121. The Origin of Major Solar Activity: Collisional Shearing between Nonconjugated Polarities of Multiple Bipoles Emerging within Active Regions

Contributed by Georgios Chintzoglou. Posted on February 28, 2019

Georgios Chintzoglou1,2, Jie Zhang3, Mark C. M. Cheung1, Maria Kazachenko4

1. Lockheed Martin Solar and Astrophysics Laboratory, 3176 Porter Dr., Palo Alto, CA 94304, USA
2. University Corporation for Atmospheric Research, Boulder, CO 80307-3000, USA
3. Department of Physics and Astronomy, George Mason University, Fairfax, VA, 22030, USA
4. Space Sciences Laboratory, University of California, Berkeley, CA, USA

Active Regions (ARs) that exhibit compact polarity inversion lines (PILs) are known to be very flare productive[1]. However, the physical mechanisms behind this statistical inference have not been demonstrated conclusively.

In Ref.[2], we presented the analysis of observations of highly flare- and CME-productive ARs in support of a new scenario for the origin of major solar activity. We examine the amount of flux canceled in the relevant segments of the internal PILs of the ARs, the “collisional PILs”, i.e. those formed by collisions of opposite polarities from different bipoles emerging within the same ARs (cPIL; orange lines in Figure 1), and also the timing of the flux cancellation with respect to flare activity. Using two well-observed flare- and CME-productive ARs composed of two conjugate bipoles that emerged (a) simultaneously (AR 11158) and (b) sequentially (AR 12017), we show that clusters of intense flaring activity correlate with the onset and duration of flux cancellation in collisional PILs. The cancellation begins at the onset of collision between the opposite-signed nonconjugated polarities and not at the PILs of the individual emerging bipoles/flux tubes (i.e. the “self PILs”; dark dotted lines in Figure 1).

Figure 1| Evolution of two highly flare- and CME-productive emerging ARs. Top panels: snapshot LOS magnetograms (saturated at ±1000 G) showing the evolution of AR 11158 as observed by SDO/HMI. Bottom panels: same for AR 12017.

Traditional measurements of the unsigned magnetic flux taken over the entire surface of a complex multipolar AR are naturally unable to reveal the action and the role the cancellation process plays during the emergence phase (e.g. Figure 2 P2 and N2 flux is increasing). To overcome this limitation, in Ref.[2] we introduced a novel measurement technique, the conjugate flux deficit method, which allows the measurement of the canceled magnetic flux and also the rate of magnetic cancellation at the collisional PIL. This method applies to multipolar ARs by means of recording the flux imbalance in their individual bipoles (in fact, a “deficit”) that develops gradually upon collision between nonconjugated polarities (Figure 2c). The fact that cancellation is measured in emerging ARs has significant implications in understanding the flare clustering and the origin of eruptive flares.

Figure 2| (a) Magnetic flux time evolution for bipole 2 of AR 11158 (conjugated polarities N2 and P2). (b) Average photospheric Doppler velocities at cPIL, (c) Magnetic flux deficit with flares overplotted (excluding the flares that were produced in the location of a parasitic bipole north of N2). (d) Plot of the oblateness (deformation) of sunspot N1, centroid distance between the colliding N1 and P2, and the resulting collisional PIL length with time.

Our proposed scenario considers cancellation occurring between colliding flux tubes that emerge simultaneously or sequentially, forming a single flare-productive AR (Figure 3, t0 panels). During the emergence stage, each flux tube manifests itself as a bipolar magnetic region (BMR), i.e. two conjugate opposite sign polarities. As the emergence phase progresses, each BMR’s conjugate polarities undergo mutual separation, following the canonical diverging polarity motions in each bipole as seen on the surface (we call this “self-separation”). As a result, collision (convergence) and shearing may occur between opposite-signed nonconjugated polarities, forming a collisional PIL and lasting for as long as they are in close contact and while the bipoles self-separate (Figure 3, t > tcollision onset). Activity follows shortly after the onset of collision (∼12 hr) and persists for the duration of the collision. This process develops self-consistently owing to the proper motions of the nonconjugated polarities. To emphasize the essence of these proper motions in leading to collision and to differentiate this driving mechanism from the cancellation scenario[3] that involves only a single conjugated bipole, we introduce the term “collisional shearing” to refer to the observed process. As a result of collisional shearing, cancellation occurs in the collisional PIL. This is consistent with photospheric cancellation, by submergence of flux at the collisional PIL (Figure 2b).

Figure 3| The collisional shearing processes describing the formation of multipolar flare-productive ARs.

We emphasize that virtually no flaring or eruptive activity occurs at the self-PILs separating the conjugated bipoles. The intense flaring and eruptive activity produced in the corona are rooted at the collisional PIL, effects consistent with the formation of stressed and twisted magnetic segments (i.e., magnetic flux ropes) that progressively reconnect and coalesce into more monolithic flux systems (e.g., Ref.[4]). Observational evidence and data-driven modeling[2] provide additional support to the creation of stressed and twisted magnetic structures at the collisional PIL, well before they drive eruptions. This view, given that it requires at least two bipoles, is inconsistent with the CSHKP standard model and the MHD emergence-based and cancellation-based, “data-inspired” 3D models of a single bipole[2].

Our results suggest that the quantification of magnetic cancellation driven by collisional shearing needs to be taken into consideration in order to improve the prediction of solar energetic events and space weather.


[1] Schrijver, C. J. 2007, ApJ, 655, L117
[2] Chintzoglou, G., Zhang, J., Cheung, M.C.M., & Kazachenko, M. 2019, ApJ, 871, 67
[3] van Ballegooijen A. A., & Martens P. C. H. 1989, ApJ, 343, 971
[4] Chintzoglou, G., Patsourakos, S., & Vourlidas, A. 2015, ApJ, 809, 34

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