The relationship between solar flare ribbons and magnetic field changes in the solar photosphere

ABSTRACT The changing magnetic field in solar flares has a complex association with the ultraviolet (UV) emission of flare ribbons. These ribbons are visual markers indicating the chromospheric endpoints of the magnetic field that is undergoing coronal reconnection, the process thought to be the driver of flares. The relationship between these field changes and ribbons is not entirely clear. In this work, we aim to investigate the spatially and temporally varying magnetic field behaviour exclusively within flare ribbon regions. We studied six M-class flares to understand the relationship between the line-of-sight magnetic field changes in the photosphere and the flare ribbons’ UV emission, using 45 s-cadence magnetograms obtained from the Solar Dynamics Observatory (SDO) Helioseismic and Magnetic Imager, and 1600 Å images from the SDO Atmospheric Imaging Assembly. Our findings reveal that the strongest changes in the magnetic field can either precede or lag the UV peaks in different locations, but that in 4 of the 6 flares, on average the magnetic field changes preceded the UV peak typically by a few minutes. We report oscillations in the line-of-sight magnetic field which might not be a flare-related change, as well as the previously studied monotonic changes. We have identified a moderate correlation between the absolute magnetic field strength ($|$B$|$) and the rate of magnetic field change ($|$dB/dt$|$), indicating that on average stronger magnetic fields experience higher rates-of-change, up to a maximum of 8.6 $\rm {G\, min^{-1}}$ on average. These results provide insight into the dynamic magnetic processes governing flare evolution, caused by the reconfiguration of the magnetic field, and release of magnetic energy that results in the generation of ribbon radiation.