SOLARNET Conference: The Many Scales of the Magnetic Sun

Abrupt and permanent photospheric magnetic field changes have been observed in many flares. It is believed that such changes are related to the reconfiguration of magnetic field lines, however, the real origin is still unclear. I will present the analysis of 37 flares observed by the Solar Dynamics Observatory (SDO). The characteristics of the magnetic field changes in the flare events are determined using HMI high-cadence vector magnetogram (135 s), whereas the ribbon properties are derived from co-aligned AIA 1600 Å images. We find that pixels swept up by ribbons do not always exhibit permanent changes in the field. However, when they do, ribbon emission typically occurs several minutes before the start of field changes. The changes in the properties of the field show no relation to the size of active regions but are strongly related to the flare-ribbon properties. We find that the duration of permanent changes in the field is strongly coupled with the duration of the flare. Our results suggest that changes in photospheric magnetic fields are caused by a combination of two scenarios: contraction of flare loops driven by magnetic reconnection and coronal implosion.