The dawn‐dusk plasma sheet asymmetry of energetic particles: An Interball perspective

Abstract

Abstract. We systematically study the energetic particle (-•25 to 850 keV) dawn-dusk plasma sheet asymmetry using both ion and electron spectra composed of 5 6 energy channels. These spectra, corresponding to an average duration of-•8 hours, have been provided by the Interball tail probe and cover the extent between 15 and 28 R• away from the Earth. The events are classified in four Y coordinate categories: the "dawnside" with Yas•<-13 R•, the "center dawn" with 0>Y>-13 R•, the "center dusk" with 0<Y<13 R•, and the "duskside" with Y> 13 R•. The asymmetry is profound between the two extreme dawn-dusk flanks and weaker between the two central plasma sheet regions. In all the energy channels the dusk flank ion fluxes exceed those of electrons. At the dawnside the energetic electron fluxes frequently outnumber those of ions above a bottom energy threshold. During quite times (Kp<2) the ion (electron) fluxes in the duskside (dawnside) plasma sheet exceed by far those in the dawn (dusk) flank. In general, the ion and electron fluxes significantly increase as the Kp index steps upward. However, owing to the dawn-dusk asymmetry, one can observe higher electron fluxes at dawn with low Kp than at dusk with high Kp. On the basis of two case studies, it is shown that ongoing substorm processes and plasma temperature transitions temporarily affect the spatial dawn-dusk asynunetry and the associated spectra. In one case study, via successive spectra it was concluded that the betatron acceleration mechanism is probably the dominant energization process, although the observed spectra did not preserve their spectral shape. This is interpreted as due to the action of energy-dependent large-scale drifts across the tail that strongly modify the initial source spectra. The dawn-dusk asymmetry and all the spectral characteristics from inside the plasma sheet are transmitted out into the magnetosheath, and this may help us (in the future) to discriminate between merging and leakage processes taking place over the magnetopause. PrologueIn this work, past results conceming the dawn-dusk plasma sheet asymmetry are generalized to the full range of energies covered by the Interball tail particle detector. The advantage here is that the whole energetic particle spectrum (composed of •56 points) is available for the first time. Thus this study uses ion and electron spectra to show a more precise picture of energetic particle populations across the tail. The spectra visualize the average spatial distribution of "preexisting populations," i.e., of populations that will be further involved in magnetospheric substorm processes. Then with two illustrative case studies, the temporarily affected dawn-dusk asymmetry is approached. Although the latter study is not thorough, it demonstrates how "the cross-tail large-scale drifts" probably modify the initial spectra produced by Early systematic studies of energetic particle bursts inside the plasma sheet were made mostly using the IMP 7 and 8 spacecraft by Keath et al. [1976], Sarris et al. [1976], Kri...

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