This project was focused on studying the extreme Solar Energetic Particles (SEP) measured by the the Alpha Magnetic Spectrometer on board the International Space Station (ISS). This award had the intellectual merit of studing the most energetic SEP fluxes and to asses differences compared to the low SEP fluxes. SEP are accelerated by the Sun in explosive events such as solar Flares and Coronal Mass Ejections (CMEs). During 10-years mission, AMS has detected 28 proton SEP events and two helium SEP events . We found that all AMS SEP events were associated with high energy X- and M-class flares and fast CMEs. The helium SEP events are the most energetic of the AMS SEP list. The majority of the AMS events were associated with solar flares located toward the western limb of the Sun, but they cover all longitudinal expanses of the solar disk close to the equator including a few the backside events.

AMS typically measures SEP as an excess over the Galactic Cosmic Ray spectra below 3 GV. This corresponds to the most energetic SEP events. We performed extensive studies of the detector performances, the geomagnetic rigidity cutoff, the GCR flux background with long- and short-time scale evolution including Forbush decrease (FD) events. This analysis had broader impacts on better understanding the GCR time evolution caused by the Sun: the long-term variation of GCR due to solar modulation related to the 11-years solar cycle and the short-term variation of solar modulation caused by shocks ahead of Inerplanetary CMEs. These studies produced three important AMS publications: 1) monthly proton and helium fluxes, 2) daily proton fluxes, 3) daily helium fluxes.

The AMS GCR analysis enable us to extract the SEP flux on daily and sub-daily bases and to perform the time evolution of the events. Overall, inspection of the AMS SEP fluxes showed that the ground level enhancement (GLE) events, that can be measured on the ground by neutron monitors, are not qualitatively different from other SEP events. They exist on a continuum of SEP events, and may have some quantitatively extreme properties, such as a particularly hard spectrum or particularly high intensity. However, they exist as an otherwise non-remarkable subset of large SEP events.

At the daily level, we found that the AMS SEP proton and helium spectrum can consistently be fit by a single power law. In addition, the intensity at a given rigidity was positively correlated with the CME speed of the associated CME. These two factors give some suggestion that diffusive shock acceleration might be the acceleration mechanism for these particles. Interestingly, analysis of the spectral index across events showed a unexpected relationship in which higher CME speed was associated with a softer spectrum.

Sub-daily AMS SEP fluxes enabled an investigation of the time evolution of high energy SEP events. They showed a consistent behavior in which the spectral index of an event was stable, but the intensity decayed in time. The consistency of this result across events suggests that the spectral index of the sub-daily SEP fluxes is likely attributable to non-local factors, either particle acceleration or initial transport from the acceleration site to the near-earth environment.

We also performed a multi-spacecraft analysis of the SEP proton flux. We used data from SOHO/ERNE, GOES/EPAD, GOES/HEPAD and AMS. We observed that the entire spectra cannot be fitted by a single power law and that the low energy range spectra have a different time evolution compare to the high energy one. These facts suggest that the acceleration and/or propagation mechanisms of high and low SEPs might be different.

Finally, education and public outreach was one of the broader impacts of this project. To increase diversity and inclusion in science, technology, engineering, and mathematics (STEM) we organized multiple events for mid- and high- school students. We will developed new activities that enriched the NSF Quarknet program in Hawaii. Students learned about the science involved in this project and participated in the Hawaii Student/Teacher Astronomy Research (HI-STAR) on Maui, a program funded by the NSF for the past ten years. In Hawaii, we were be able to reach minority students, especially Native Hawaiians and other Pacific Islanders, who are typically underrepresented in STEM fields. The PI of this project is woman, and most of these funds were be used to support two female postdoctoral research associates, further supporting the opportunities for women in STEM fields.

Last Modified: 09/30/2022
Modified by: Veronica Bindi