Response of the Photospheric Magnetic Field to the X2.2 Flare on 2011 February 15

Abstract

It is well known that the long-term evolution of the photospheric magnetic field plays an important role in building up free energy to power solar eruptions. Observations, despite being controversial, have also revealed a rapid and permanent variation of the photospheric magnetic field in response to the coronal magnetic field restructuring during the eruption. The Helioseismic and Magnetic Imager instrument (HMI) on board the newly launched Solar Dynamics Observatory produces seeing-free full-disk vector magnetograms at consistently high resolution and high cadence, which finally makes possible an unambiguous and comprehensive study of this important back-reaction process. In this study, we present a near disk-center, GOES-class X2.2 flare, which occurred in NOAA AR 11158 on 2011 February 15. Using the magnetic field measurements made by HMI, we obtained the first solid evidence of a rapid (in about 30 minutes) and irreversible enhancement in the horizontal magnetic field at the flaring magnetic polarity inversion line (PIL) by a magnitude of ∼30%. It is also shown that the photospheric field becomes more sheared and more inclined. This field evolution is unequivocally associated with the flare occurrence in this sigmoidal active region, with the enhancement area located in between the two chromospheric flare ribbons and the initial conjugate hard X-ray footpoints. These results strongly corroborate our previous conjecture that the photospheric magnetic field near the PIL must become more horizontal after eruptions, which could be related to the newly formed low-lying fields resulting from the tether-cutting reconnection.

Three-dimensional magnetic reconnection and its application to solar flares

“…Local changes in the horizontal photospheric magnetic fields were also recently reported (e.g. Wang et al 2002;Sun et al 2012;Wang et al 2012;Petrie 2013), while in those studies, the vertical field component does not change much.…”

The magnetic field in the solar atmosphere

“…The sign of this change remains inconclusive, however, since both, an increase of the shear (Wang 1992;Chen et al 1994;Wang et al 1994Wang et al , 2012aLiu et al 2005;Petrie 2012), as well as a decrease has been found during flares (Wang 2006). To explain such contradictory results, Dun et al (2007) proposed that the measures for the non-potentiality of a magnetic field may take on different values from one portion of an AR to another.…”

Rapid Penumbra and Lorentz Force Changes in an X1.0 Solar Flare

“…In more recent years, high-resolution white-light sunspot observations revealed some changes in photospheric fine structures caused by flares, e.g., disappearing penumbra fibrils and transition bright grains could evolve into faculae (Wang et al 2012a), granulation pattern could evolve to alternating dark and bright penumbra fibril structures (Wang et al 2013), and there were remarkable high-speed flows along the flaring PILs (Shimizu et al 2014). On the other hand, the photospheric vector magnetic-field observations showed that the horizontal magnetic fields increased rapidly and permanently around the central PILs (Wang & Liu 2010;Liu et al 2012;Petrie 2012;Sun et al 2012;Wang et al 2012bWang et al , 2012c, while they decreased significantly in the outer regions (Wang et al 2009;Li et al 2011;Sun et al 2012). These results thus suggested that, after major flares, the magnetic fields often became more horizontal in the central PIL regions but more vertical in the decaying penumbra regions.…”

“…In addition, the LF changes can be understood as a proxy for the pressure changes from the upper atmosphere to the photosphere. More recently, it was found that during major flares the sudden photospheric magnetic structure changes in their central regions close to the main flaring PILs might have been accompanied by obvious LF changes, such as the downward LF changes Petrie 2012;Sun et al 2012;Wang et al 2012bWang et al , 2012c, the horizontal LF changes providing torque for rapid sunspot rotation (Wang et al 2014) or abrupt torsional force for relaxing magnetic twist near the PILs (Petrie 2013), and so on. Besides the central regions, as mentioned above, the peripheral penumbra regions can decay clearly in some major flares.…”