Rapid Penumbral Decay following Three X-Class Solar Flares

Abstract

We show strong evidence that penumbral segments decayed rapidly and permanently right after three X-class solar flares. Two of the three events occurred very recently in NOAA Active Region 10486, an X17 flare on 2003 October 28 and an X10 flare on 2003 October 29. The third X2.3 flare was observed in solar active region NOAA AR 9026 on 2000 June 6. The locus of penumbral decay is related to flare emission, albeit with distinct differences for each event. We present difference images highlighting the rapid changes between pre- and postflare states of the flaring active region, which show distinct decaying penumbral segments and neighboring umbral cores becoming darker. Because of the lack of spectroscopic data, we cannot exclude the possibility that the observed changes are due to changes in the temperature structure of the flaring atmosphere, or to a corresponding reduction in opacity for a section of both umbra and penumbra. However, we argue against this possibility because the observed intensity changes are permanent, not transient. We instead propose a possible explanation that magnetic fields change from a highly inclined to a more vertical configuration within approximately 1 hr after the flares; i.e., part of the penumbral magnetic field is converted into umbral fields.