Statistical Characteristics of the Proton Isotropy Boundary

Abstract We present a statistical study of 272 proton isotropy boundary (IB) events, spanning 50 keV up to 2 MeV observed by ELFIN on the nightside magnetosphere, extending the energy range beyond the 120 keV population considered in prior works. Viewed from Low Earth Orbit, the IB is the magnetic latitude poleward of which persistently isotropic pitch‐angle distributions () are first detected. We characterize the IB distribution in local time, energy, geomagnetic activity, and 50 keV precipitation from isotropic protons. We find these IBs primarily exhibit negative energy‐latitude dispersion patterns consistent with equatorial magnetic field‐line curvature (FLC) scattering, with a 20%–30% chance of any particular energy channel exhibiting positive dispersion. The lowest latitude and most energetic IBs were detected in the pre‐midnight sector, consistent with the typical location of maximal cross‐tail current‐sheet thinning. We identify that proton IBs form the lower‐latitude boundary of a region with significant FLC‐caused proton precipitation between the outer ring current and inner edge of the plasma sheet (“PS2RC”), resulting in perpetual loss of protons exceeding typical plasma sheet energies. We show this 50 keV precipitation is often sufficiently intense and distributed to produce ionization enhancements over a range of altitudes at auroral/sub‐auroral latitudes. We also compare proton IB properties to those of electron IBs observed by ELFIN, finding similar trends across local time and activity. These results demonstrate that the IB and precipitation of isotropic particles in their poleward vicinity can be geophysically significant in connecting the magnetosphere and ionosphere.