EVOLUTION OF MAGNETIC FIELD AND ENERGY IN A MAJOR ERUPTIVE ACTIVE REGION BASED ONSDO/HMI OBSERVATION

Abstract

We report the evolution of magnetic field and its energy in NOAA active region 11158 over 5 days based on a vector magnetogram series from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO ). Fast flux emergence and strong shearing motion led to a quadrupolar sunspot complex that produced several major eruptions, including the first X-class flare of Solar Cycle 24. Extrapolated non-linear force-free coronal fields show substantial electric current and free energy increase during early flux emergence near a low-lying sigmoidal filament with sheared kilogauss field in the filament channel. The computed magnetic free energy reaches a maximum of ∼2.6 × 10 32 erg, about 50% of which is stored below 6 Mm. It decreases by ∼0.3 × 10 32 erg within 1 hour of the X-class flare, which is likely an underestimation of the actual energy loss. During the flare, the photospheric field changed rapidly: horizontal field was enhanced by 28% in the core region, becoming more inclined and more parallel to the polarity inversion line. Such change is consistent with the conjectured coronal field "implosion", and is supported by the coronal loop retraction observed by the Atmospheric Imaging Assembly (AIA). The extrapolated field becomes more "compact" after the flare, with shorter loops in the core region, probably because of reconnection. The coronal field becomes slightly more sheared in the lowest layer, relaxes faster with height, and is overall less energetic.

Three-Dimensional Nonlinear Force-Free Field Reconstruction of Solar Active Region 11158 by Direct Boundary Integral Equation

“…Around the PIL, there are some observed EUV loops connecting P2 with N1. Our extrapolation has obtained a series of small and low calculated magnetic field lines along the PIL and connecting the regions on both sides, which are agreeable with the EUV loop structures in general and the filament structure marked in Figure 1 in Sun et al (2012), as shown in the last two rows in Figure 7. However, we did not obtain higher-lying calculated magnetic field lines over the filament channel connecting footpoints with opposite magnetic polarities P2 and N1.…”

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.…”

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.…”