Understanding Solar Energetic Proton Effects in Space and Ground-level from GPS-based Measurements

Abstract:

Arrivals of solar energetic protons (SEPs) in the near-Earth space cause severe radiation storms, which directly impact satellites orbiting in medium and high altitude orbits, disrupt transpolar flights sometimes, and even affect electronics and humans on the ground during the rare but significant ground-level enhancements (GLE) of neutrons. Therefore, understanding the effects of SEP events is critical for the space research community. In this work, we demonstrate that the energetic particle instruments on board GPS satellites form a powerful global prompt proton sensor network (GPPSn) that provides an unprecedented opportunity to monitor and characterize solar protons targeting the Earth. The inclined medium-Earth-orbits of the GPS constellation have the unique advantage of allowing SEPs to be simultaneously measured from multiple points in both open- and closed-field line regions. Examining two example intervals of solar proton events in 2012 and 2017, we showcase how GPS proton data are prepared, calibrated and utilized to reveal important features of solar protons, including their source, acceleration/scattering by interplanetary shocks, the relative position of Earth when impinged by these shocks, the shape of solar particle fronts, the access of solar protons inside the dynamic geomagnetic field, as well temporally-varying proton distributions in both energy and space. By comparing to Van Allen Probes data, GPS observations of protons at ~10s-150 MeV are further demonstrated not only to be useful for qualitatively monitoring the dynamics of solar protons, but also for quantitative scientific research including determining cutoff L-shells. Finally, by comparing GPS proton measurements at high energy (> ~400 MeV) during SEP events to observations from ground neutron monitor stations, we confirm the high correlation between relativistic proton intensities and ground-level neutron enchantments in all GLE events since 2003. Our results establish that this GPPSn can join forces with other existing solar proton monitors and contribute to observing, warning, understanding and ultimately forecasting the incoming solar energetic proton events including extreme ones.