Photospheric and Coronal Observations of Abrupt Magnetic Restructuring in Two Flaring Active Regions

Abstract

For two major X-class flares observed by the Solar Dynamics Observatory (SDO) and the Solar TErrestrial RElations Observatory Ahead (STEREO-A) spacecraft when they were close to quadrature, we compare major, abrupt changes in the photospheric magnetic vector field to changes in the observed coronal magnetic structure during the two flares. The Lorentz force changes in strong photospheric fields within active regions are estimated from time series of SDO Helioseismic and Magnetic Imager (HMI) vector magnetograms. These show that the major changes occurred in each case near the main neutral line of the region and in two neighboring twisted opposite-polarity sunspots. In each case the horizontal parallel field strengthened significantly near the neutral line while the azimuthal field in the sunspots decreased, suggesting that a flux rope joining the two sunspots collapsed across the neutral line with reduced magnetic pressure because of a reduced field twist component. At the same time, the coronal extreme ultraviolet (EUV) loop structure was observed by the Atmospheric Imaging Assembly (AIA) onboard SDO and the Extreme Ultraviolet Imager (EUVI) on STEREO-A to decrease significantly in height during each eruption, discontinuous changes signifying ejection of magnetized plasma, and outward-propagating continuous but abrupt changes consistent with loop contraction. An asymmetry in the observed EUV loop changes during one of the flares matches an asymmetry in the photospheric magnetic changes associated with that flare. The observations are discussed in terms of the wellknown tether-cutting and breakout flare initiation models.

Expanding and Contracting Coronal Loops as Evidence of Vortex Flows Induced by Solar Eruptions

“…This is similar to the previous reports of observed contraction events seen near disk center (e.g., Sun et al 2012;Gosain 2012;Simões et al 2013;Petrie 2016). Furthermore, the expansion for these loops is slower than the subsequent contraction, similarly as in the our observation (see also Figure 6b of Wang et al 2016) and other observations mentioned.…”

“…Observational evidence suggests that while the contracting loop motions in flares are not common, they may happen at any phase of solar flares, including the early, impulsive, and gradual one (e.g., Liu et al 2012;Sun et al 2012;Gosain 2012;Simões et al 2013;Yan et al 2013;Shen et al 2014;Kushwaha et al 2015;Thalmann et al 2016;Petrie 2016). Gosain (2012) and Simões et al (2013) have shown that the onset of the contraction depends on the location of the loops with respect to the flare: loops located progressively further away from the flare contract later.…”

Observation of a reversal of rotation in a sunspot during a solar flare

“…The authors40 noted that the sunspots may connect the two ends of the flux rope associated with the flare, and suggested that the decrease in the horizontal field of the sunspots could be explained by the elimination from the flux rope of some of the twist component of the field4142. In contrast to the previously studied case, the sunspot presented here was swept by the flare ribbon.…”

Unambiguous Evidence of Coronal Implosions during Solar Eruptions and Flares

“…As these peripheral loop contractions were always observed face-on and accompanied by eruptions from central magnetic structures (like a filament or an arcade eruption), the possibility could not be ruled out that apparent contraction is a projection effect due to the inclination of the loop plane pushed by the erupting structure, rather than a real contraction (from our survey experience, loop inclining is indeed more commonly observed when the loops are viewed with an edgeon state at the solar limb, and even some of them do not restore back to their original locations). As far as we know, only Petrie (2016) reported edge-on loop contractions in two active regions from the perspective of the Solar TErrestrial RElations Observatory (STEREO) in 195 Å, but due to the short interval of the process and the long cadence (∼5 minutes), the dynamics was not persistently revealed and not clear enough to be well studied. In addition, both the Petrie (2016) events show dramatic eruptions, but in this paper we also show a new type of loop contraction observed edge-on without violent eruptions.…”