Ground level enhancements (GLEs) are extreme solar energetic particle (SEP)
events that are of particular importance in space weather. In solar cycle 24,
two GLEs were recorded on 2012 May 17 (GLE 71) and 2017 September 10 (GLE 72),
respectively, by a range of advanced modern instruments. Here we conduct a
comparative analysis of the two events by focusing on the effects of
large-scale magnetic field configuration near active regions on particle
acceleration and release. Although the active regions both located near the
western limb, temporal variations of SEP intensities and energy spectra
measured in-situ display different behaviors at early stages. By combining a
potential field model, we find the CME in GLE 71 originated below the streamer
belt, while in GLE 72 near the edge of the streamer belt. We reconstruct the
CME shock fronts with an ellipsoid model based on nearly simultaneous
coronagraph images from multi-viewpoints, and further derive the 3D shock
geometry at the GLE onset. The highest-energy particles are primarily
accelerated in the shock-streamer interaction regions, i.e., likely at the nose
of the shock in GLE 71 and the eastern flank in GLE 72, due to
quasi-perpendicular shock geometry and confinement of closed fields.
Subsequently, they are released to the field lines connecting to near-Earth
spacecraft when the shocks move through the streamer cusp region. This suggests
that magnetic structures in the corona, especially shock-streamer interactions,
may have played an important role in the acceleration and release of the
highest-energy particles in the two events.Comment: Accepted for publication in Ap