Abrupt changes of the photospheric magnetic field in active regions and the impulsive phase of solar flares

Abstract

We compared time profiles of changes of the unsigned photospheric magnetic flux in active regions with those of their associated soft X-ray (SXR) bursts for a sample of 75 ≥ M5 flares well observed by Global Oscillation Network Group longitudinal magnetographs. Sixty-six of these events had stepwise changes in the spatially integrated unsigned flux during the SXR flares. In superposed epoch plots for these 66 events, there is a sharp increase in the unsigned magnetic flux coincident with the onset of the flare impulsive phase while the end of the stepwise change corresponds to the time of peak SXR emission. We substantiated this result with a histogram-based comparison of the timing of flux steps (onset, midpoint of step, and end) for representative points in the flaring regions with their associated SXR event time markers (flare onset, onset of impulsive phase, time of peak logarithmic derivative, maximum). On an individual event basis, the principal part of the stepwise magnetic flux change occurred during the main rise phase of the SXR burst (impulsive phase onset to SXR peak) for 60% of the 66 cases. We find a close timing agreement between magnetic flux steps and >100keV emission for the three largest hard X-ray (>100keV) bursts in our sample. These results identify the abrupt changes in photospheric magnetic fields as an impulsive phase phenomenon and indicate that the coronal magnetic field changes that drive flares are rapidly transmitted to the photosphere. © 2012. The American Astronomical Society. All rights reserved.