UCLA

In this dissertation, I investigate the observational characteristics of 50 keV to ∼5 MeV electron and 50 keV to ∼2 MeV proton isotropy boundaries in Earth’s magnetosphere. Viewed from Low Earth Orbit, the isotropy boundary (IB) is the magnetic latitude poleward of which persistently isotropic pitch-angle distributions (Jprec/Jperp ∼ 1) are first detected, representing a fundamental transition from an adiabatic “inner magnetosphere” to a non-adiabatic “outer magnetosphere.” The IB is a near-instantaneous tracer of the equatorial magnetospheric field configuration, and provides a means to remote-sense its evolution under any geomagnetic conditions. Here, I use particle data from the ELFIN mission to characterize the IB distribution in local time, energy, geomagnetic activity, and ≥50 keV precipitation from isotropic particles. I find these IBs primarily exhibit negative energy-latitude dispersion patterns consistent with equatorialmagnetic field-line curvature (FLC) scattering, with a 10%-30% chance of any particular energy channel exhibiting mesoscale-embedded positive dispersion structures, associated with wave-particle interactions and localized Bz gradients. The lowest latitude and most energetic IBs were in the pre-midnight sector, consistent with the location of maximal cross-tail current-sheet thinning. I identify that electron and proton IBs form the lower-latitude boundary of an FLC-dominated transition region, separating the outer radiation belt/ring current from the inner edge of the plasma sheet (“PS2ORB” and “PS2RC”), resulting in perpetual loss of electrons and protons exceeding typical plasma sheet energies. I 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. Lastly, I use the information-theoretic technique of Mutual Information (MI) to characterize the drivers of IB characteristics in the solar wind and magnetosphere. From these observables, I construct an empirical predictive model of IB properties, which had previously never been reported for electrons, and extends the previously reported <200 keV proton IB models up to MeV energies. These results demonstrate a deep connection between the IB latitude, particle precipitation, andthe evolution of the magnetosphere.