Rapid Changes of Photospheric Magnetic Field after Tether-Cutting Reconnection and Magnetic Implosion

TL;DR

This study investigates rapid photospheric magnetic field changes during a solar flare, linking them to tether-cutting reconnection and magnetic implosion through multi-instrument observations and NLFFF modeling.

Contribution

It provides detailed analysis of magnetic field evolution during a flare, highlighting the role of tether-cutting reconnection and coronal field implosion.

Findings

28% increase in horizontal magnetic field strength in the flare region

Coronal currents collapsed downward after the sigmoid eruption

Magnetic field changes are consistent with tether-cutting reconnection and implosion models

Abstract

The rapid, irreversible change of the photospheric magnetic field has been recognized as an important element of the solar flare process. This Letter reports such a rapid change of magnetic fields during the 2011 February 13 M6.6 flare in NOAA AR 11158 that we found from the vector magnetograms of the Helioseismic and Magnetic Imager with 12-min cadence. High-resolution magnetograms of Hinode that are available at ~-5.5, -1.5, 1.5, and 4 hrs relative to the flare maximum are used to reconstruct three-dimensional coronal magnetic field under the nonlinear force-free field (NLFFF) assumption. UV and hard X-ray images are also used to illuminate the magnetic field evolution and energy release. The rapid change is mainly detected by HMI in a compact region lying in the center of the magnetic sigmoid, where the mean horizontal field strength exhibited a significant increase by 28%. The region lies between the initial strong UV and hard X-ray sources in the chromosphere, which are cospatial with the central feet of the sigmoid according to the NLFFF model. The NLFFF model further shows that strong coronal currents are concentrated immediately above the region, and that more intriguingly, the coronal current system underwent an apparent downward collapse after the sigmoid eruption. These results are discussed in favor of both the tether-cutting reconnection producing the flare and the ensuing implosion of the coronal field resulting from the energy release.