Statistical Characteristics of the Electron Isotropy Boundary

Abstract

Utilizing observations from the ELFIN satellites, we present a statistical study of ∼2000 events in 2019‐2020 characterizing the occurrence in magnetic local time (MLT) and latitude of ≥50 keV electron isotropy boundaries (IBs) and associated electron precipitation. The isotropy boundary of an electron of a given energy is the magnetic latitude poleward of which persistent isotropized pitch‐angle distributions (Jprec/Jperp ∼ 1) are first observed to occur, interpreted as resulting from magnetic field‐line curvature scattering (FLCS) in the equatorial magnetosphere. We find that energetic electron IBs can be well‐recognized on the nightside from dusk until dawn, under all geomagnetic activity conditions, with a peak occurrence rate of almost 90% near ∼22 hours in MLT, remaining above 80% from 21 to 01 MLT. The observed IBs span IGRF magnetic latitudes of 60°‐74° with a maximum occurrence between 66°‐71° (L of 6‐8), trending toward lower latitudes and pre‐midnight local times with activity. The precipitating energy flux of ≥50 keV electrons averaged over the IB‐associated latitudes varies over four orders of magnitude, up to 1 erg/cm2‐s, and often includes wide‐energy electron spectra exceeding 1 MeV. The IB‐associated energies and precipitating fluxes also exhibit peak values near midnight for low activity, shifting toward pre‐midnight for elevated activity. The average total precipitating power deposited over the high‐latitude nightside atmosphere (55° to 80°; IGRF L ≥3) attributed to IBs is 10‐20%, or 10 MW, but at times can approach 100% of the total ≥50 keV electron energy deposition over the entire sub‐auroral and auroral zone region, exceeding 1 GW.

Dispersed Relativistic Electron Precipitation Patterns Between the Ion and Electron Isotropy Boundaries

“…ELFIN electron and ion trapped flux measurements falling close to noise level at 300 keV indicate the plasma sheet proper: the electron plasma sheet starts in time from (extends poleward of) 23:41:30 UT (Panels (d1, d2)) and the ion plasma sheet starts from ∼23:41:20 UT (Panels (e1, e2)). These are poleward of ELFIN's crossing of the electron and ion isotropy boundaries (see also Wilkins et al., 2023), which are evident in the dispersion with latitude of the minimum energy where precipitation ratios become ∼1 for electrons and ions, at 23:41:00 ‐ 23:41:20UT for electrons and 23:40:00 ‐ 23:40:30UT for ions respectively, the actual time depending on energy. The comparison of ELFIN electron flux spectra with MMS spectra in Panel (f) confirms that MMS flux measurements at 23:46:00 from the plasma sheet are quite close to ELFIN's near‐Earth plasma sheet measurements.…”

“…Third, electron field line curvature scattering occurs when the electron gyroradius becomes comparable to the field line curvature radius (see theoretical models in Birmingham, 1984; Delcourt et al., 1994, 1995; Artemyev et al., 2015). As the gyroradius (for a fixed electron energy) increases rapidly and the field line curvature radius decreases rapidly with equatorial distance from Earth, curvature scattering exhibits characteristic energy versus L ‐shell dispersion: lower energy electrons are scattered and thus isotropize in pitch‐angle at higher L ‐shells; higher energy electrons do so at lower L ‐shells (Sergeev et al., 2012; Sivadas et al., 2019; Wilkins et al., 2023; Yahnin et al., 1997). For a given energy, the latitude at which isotropy is first observed in a poleward crossing of the ionospheric footpoint of the field lines is called the isotropy boundary for that energy (Imhof et al., 1977; Sergeev et al., 1983).…”

“…For a given energy, the latitude at which isotropy is first observed in a poleward crossing of the ionospheric footpoint of the field lines is called the isotropy boundary for that energy (Imhof et al., 1977; Sergeev et al., 1983). For electrons of energies from tens of keV to a few MeV this boundary maps anywhere from the inner edge of the electron plasma sheet to the outer edge of outer radiation belt (Bloch et al., 2021; Dubyagin et al., 2002; Newell et al., 1998; Sergeev et al., 1993; Wilkins et al., 2023). For ions of similar energies this boundary maps significantly Earthward of the corresponding electron boundary, well within the outer edge of the outer radiation belt.…”

Ionospheric Plasma Density Gradients Associated With Night‐Side Energetic Electron Precipitation

“…These 27 events, selected based on the simultaneous TEC coverage, are similar to usual ELFIN observations of whistler‐mode wave driven precipitation. Figures 1a–1d shows a typical example of such events observed in the heart of the outer radiation belt (∼01:50:15UT), as marked by the vertical dashed line, earthward from the plasma sheet; the plasma sheet is observed at low altitudes (before ∼01:49:30UT) as a prolonged region filled by <200 keV, isotropic ( j prec / j trap ∼ 1) precipitation driven by curvature scattering (Artemyev et al., 2022; Wilkins et al., 2023).…”

“…The second subset consists of 16 events with a precipitation pattern typical of the plasma sheet; the full list of events can be found in Table S2 in Supporting Information . The equatorial (inner) edge of such precipitation is the so‐called isotropy boundary (Imhof et al., 1977; Sergeev et al., 1983; Sergeev & Tsyganenko, 1982) where a combination of curvature scattering around the equatorial current sheet and high fluxes of relativistic electrons at the outer edge of the radiation belt provides a very distinct, localized (a fraction of a degree in latitude) burst of isotropic ( j prec / j trap ∼ 1) electron precipitation often up to MeV (Artemyev et al., 2022; Wilkins et al., 2023); note that these 16 events, selected based on TEC coverage, are similar to usual ELFIN observations of the isotropy boundary. A typical example of the isotropy boundary observed by ELFIN is shown in Figure 1 (center column), where this boundary (at ∼04:09:45UT) separates the outer radiation belt, with transient precipitation bursts of sub‐MeV electrons scattered by whistler‐mode waves, and the plasma sheet, with isotropic precipitation of <200 keV electrons.…”