Statistical significance of association between whistler-mode chorus enhancements and enhanced convection periods during high-speed streams

Abstract

During high-speed solar wind streams, substorms occur repetitively and relativistic electron fluxes enhance significantly. It has recently been proposed that enhanced dawnside chorus waves lead to the energization of the relativistic electrons and that they are associated with the periods of enhanced convection that precede substorm expansions, rather than with the expansions themselves. In this paper, we have evaluated the statistical significance of this association using a total of 657 substorms during high-speed solar wind streams observed by the ACE spacecraft and whistler-mode chorus waves observed from the VLF/ELF Logger Experiment (VELOX) at Halley station, Antarctica. We find that ∼66% of the substorm events identified at 0400-1400 MLT show the association with the chorus enhancement that starts to increase ∼35 min, on average, prior to substorm onsets and remains elevated until declining back to near the preenhancement level in ∼ 16 min, on average, after substorm onsets. Our statistical results suggest that a large number of the chorus wave enhancements at dawn to postnoon local times occur during the enhanced convection period of the substorm growth phase. This is distinguished from the chorus wave enhancement near midnight that is caused by substorm-injected electrons after onsets. We find that ∼59% of the events identified at 2200-0200 MLT show chorus enhancements that start on average ∼6 min after substorm onsets and remain elevated for ∼32 min on average. Copyright 2007 by the American Geophysical Union.