The Role of a Tiny Brightening in a Huge Geoeffective Solar Eruption Leading to the St. Patrick's Day Storm

The largest magnetic storm in solar cycle 24 was caused by a fast coronal mass ejection (CME) that was related to a small C9.1 flare that occurred on 2015 March 15 in solar active region (AR) NOAA 12297. The purpose of this study is to understand the onset mechanism of the geoeffective huge solar eruption. We focused on the C2.4 flare that occurred prior to the C9.1 flare of the filament eruption. The magnetic field structure in the AR was complicated: there were several filaments, including the one that erupted and caused the CME. We hence carefully investigated the photospheric magnetic field, brightenings observed in the solar atmosphere, and the 3D coronal magnetic field extrapolated from nonlinear force-free field modeling, using data from Hinode and Solar Dynamics Observatory. We found three intriguing points. (1) There was a compact but noticeably highly twisted magnetic field structure that is represented by a small filament in the C2.4 flaring region, where a tiny precursor brightening (PB) was observed before the C2.4 flare. (2) The C2.4 flaring region is located in the vicinity of a foot point of the closed field that prohibits the filament from erupting. (3) The filament shows a sudden eruption after the C2.4 flare and the accompanying small filament eruption. From our analysis, we suggest that a small magnetic disturbance that was represented by the tiny PB at the time of the C2.4 flare is related to the trigger of the huge filament eruption.