Quantitative assessment of protons during the solar proton events of September 2017

Abstract

(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 eneErgies  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 EaErth is L∼4.4 for 102.6 MeV energiEes on 10th September 2017, while lower energy protons i.e., 25 MeV are observed deepE up to L∼3.4 E 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 ∼2 days. Thus, we quantify the temporal flux variability in terEms of L -value, energy and MLT.

(Plain Language Summary) During a solar energetic particle (SEP) event, energetic electrons and ions flood the heliosphere causing severe damage to satellites, radio communication and humans in space. The Earth's magnetic field controls the dynamics of these particles to near-Earth space. One such unique event was observed in September 2017 for which the energy spectra and quantification of the proton fluxes spanning from 1.5 to 433 MeV using multi-satellite observations is studied. This was the largest proton event of the solar cycle 24 with three M-class and four X-class flares were observed by near-Earth spacecrafts. Proton fluxes were quantified at different locations like L1 point, geostationary orbit, inner magnetosphere, and low altitudes. The extent of flux enhancements, its access into the Earth's magnetosphere, MLT dependence and time to reach maximum fluxes are computed and compared before and after the SEPs arrived. We show that the multiple spacecraft observations are the key tool to quantify the temporal flux variability in terms of L-value, energy and MLT.