Quantitative Assessment of Protons During the Solar Proton Events of September 2017

Abstract

We present multi-spacecraft observations of the proton fluxes spanning from 1.5 to 433 MeV for the largest solar proton event of solar cycle 24, i.e., September 7 and 10, 2017. In September 2017, M5.5 flare on September 4, X9.3 flare on September 6 and X8.2 flare on September 10 gave rise to solar proton event when observed by near-Earth spacecrafts. On September 7 and September 10, 2017, a strong enhancement in the proton intensities was observed by Advanced Composition Explorer (ACE) and WIND at L1 and Van Allen Probes, GOES-15 and POES-19 in the Earth's inner magnetosphere. Below geosynchronous orbit, Van Allen Probes and POES-19 show that no significant proton flux was observed with energies ≤25 MeV on September 4, while the fluxes peaked 3 to 7-times during September 7 and by ∼25 times during the third proton flux event on September 10, 2017. Van Allen Probe-A observation shows that the closest distance that solar proton fluxes could approach the Earth is L∼4.4 for 102.6 MeV energies on 10^th September 2017, while lower energy protons i.e., 25 MeV are observed deep up to L∼3.4 on 11^th September 2017. POES-19 observations show that there is no particular magnetic local time (MLT) dependence of the solar proton flux and is symmetric everywhere at high and low latitudes. The measurements from multiple spacecrafts located in the different regions of the Earth's magnetosphere show that the increased level of solar proton flux population persisted for ∼2 days. Thus, we quantify the temporal flux variability in terms of L-value, energy and MLT.