Solar Energetic Particle Observations with the PAMELA Experiment
2019 August 02
Despite the progress made over the past decades, the physical mechanisms underlying the origin of solar energetic particles (SEPs) are still debated. The largest uncertainties concern the most energetic ($\gtrsim$500 MeV) SEP events, which are difficult to characterize due to the relatively few and indirect observations such as those made by neutron monitors. The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) satellite experiment has recently offered a unique opportunity to study SEPs with energies between 80 MeV and few GeV, including their energy spectra, composition and pitch angle distributions. In particular, PAMELA has measured for the first time with good accuracy the spectral features at moderate and high energies for 26 SEP events occurring between 2006 December and 2014 September, providing important constraints for current SEP models. Reported spectral shapes exhibit a high-energy rollover that can be attributed to particles escaping the shock region during acceleration, as a consequence of its limited extension and lifetime. PAMELA observations also allow the relationship between low-energy SEPs detected by in-situ spacecraft and the high-energy SEPs registered by the worldwide network of neutron monitors during the rare ground-level enhancements (GLEs) to be investigated. No qualitative distinction between the spectra of GLE and non-GLE events was observed, suggesting that GLEs are not a separate class, but rather are a subset of a continuous distribution of SEP events that are more intense at high energies. In this work we combine data from PAMELA and other near-Earth spacecraft in order to determine the SEP spectral shapes at 1 AU from tens of keV to a few GeV.