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

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 similar to 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 similar to 4.4 for 102.6 MeV energies on 10th September 2017, while lower energy protons i.e., 25 MeV are observed deep up to L similar to 3.4 on 11th 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 similar to 2 days. Thus, we quantify the temporal flux variability in terms of L-value, energy and MLT.

Automated summary

The study presents the observations of proton fluxes during the solar proton event on September 7 and 10, 2017, detected by multiple spacecrafts across different regions of Earth's magnetosphere. Variations in solar proton flux distribution were influenced by different levels of solar flare activity. The study quantifies the temporal flux variability in terms of L-value, energy, and magnetic local time (MLT) in the Earth's magnetosphere.