As AR12192 crossed the Sun in the second half of October, it not only grew unusually large, but it hosted six GOES class-X flares, the strongest class; the writer logged one of the six. As well, an M8.7 flare was logged on Oct. 21st. This is an account of the X3.1 flare of Oct.24th.
The writer aims to log as much detail as possible of the spot group hosting the flare, hoping to understand the context of these energetic events. Detail of the group’s umbrae and penumbrae and the heliocentric coordinates of major features are recorded. As well, Mt. Wilson’s daily magnetograph data is sampled and added to the sunspot sketch (Fig1). The writer’s sketch is made ~5 hours after Mt Wilson’s worker completes theirs (also in pencil), adding one more to their vast archive. Now mostly digitised, it may be browsed at leisure: a treasure trove! There you will find the biggest spot yet recorded, in April 1947, and twice the area of 12192; the second biggest was in late Jan. 1946. Yes, the biggest spots in 140 years are concentrated around the mid-20th century – but why?
X3.1 flare. With complex spots like 12192 it helps to have the WL sketch ready when a flare erupts: flare ribbons can then be mapped with some accuracy. Since flares change rapidly, transparent overlays record flare motions relative to the more-or-less ‘fixed’ sunspots. Other transients like surges and filaments can be similarly logged.
However, at times of high activity, flares may erupt before the WL sketch is done, thus the X3.1 peak went unnoticed (in H-alpha) and its initial stages missed. Fig 2 shows the flare ~40m after its peak when it had faded to X1.1 but was still large and bright, visual class 2b. This was the biggest of the six X-class flares.
- Big surges had been ejecting from the group’s dominant (f) spot (at –13,242) from the 22nd, and on the 23rd some reached a point 30º lat south of the group – before retracting to their start point –a distance of 370Mm! We noted that the surges emerged (as they do) from the margins of the big (f) spot– and magnetograph data show a curious ‘collar’ of opposite polarity around the big spot’s penumbra– this seemed to promote surging.
Flare geometry. The X3 flare did not much involve the big (f) spot but erupted mainly along a ~N-S line, from the tangle of ‘red’ (p) spots at -10,251, southwards to –25,247. The SDO HMI magnetogram was used to (roughly) plot the group’s inversion line – the boundary between unlike polarities. Its complex windings are mapped as blue lines x-x’ and y-y’. Note the cluster of opposite polarities within the big (f) spot on its west side – one of R18 polarity – line x-x’ winds around it (arrow). Minor flare ribbons traced the gaps between umbrae and the ‘light bridge’ of the big (f) spot (Fig2).
Perhaps the flare arose on the N and W margins of line y-y’ and, as events proceeded, spread away from there. The Oct. 21st M8.7 flare developed toward this region too, SW of the group – where surges also were targeted.
There seems no doubt that AR12192 reached its huge size by a fusion of multiple groups that began back in Sep. when the group was AR12172, and when 12173 emerged nearby – and merged in some way during its far side transit. It has hosted some amazing H-alpha transients – and may well have more on its mid-November return.
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.