Interplanetary Protons versus Interacting Protons in the 2017 September 10 Solar Eruptive Event

Abstract

We analyze the relativistic proton emission from the Sun during the eruptive event on 2017 September 10, which
caused a ground-level enhancement (GLE 72) registered by the worldwide network of neutron monitors. Using the
neutron monitor data and interplanetary transport modeling both along and across interplanetary magnetic field
(IMF) lines, we deduce parameters of the proton injection into the interplanetary medium. The inferred injection
profile of the interplanetary protons is compared with the profile of the >100 MeV γ-ray emission observed by the
Fermi Large Area Telescope, attributed to pion production from the interaction of >300 MeV protons at the Sun.
GLE 72 started with a prompt component that arrived along the IMF lines. This was followed by a more prolonged
enhancement caused by protons arriving at the Earth across the IMF lines from the southwest. The interplanetary
proton event is modeled using two sources—one source at the root of the Earth-connected IMF line and another
source situated near the solar western limb. The maximum phase of the second injection of interplanetary protons
coincides with the maximum phase of the prolonged >100 MeV γ-ray emission that originated from a small area at
the solar western limb, below the current sheet trailing the associated coronal mass ejection (CME). A possible
common source of interacting protons and interplanetary protons is discussed in terms of proton acceleration at the
CME bow shock versus coronal (re-)acceleration in the wake of the CME.