Investigation of Solar Proton Access into the Inner Magnetosphere on 11 September 2017

Abstract:

Solar energetic protons (SEPs) are shielded by the Earth’s magnetosphere and are known to exhibit an east-west asymmetry effect. SEP access to the inner magnetosphere at low latitude and the cutoff energies for protons arriving from different directions at different altitudes over the inner magnetosphere remains largely unquantified. SEPs arriving from the west have their gyrocenters at a larger radial distance from the earth where the magnetic shielding is weaker and lower energy protons have access while those arriving from the east must penetrate a stronger magnetic field region since their gyrocenters are within the orbit of the spacecraft, thus only higher energy protons access the spacecraft from the east. This phenomenon has been modeled at geosynchronous orbit (Kress et al., 2013; 2015), but for the first time we are able to model the cutoff at varying radial distance along the trajectory of the spinning Van Allen Probes spacecraft and compare with observations at varying radial distance from earth. In this study, solar proton access to the inner magnetosphere on 11 September 2017 is quantitatively examined using a Tsyganeko plus IGRF magnetic field model. The measured proton flux is shown to depend largely on particle arrival direction. The maximum and minimum cutoff rigidities along the Van Allen Probes orbit are numerically computed by extending the Dartmouth geomagnetic cutoff code [Kress et al., 2010] for arbitrary direction. The numerically calculated cutoff rigidity is approximately consistent with observations, showing that proton access into the inner magnetosphere depends largely on the angle between particle arrival direction and magnetic west. The effects of the earlier 7 – 9 September geomagnetic storm (minimum Dst = - 124 nT) on cutoff suppression for a weaker SEP event and extension of SEP access to lower latitudes, which can affect airline routes as an immediate space weather consequence, will also be examined.

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