RAPID TRANSITION OF UNCOMBED PENUMBRAE TO FACULAE DURING LARGE FLARES

In the past two decades, the complex nature of sunspots has been disclosed with high-resolution observations. One of the most important findings is the "uncombed" penumbral structure, where a more horizontal magnetic component carrying most of Evershed flows is embedded in a more vertical magnetic background. The penumbral bright grains are locations of hot upflows and dark fibrils are locations of horizontal flows that are guided by a nearly horizontal magnetic field. On the other hand, it was found that flares may change the topology of sunspots in delta configuration: the structure at the flaring polarity inversion line becomes darkened while sections of peripheral penumbrae may disappear quickly and permanently associated with flares. The high spatial and temporal resolution observations obtained with the Hinode/Solar Optical Telescope provide an excellent opportunity to study the evolution of penumbral fine structures associated with major flares. Taking advantage of two near-limb events, we found that in sections of peripheral penumbrae swept by flare ribbons the dark fibrils completely disappear, while the bright grains evolve into faculae that are signatures of vertical magnetic flux tubes. The corresponding magnetic fluxes measured in the decaying penumbrae show stepwise changes temporally correlated with the flares. These observations suggest that the horizontal magnetic field component of the penumbra could be straightened upward (i.e., turning from horizontal to vertical) due to magnetic field restructuring associated with flares, which results in the transition of penumbrae to faculae.