Hinode/EIS Coronal Magnetic Field Measurements at the Onset of a C2 Flare

Abstract

Abstract We study Hinode/EIS observations of an active region taken before, during, and after a small C2.0 flare in order to monitor the evolution of the magnetic field and its relation to the flare event. We find that while the flare left the active region itself unaltered, the event included a large magnetic field enhancement (MFE), which consisted of a large magnetic field strength increase to values just short of 500 G in a rather small region where no magnetic field was measured before. This MFE is observed during the impulsive phase of the flare at the footpoints of flare loops, its magnetic energy is sufficient to power the radiative losses of the entire flare, and has completely dissipated after the flare. We argue that the MFE might occur at the location of the reconnection event triggering the flare, and note that it formed within 22 minutes of the flare start (as given by the EIS raster return time). These results open the door to a new line of studies aimed at determining whether MFEs can be flare precursor events or used for Space Weather forecasts, what advance warning time they could provide and if this time is long enough to allow for mitigation procedures to be implemented; as well as to explore which physical processes lead to MFE formation and dissipation, whether such processes are the same in both long-duration and impulsive flares, and whether they can be predicted by theoretical models.