  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 15
March 2015 in solar active region (AR) NOAA 12297. The purpose of this study is
to understand the onset mechanism of the geo-effective 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 three-dimensional 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 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 accompanying small filament eruption. From our
analysis, we suggest that a small magnetic disturbance that was represented by
the tiny precursor brightening at the time of the C2.4 flare is related to the
trigger of the huge filament eruption.
