Statistical Characteristics of the Electron Isotropy Boundary

Utilizing observations from the ELFIN satellites, we present a statistical study of $\sim$2000 events in 2019-2020 characterizing the occurrence in magnetic local time (MLT) and latitude of $\geq$50 keV electron isotropy boundaries (IBs) at Earth, and the dependence of associated precipitation on geomagnetic activity. The isotropy boundary for an electron of a given energy is the magnetic latitude poleward of which persistent isotropized pitch-angle distributions ($J_{prec}/J_{perp}\sim 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 $\sim$22 hours in MLT, remaining above 80% from 21 to 01 MLT. The IBs span a wide range of IGRF magnetic latitudes from $60^\circ$-$74^\circ$, with a maximum occurrence between $66^\circ$-$71^\circ$ (L of 6-8), shifting to lower latitudes and pre-midnight local times with activity. The precipitating energy flux of $\geq$50 keV electrons averaged over the IB-associated latitudes varies over four orders of magnitude, up to $\sim$1 erg/cm$^2$-s, and often includes electron energies exceeding 1 MeV. The local time distribution of IB-associated energies and precipitating fluxes also exhibit peak values near midnight for low activity, shifting toward pre-midnight for elevated activity. The percentage of the total energy deposited over the high-latitude regions ($55^\circ$ to $80^\circ$; or IGRF $L\gtrsim 3$) attributed to IBs is 10-20%, on average, or about 10 MW of total atmospheric power input, but at times can be up to $\sim$100% of the total $\geq$50 keV electron energy deposition over the entire sub-auroral and auroral zone region, exceeding 1 GW in atmospheric power input.