The Great Solar Active Region of October 2014
While preparing a public talk on “rogue sunspots” and their role in causing solar cycle fluctuations, I wanted to get a feeling for how frequently “very large” \(10^{23}\,\textrm{Mx}\) regions like that studied by Nagy et al. (2017) might occur on the real Sun. Those authors point to NOAA Active Region 12191 as having a flux about \(1.6\times 10^{23}\,\textrm{Mx}.\) Observed in Carrington Rotation 2156, in October 2014 during the descending phase of Cycle 24, this was apparently the largest since AR 6368 in November 1990.
In this post, I’m just collecting some references about this brute of an active region as I think it is a nice case study.
Here are some observations of AR 12191 on 23rd October 2014 from AIA and HMI (http://suntoday.lmsal.com/):
The whopping region is predominantly bipolar, albeit with some additional structure.
The activity of the region was studied by Sun et al., 2015. Interestingly, despite being very energetic with six X-class flares during its disk passage (and fast horizontal flows observed with helioseismology – Jain et al., 2017), the region produced no obvious CMEs in this time. This seems to be because there was a strong overlying field above the non-potential core, causing all of the eruptions to be confined.
It is clear that the region emerged with negative helicity, i.e. opposite to the majority sign for the southern hemisphere, although not particularly unusual. This negative helicity is evident from the dextral chirality of EUV structures (e.g. McMaken & Petrie, 2017 or Figure 1c of Sun et al., 2015). McMaken & Petrie, 2017 show that this visual impression is confirmed by calculating (essentially) the current helicity density \(\alpha = j_z/B_z.\) It is also confirmed by the recent arXiv preprint of Pipin et al., who compute the helicity density {} in the poloidal-toroidal gauge using HMI observations (see their Figure 3).
What McMaken & Petrie, 2017 show, however, is that the helicity is mixed in sign within the region, and indeed the overall sign is gradually reversed over the course of several solar rotations. This leads to the formation of a clearly sinistral filament, consistent with the hemispheric pattern, by Carrington Rotation 2160 (see their Figure 5). I think this behaviour would be expected from magneto-frictional simulations in this case, although I haven’t tested this numerically.
From the solar cycle point of view (my original interest), the observations of McMaken & Petrie, 2017 suggest that AR12192 didn’t cause a huge perturbation to the polar field. Rather, the huge positive and negative flux seemed to balance out, although they suggest that it may have had some weakening effect on the polar field.
To investigate this last point, I looked into the axial dipole moment measured for this region in NSO magnetograms by my PhD student Tim Whitbread, and now available online at the Solar Dynamo Dataverse. In Tim’s dataset (which is lower resolution than the HMI observations), this region has a magnetic flux \(-2.6\times 10^{22}\,\textrm{Mx}.\) At its time of observation, he measured its axial dipole moment to be \(-0.027\,\textrm{G},\) which is (indeed) the correct sign to weaken the polar field of the cycle. This is actually quite strong (see Tim’s paper), and at its latitude of \(-13.5^\circ\) we would expect its effect on the end-of-cycle polar field to also be about \(-0.027\,\textrm{G}.\) This would suggest that the region may have really been a “rogue region” in the sense that it had a significant weakening effect on the polar field.