Statistical Characteristics of the Electron Isotropy Boundary

Utilizing observations from the Electron Losses and Fields Investigation satellites, we present a statistical study of ∼2,000 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 ( J prec / J perp ∼ 1) are first observed to occur, interpreted as resulting from magnetic field‐line curvature scattering 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 hr in MLT, remaining above 80% from 21 to 01 MLT. The observed IBs span International Geophysical Reference Field (IGRF) magnetic latitudes of 60°–74° with a maximum occurrence between 66° and 71° ( L of 6–8), trending toward lower latitudes and premidnight 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/cm 2 ‐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 premidnight for elevated activity. The average total precipitating power deposited over the high‐latitude nightside atmosphere (55°–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 subauroral and auroral zone region, exceeding 1 GW.