X-ray flux from the Sun from 23 to 26 October 2013. Courtesy US National Weather Service Space Weather Prediction Center
The transit of two large and active spot groups across the Sun on the third week of October meant dozens of C-class flares, punctuated by an M-class every half day or so. When on the 25th the two groups were joined by a third that immediately unleashed two X-class flares, the GOES X-ray flux (Fig1) went ‘crazy’! Keeping track of which spot group was flaring at a given moment was difficult: at times two groups flared simultaneously!
While some were powerful flares they were mostly short-lived, over in minutes rather than hours. On the 25th (local) two lesser C-class were recorded, one peaked at 22:10 the other at 23:26 (Oct 24th, UT). An M2.9 would have been seen had the session lasted another three hours!
A flare from sunspot group AR11875. Sketch and copyright Harry Roberts ©, all rights reserved
AR11875 flaring. Fig2 shows the C5.7 flare that peaked at 22:10UT, the other C2.1 in southern group AR11877 at 23:26UT, is not shown.
If flares seem likely the writer’s H-alpha filter is regularly tuned across the central band (6563Å) as flares occur when a filament erupts near the ‘inversion line’ of a sunspot group. The line separates regions of opposite polarity in complex groups like AR11875 (Hale class Beta-Gamma-Delta) and it threads its way between uneven magnetic forces pulling in different directions.
ARF. Active region filaments erupt very fast and are strongly Doppler shifted, but can be detected in the Lumicon filter due to its wide tuning range. Yet despite searching the ‘wings’ of the H-alpha line no filament was seen, and the flare appeared without any warning.
It began as an incandescent ribbon that threaded its way between lesser spots in the wake of the giant preceding (p) spot of the group (sited at +6,33, rhs). Before a log could be completed the flare had doubled into twin ‘flare ribbons’ that separate rapidly at 100km/sec, as I recall.
The ribbons are in fact ‘foot points’ for an ‘arcade’ of flare arches, which grow taller as the ribbons separate. In this case I could not see the tops of the arches, just the bright foot points, as our line of sight was directly above the arcade.
Spot polarities. I have added these from the Mt Wilson daily drawing (© Regents of the University of California LA, Mt Wilson Obs), made some 5 hours earlier, so as to clarify the flare’s magnetic ‘surroundings’. We see that the big preceding (p) spot (rhs) has strong violet polarity of 2500G, but just ahead of it are two ‘interloper’ red polarities: how did they get there? Red spots are meant to be far to the ‘rear’, at the eastern end (lhs) of the group.
Lesser violet spots follow the big (p) spot, until we meet the R20 spot close to the brightest parts of the flare ribbons, the latter sited at +6,26 (i.e.6ºN, long. 26º). One more degree to the east we find an irregular penumbral mass containing mixed polarities: V19 and R21 in the one penumbra – the distinctive ‘Delta’ configuration.
It appears from the layout of the different polarities that the ‘inversion line’ lay somewhere along the path of the flare – though the eruption of the unseen filament causes sudden change and simplification of fields. Note the small dark active region filament (ARF) east of the big spot – it is perhaps sited on the inversion line too and may be a remnant of the ejected ARF that caused the flare.
‘Persistence’ was not rewarded with any ‘great’ flares – but the few lesser ones were all good examples of the dramatic transients.
AR11875. The big spot group was 13º in overall length, and the circle (Fig., upper left) shows Earth to scale. As the Delta configuration endured the group had many M-class flares and, as it neared the Sun’s west limb, it unleashed a much stronger X1 flare: a dramatic ‘exit’ to its transit across the disc.
Will it return? It may well do so: watch the eastern limb around November 12th. With white light or H-alpha ‘scopes (or both) the Sun is turning on a great display of solar physics – not to be missed!
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.