Sudden Penumbral Reappearance and Umbral Motion Induced by an M7.9 Solar Flare

Abstract

The sudden flare-related changes of sunspot structures have been recognized as the photospheric responses to the solar eruptions in the corona. In this study, we report two distinctive sunspots variations associated with the flare SOL2015-06-25T08:16 (M7.9). Along the flaring polarity inversion line (PIL), the originally decayed penumbra showed a sudden reappearance, with the horizontal fields increasing in the direction of the penumbral fibrils aligned. On the other hand, the small umbra, where the reappearing penumbra rooted, had a sudden northeastward motion, toward the north part of a large sunspot located in the other side of PIL. Based on the calculation of Lorentz force changes, the area of penumbral reappearance mainly suffered a downward pressure, while the umbra region was dominated by the northeastward lateral pressure. These observations can be well understood as a result of coronal fields contraction, which can be deduced from the nonlinear force-free field extrapolation model. It also confirms the implosion idea that the restructuring of coronal fields could impact the solar surface and interior.