Relativistic electron acceleration during HILDCAA events: are precursor CIR magnetic storms important?

Rajkumar Hajra,Rajkumar Hajra,Ondrej Santolik,Luis Eduardo Antunes Vieira,Christiano Garnett Marques Brum,Walter D Gonzalez,Ezequiel Echer,Bruce T Tsurutani

doi:10.1186/s40623-015-0280-5

Abstract

We present a comparative study of high-intensity long-duration continuous AE activity (HILDCAA) events, both isolated and those occurring in the “recovery phase” of geomagnetic storms induced by corotating interaction regions (CIRs). The aim of this study is to determine the difference, if any, in relativistic electron acceleration and magnetospheric energy deposition. All HILDCAA events in solar cycle 23 (from 1995 through 2008) are used in this study. Isolated HILDCAA events are characterized by enhanced fluxes of relativistic electrons compared to the pre-event flux levels. CIR magnetic storms followed by HILDCAA events show almost the same relativistic electron signatures. Cluster 1 spacecraft showed the presence of intense whistler-mode chorus waves in the outer magnetosphere during all HILDCAA intervals (when Cluster data were available). The storm-related HILDCAA events are characterized by slightly lower solar wind input energy and larger magnetospheric/ionospheric dissipation energy compared with the isolated events. A quantitative assessment shows that the mean ring current dissipation is ~34 % higher for the storm-related events relative to the isolated events, whereas Joule heating and auroral precipitation display no (statistically) distinguishable differences. On the average, the isolated events are found to be comparatively weaker and shorter than the storm-related events, although the geomagnetic characteristics of both classes of events bear no statistically significant difference. It is concluded that the CIR storms preceding the HILDCAAs have little to do with the acceleration of relativistic electrons. Our hypothesis is that ~10–100-keV electrons are sporadically injected into the magnetosphere during HILDCAA events, the anisotropic electrons continuously generate electromagnetic chorus plasma waves, and the chorus then continuously accelerates the high-energy portion of this electron spectrum to MeV energies.

Highlights

Relativistic (MeV) electrons in the Earth’s outer radiation belt (L > 3.5) can at times have orders of magnitude variations on time scales of few minutes to several days
We will compare relativistic electron acceleration occurring during isolated High-intensity long-duration continuous AE activity (HILDCAA) events which occur in the absence of any significant storm signatures to those occurring after geomagnetic storm main phases
The corotating interaction region (CIR) is indicated by the compressed interplanetary magnetic field (IMF) Bo and compressed plasma density from ~0417 UT on day to ~1049 UT on day

Summary

Introduction

Relativistic (MeV) electrons in the Earth’s outer radiation belt (L > 3.5) can at times have orders of magnitude variations on time scales of few minutes to several days. Hajra et al Earth, Planets and Space (2015) 67:109 the CIRs have been shown to lead to the acceleration of relativistic electrons within the Earth’s outer radiation belt (Tsurutani et al 2006; Kasahara et al 2009; Hajra et al 2014a, 2015). It is the purpose of this study to determine the possible effects of geomagnetic storms preceding HSS HILDCAA events on the acceleration of relativistic electrons. The geomagnetic characteristics and magnetospheric energy budget of the two classes of the events will be compared

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Discussion

Conclusion