The Polar spacecraft provided over a decade of quality observations in the high latitude regions of Earth's magnetosphere. After on-orbit operations terminated, some questions raised early in the mission remained unanswered. Polar made the first observations of energetic particles in the cusp, with easy access to the ring current and radiation belt. Understanding their source has potential implications for spacecraft operations and exploration activities. Cusp acceleration, possibly in the large diamagnetic cavities observed by Polar, would provide a new source of energized particles in the magnetosphere.

To analyze the cusp energetic particle (CEP) population, I converted MICS energetic particle data from telemetry to physical units by energy, species, and pitch angle, producing a final archive of MICS data.

In several case studies, I found the magnetic field and plasma pressure were anticorrelated inside, but not outside, cusp diamagnetic cavities (CDCs). There appeared to be sufficient energy in the plasma to support the cavities against collapse. The cavities and energetic particles were strongly associated, although the energetic particle density was insufficient to cause the diamagnetic depression. Energetic particles were of both ionospheric and solar wind origin, while the lower-energy particles inside CDCs were almost exclusively alpha particles, of solar wind origin.

I prepared a six-year database of cusp crossings to study CDC and CEP size, relationship to each other, and reaction to external drivers. The CDCs were large in latitude and local time, but radially shallow, not extending far from the magnetopause. CEPs were observed farther inside the magnetosphere. CEPs and CDCs were observed together on the same pass with a greater frequency than expected from random distribution.

I found only minor contribution to CEPs from energization in substorms. Cusp observations showed no difference in CEP frequency between connection to the quasi-parallel and quasi-perpendicular bow shock, indicating the shock was not a significant source of CEPs. Combining this lack of response to other sources with the strong association between CDCs and CEPs implies that CDC acceleration is a potential source of energetic particles in the magnetosphere, requiring further investigation.