Solar Energetic Proton Access to the Magnetosphere During the 10\textendash14 September 2017 Particle Event

We explore the penetration of >60 MeV protons into the magnetosphere during the 10\textendash14 September 2017 solar energetic particle event. Solar energetic particles can cause single event effects and total dose degradation in spacecraft electronics. Therefore, it is important for satellite anomaly analysis to understand how deep into the magnetosphere these particles penetrate. Whereas most studies of geomagnetic cutoffs use low-altitude data, we use data from the Relativistic Proton Spectrometer on National Aeronautics and Space Administration\textquoterights Van Allen Probes, which is in a high-altitude, elliptical orbit. We determine how the penetration depends on particle energy, location, and direction of incidence. We evaluate multiple published models of the geomagnetic cutoff to determine how well these models constrain the spectrum at the location of a spacecraft inside the magnetosphere given data outside the magnetosphere. We show that, compared to cutoff models, low-altitude proton measurements are far superior for near-real-time monitoring of the geomagnetic cutoff in support of high-altitude anomaly resolution.