Researchers from the Yunnan Observatories (YNAO) of the Chinese Academy of Sciences and Yunnan Normal University have recently investigated how an X1.5 solar flare can cause a significant rearrangement of the sunspot magnetic field after a detailed analysis on the effects of a solar flare that occurred between May 9 and 10, 2022.

The results were published in the latest issue of Monthly Notices of the Royal Astronomical Society, entitled "Rearrangement of sunspot magnetic field caused by an X1.5 solar flare".

Solar flares are a sudden release of energy caused by magnetic reconnection, occurring on timescales as short as a few minutes. During this process, the coronal magnetic field is rapidly reconfigured, including the eruption of flux ropes above and the formation of post-flare loops below the reconnecting current sheet.

Studies of sudden changes in sunspot structures associated with flares in recent years revealed that after a flare, sunspot penumbral structures may increase in size or become darker near the polar inversion line, whereas they usually decay or disappear in the outer regions relative to the polar inversion line.

Further studies of the magnetic field in the photosphere show that the distribution of the horizontal magnetic field along the polar reversal line becomes stronger after the eruption, while the region away from the polarity inversion line decays, i.e., the magnetic field near the polarity inversion line becomes more horizontal, while the relative peripheral region, in contrast, becomes more vertical.

Using imaging data from the Solar Dynamics Observatory and the 1-meter New Vacuum Solar Telescope, researchers found a circular filament formation event driven by the rotation of a sunspot, and observed that the subsequent eruption of this filament had a significant feedback effect on the sunspot structure, which was manifested in a significant contraction of the sunspot.

Flare outbursts have significant feedback effects on sunspot structure, and the magnetic field in the outer regions converged towards the polarity reversal line, leading to an increase in magnetic field strength at the polarity inversion line and a decrease in the magnetic field in the outer regions.

This magnetic field rearrangement was directly related to the change of the strong horizontal Lorentz force towards the polarity inversion line that occurred during flares. This indicated that solar flares, as a typical solar eruptive activity, not only changed the local magnetic field structure in a short period but also influenced the sunspot evolution process in the long term.

“This study further revealed the intrinsic relationship between solar filament eruptions, solar flares and sunspot changes, and advanced the understanding of the fine physical processes of solar flares,” said YAN Xiaoli, the co-author of the study, also a researcher from YNAO.

Figure 1: Overview image of the active area. (a): NVST Hα image. (b): NVST TiO and HMI intensity images. (c): HMI magnetogram. (d): AIA 1600 angstroms image. (Image by YNAO)