MMS observation of inverse energy dispersion in shock drift accelerated ions

Abstract

The four Magnetospheric Multiscale (MMS) spacecraft observed a ∼1 min burst of energetic ions (50–1000 keV) in the region upstream from the subsolar quasi‐perpendicular bow shock on 6 December 2015. The composition, flux levels, and spectral indices of these energetic protons, helium, and oxygen ions greatly resemble those seen in the outer magnetosphere earlier while MMS crossed the magnetopause and differ significantly from those simultaneously observed far upstream by Advanced Composition Explorer (ACE). However, the event cannot be explained solely in terms of leakage from the magnetosphere. The strongly southward orientation of the interplanetary magnetic field (IMF) lines at the time of the event precludes any connection to the magnetosphere. This point is confirmed by the presence of energetic electrons, known to occur on magnetic field lines that graze the bow shock rather than connect to the magnetosphere. We suggest that the ions gradient drifted out of the nearby quasi‐parallel foreshock and into the quasi‐perpendicular bow shock. Each of the ion species exhibited an inverse energy dispersion. As predicted by models for shock drift acceleration, the energies of the ions increased as θBn, the angle between the IMF and the shock normal, increased. Finally, we note that a similar event was observed a few minutes later in the subsolar magnetosheath, indicating that such events can be swept downstream of the bow shock.

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

“…In an attempt to highlight the novelty of the statistical study presented here amidst the extremely rich history of the study of energetic electrons beyond the magnetopause, a substantial (but admittedly nonexhaustive) literature review is provided in section 1.1. It must be emphasized that this paper (and the review included) will focus specifically on observations of energetic (greater than tens of keV) electrons observed in the magnetosheath and upstream of the bow shock and is independent of the extremely rich literature regarding the well‐documented history of lower energy (eV to several keV) particles at/beyond the magnetopause [e.g., Freeman et al , ; Montgomery et al , ; Scarf et al , ; Hones et al , ; Scudder et al , ; Reasoner , ; Eastman et al , ; Cowley , ; Fuselier et al , , ; Lefebvre et al , ; Lee et al , , and references therein] and observations of escaping magnetospheric ions [e.g., West and Buck , ; Anagnostopoulos et al , ; Sibeck et al , , ; Zong et al , ; Eccles and Fritz , ; Kronberg et al , ; Westlake et al , , and references therein], upstream ion events [e.g., Asbridge et al , ; Lin et al , ; Scholer et al , , ; Desai et al , , and references therein], and shock‐related ion acceleration [e.g., Ipavich et al , ; Gosling , ; Armstrong et al , ; Forman and Webb , ; Scholer , ; Thomsen , ; Sibeck et al , ; Burgess et al , ; Lee et al , , and references therein]. We focus here on electrons because ions have a broader array of escape mechanisms [ Mauk et al , ] that make their study substantially more complex.…”