Authors: Ashraf Moradi (University of Arizona), Joe Giacalone (University of Arizona)
Solar protons with rigidities higher than 1 GV (432 MeV) penetrate Earth’s atmosphere, generate secondary neutrons that reach the ground level neutron monitors. These events are called ground-level enhancement (GLE). The time-intensity profiles of the usual GLE events show a sharp rise followed by a slow decay. However, in 1989 OCT 22 (GLE#44), an unusual time-intensity profile was observed in several ground-level stations (Cramp 1997). The time-intensity profile showed a very sharp initial spike followed by a second smaller spike with a substantially slower rise and slower decay. Cramp (1997), also, obtained the pitch-angle distribution of the event. The pitch-angle distributions of the first arriving protons had a strong peak along the local IMF. Later, there was two peaks in the pitch-angle distribution, one along the local IMF and the other in the opposite direction. Moradi & Giacalone (2022) adopted Giacalone (2004) fluctuating interplanetary magnetic field (IMF) and showed that the turbulent IMF can explain the unusual time-intensity profile of GLE#44. In this study, we adopt the same simulation from Moradi & Giacalone (2022) to find and compare the pitch-angle distributions of solar energetic protons at 1 au in the average Parker spiral and large-scale turbulent interplanetary magnetic.
We apply a 3D fully relativistic test particle model. The pitch-angle scattering is included by applying the ad hoc scattering. We investigate the pitch-angle distributions of the 500MeV solar protons at 1 au in a weak pitch-angle scattering (λ|| = 1 au) with various parameters: Five IMF including Parker field and turbulent IMF with two realizations and two fluctuating rms velocity amplitude (Vrms) of 0.6 and 2 km/s, and A+ and A- field polarity. We release these solar protons in a 10˚x10˚ area near the HCS (N1˚), relatively far ( N10˚) and far from HCS (N20˚ and N30˚).
We find that the pitch-angle distribution at 1 au in the turbulent IMF, can vary significantly depending on the observer’s location, IMF Parameters, and IMF polarity. We also find that in the case of the “weak” scattering and A- polarity, the same λ|| and IMF polarity as the GLE#44 in 1989 OCT 22 event, both outward and reflected fluxes of protons can exist at 1 au in some observer’s locations.