Chang Liu & Haimin Wang
Space Weather Research Laboratory, New Jersey Institute of Technology, University Heights, Newark, NJ 07102-1982, USA

The concept of “implosions” in coronal transients predicts an inward contraction of coronal field that would occur simultaneously with the magnetic energy release^[1]. A direct consequence of coronal implosions would be a more horizontal configuration of the photospheric magnetic field. This is supported by the finding that the transverse magnetic field around the magnetic polarity inversion line (PIL) at the center of the flaring region often exhibits a rapid and persistent enhancement immediately after flares/CMEs^[2]. Intuitively, such an inward collapse of the central magnetic field may also be accompanied by an upward turning of the peripheral field in the active region. A related observation could be the decay of sunspot penumbrae in the outer areas of flaring regions^[3]. Obviously, the implosion process can have multiple impacts on the low solar atmosphere, which have not yet been fully explored.

Figure 1 | HMI observations of the X2.1 (top) and X1.8 (bottom) flares. In (c) and (f), the dotted line is the PIL and the solid line indicates the vertical slice used in Figure 2.

As an effort in this direction, this nugget reports on a comparative investigation of the homologous 2011 September 6 X2.1 and September 7 X1.8 flares in NOAA AR 11283. Both flares are initiated around the highly sheared PIL of the AR (Figures 1b and 1e), along which a flare-related, stepwise increases (26% and 38%) of the horizontal field Bh is clearly observed (Figures 1c and 1f). More interestingly, the central area of Bh enhancement is surrounded by the ring-like region with decreasing Bh, which largely corresponds to the peripheral penumbral region (cf. Figures 1a and 1d) and is more significant on the northern side. To better delineate the entire evolution, we also calculate the centroid (flux-weighted average) separation between the main positive and negative fields. The result shows an obvious decrease of the centroid separation by 0.85 Mm and 1.4 Mm immediately after the X2.1 and X1.8 flares, respectively, for a short time period, before the long-term evolution trend is restored (Figure 3a). We propose that this can be a surface signature of coronal implosions.

Figure 2 | (a–d) HMI and AIA images right before the X2.1 and X1.8 flares, superimposed with selected NLFFF lines and magnetic contours, respectively. The white line in (a) and (c) is the bottom of the rotating vertical slice, over which the distributions of Jh are plotted in (e)–(l). The slice rotates counterclockwise and maintains an orientation perpendicular to the central PIL. The thick arrow in (e) indicates the FR orientation in the cross section.

Using a nonlinear force-free field (NLFFF) extrapolation technique, it is found that compared to the X2.1 flare (Figures 2a and 2b), the X1.8 flare could be associated with a more well-formed, twisted flux rope (FR) as also reflected by an apparently thicker filament (Figures 2c and 2d). We employ a vertical slice cutting through the middle of the FR to examine its evolution (Figures 2e–2l; also see a movie [http://solar.njit.edu/~cliu/2012090607/FR_20110906_07.mp4]). We see that (1) the twisted FR collapses toward the surface after the X2.1 flare, then rises gradually in 1 day reaching a higher altitude, and collapses again after the X1.8 flare. Both the distance and speed of the falling motion in the X1.8 flare are twice as large as those in the X2.1 flare (Figures 3b and 3c), indicating a more violent implosion and agreeing with a more significant change of the photospheric magnetic field (both the Bh enhancement and decrease of centroid separation). (2) The FR is attached to the photosphere before the X2.1 flare; in contrast, it is already elevated off the surface before the X1.8 flare. A full FR eruption could occur in the latter event, as little signatures of FRs remain afterward. (3) The FR is not symmetric in its central vertical cross section but leans northward at 66 deg relative to the surface. Together with the ambient fields, they turn southward rapidly after both flares, echoing the observed Bh decrease in the peripheral regions on the surface.

Figure 3 | Temporal evolution of the flux-weighted centroid separation between the two magnetic polarities on the surface (a), the FR height (b), and the FR speed (c). The vertical dotted lines indicate the peak times of the hard X-ray emission.

In summary, our observational and model results portray a coherent picture of implosions in the low corona, in which the central field collapses toward the photosphere while the peripheral field turns to a more vertical configuration. A surface signature is that the separation between the major magnetic concentrations with opposite polarities also decrease. Moreover, the implosion process appears to be more abrupt when associated with a full FR eruption. Details of this work and further analysis on the flare energy release and helioseismic response can be found in Ref.[4].

References

[1] Hudson, H. S. 2000, ApJ Lett, 531, L75
[2] Wang, H. & Liu, C. 2010, ApJ Lett, 716, L195
[3] Liu, C., Deng, N., Liu, Y., Falconer, D., Goode, P. R., Denker, C. & Wang, H. 2005,ApJ, 622, 722
[4] Liu, C., Deng, N., Lee J., Wiegelmann, T., Jiang C., Dennis B. R., Su Y., Donea A., & Wang H. 2014, ApJ, 795, 128