Rapid Transition of Uncombed Penumbrae to Faculae during Large Flares

TL;DR

This study observes how large solar flares cause the uncombed penumbral structures in sunspots to rapidly transform into faculae, indicating significant magnetic field restructuring during flares.

Contribution

It provides high-resolution observational evidence of penumbral transformation into faculae during flares, linking magnetic field changes to flare activity.

Findings

Dark fibrils disappear in flare-affected penumbrae

Bright grains evolve into faculae during flares

Magnetic fluxes show stepwise changes correlated with flares

Abstract

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 (Solanki & Montavon 1993). The penumbral bright grains are locations of hot upflows and dark fibrils are locations of horizontal flows that are guided by 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 (Liu et al. 2005). The high spatial and temporal resolution observations obtained with Hinode/ SOT provide an excellent opportunity to study the evolution of penumbral fine structure 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.