Statistical analysis of MMS observations of energetic electron escape observed at/beyond the dayside magnetopause

Abstract

Observations from the Energetic Particle Detector (EPD) instrument suite aboard the Magnetospheric Multiscale (MMS) spacecraft show that energetic (greater than tens of keV) magnetospheric particle escape into the magnetosheath occurs commonly across the dayside. This includes the surprisingly frequent observation of magnetospheric electrons in the duskside magnetosheath, an unexpected result given assumptions regarding magnetic drift shadowing. The 238 events identified in the 40 keV electron energy channel during the first MMS dayside season that exhibit strongly anisotropic pitch angle distributions indicating monohemispheric field‐aligned streaming away from the magnetopause. A review of the extremely rich literature of energetic electron observations beyond the magnetopause is provided to place these new observations into historical context. Despite the extensive history of such research, these new observations provide a more comprehensive data set that includes unprecedented magnetic local time (MLT) coverage of the dayside equatorial magnetopause/magnetosheath. These data clearly highlight the common escape of energetic electrons along magnetic field lines concluded to have been reconnected across the magnetopause. While these streaming escape events agree with prior studies which show strong correlation with geomagnetic activity (suggesting a magnetotail source) and occur most frequently during periods of southward IMF, the high number of duskside events is unexpected and previously unobserved. Although the lowest electron energy channel was the focus of this study, the events reported here exhibit pitch angle anisotropies indicative of streaming up to 200 keV, which could represent the magnetopause loss of >1 MeV electrons from the outer radiation belt.

First MMS Observation of Energetic Particles Trapped in High‐Latitude Magnetic Field Depressions

“…Example of Cluster spacecraft trajectory during northern (a) and southern (d) cusp diamagnetic cavity crossings, respectively. (b, c, e, and f–i) MMS locations close to Southern Hemisphere magnetospheric boundaries calculated from T96 model in x , z GSM plane during electron leakage events observed by Cohen et al (). The interplanetary magnetic field vector in GSM coordinates during each leakage event is marked in each panel and distance between MMS to y = 0 plane is shown in bottom corner of each panel.…”

“…Cluster trajectory during encounters of the "traditional" diamagnetic cavities (DMCs) at Northern (Nykyri, Otto, Adamson, Dougal, & Mumme, 2011) and Southern Hemispheres are shown for comparison. This shows that the MMS orbit can frequently encounter southern Cohen et al (2017). The interplanetary magnetic field vector in GSM coordinates during each leakage event is marked in each panel and distance between MMS to y = 0 plane is shown in bottom corner of each panel.…”

Evidence of Electron Acceleration at a Reconnecting Magnetopause

“…Clearly, they were on the magnetosheath side of the ion diffusion region. They should not be leaked out from magnetosphere along the newly opened field lines (see Figure i), because of the following two reasons: If these electrons (1–4 keV) were from the magnetosphere, they should have pitch angles primarily in the antiparallel direction (Cohen et al, ) because MMS crossed the north part of the X‐line (see Figure i). However, this is not the case.…”

“…1. If these electrons (1-4 keV) were from the magnetosphere, they should have pitch angles primarily in the antiparallel direction (Cohen et al, 2017) because MMS crossed the north part of the X-line (see Figure 1i). However, this is not the case.…”

Asymmetries in the Earth's dayside magnetosheath: results from global hybrid-Vlasov simulations

“…Zwan and Wolf, 1976;Wang et al, 2004), and as a function of the distance from the Sun-Earth line, with pronounced dawn-dusk asymmetries (see the reviews by Walsh et al, 2014;Dimmock et al, 2017, and references therein). One of the main sources of these dawn-dusk asymmetries is the bow shock, another one being the leakage of magnetospheric particles into the magnetosheath, which result in a dawn-dusk asymmetry of the energetic ion and electron components in the magnetosheath plasma (Anagnostopoulos et al, 2005;Cohen et al, 2017). In this paper, we concentrate on the impact of the bow shock properties on the large-scale distribution of the magnetosheath properties.…”