetitive reductions in convection. We use these substorm onsets, identified using geosynchronous particles and midlatitude H components, as indicators of preceding periods of enhanced convection and of reductions in convection. We use ground‐based chorus observations from the Halley station VLF/ELF Logger Experiment (VELOX) instrument to indicate magnetospheric chorus intensities. These data give evidence that the periods of enhanced convection that precede substorm expansions lead to the enhanced dawnside chorus wave. We also see that the enhanced solar wind densities nsw ahead of high‐speed streams are associated with significant energetic electron loss at geosynchronous orbit and that the subsequent flux increases appear to not begin until nsw drops below ∼5 cm−3 even if the solar wind speed increases earlier. The sequence of loss during the leading interval of high nsw, followed by energization during high‐speed streams, occurs whether or not the high nsw interval leads to a magnetic storm.