AR12371: Delta Groups and Flares of Solar Cycle 24. Sketch and copyright Harry Roberts ©, all rights reserved.
Regular solar observer & correspondent Harry Roberts reports on his observations of flares seen on June 20-21 (UT). These flares caused aurora that were visible from Sydney, and similar latitudes, on June 23, 2015.
AR12371: Delta Groups and Flares of Solar Cycle 24.
G. E. Hale’s magnetic classification of sunspot groups did not at first include class Delta – this category was defined by H. Kunzel in 1960 as ‘two or more umbrae of opposite magnetic polarity in a single penumbra’ (pu).
Since the start of Solar Cycle 24 (SC24) there have been (writer’s count to end April 2015) 39 Delta groups, the first being AR11045 in 2010 Feb. Between them they have hosted 29 GOES X-class flares (i.e. flares with an X-ray flux >1×10-4Wm-2). The strongest was an X6.9 in 2011 July.
In SC24 thus far, not all Delta groups produced GOES X-class flares, but all such flares were the product of Delta groups: i.e. to see an X-class flare we must monitor Delta groups closely. AR12192 of 2014 October, a Delta group, produced six X-class flares: the strongest an X3.1; also that group was the largest sunspot for over twenty years.
- This would have appeared on the Sun about 2015 June 15 – but went unseen until the 19th (UT) due to heavy cloud. By then it had been dubbed class Delta by NOAA and contained about 24 spots: its irregular shaped preceding (p) spot and jumbled large following (f) spots were typical of the class. No polarity data was posted by Mt Wilson that day but data for the 20th showed umbrae of unlike ‘sign’ in the large irregular (f) spot, along with linear umbra and multiple light bridges in the elongated (p) spot (Fig. Preflare): a classic Delta.
This was one of the few recent spots that promised imminent flaring; when the white light (WL) log was done, H-alpha patrol began at 21ST 00:45 – and, amazingly, the expected flare in fact arose at 01:30 reaching visual class 1N! Three logs show the progress of this unusual flare which had two separate peaks of M-class. AR12371 however did not host any X-class events.
Preflare: The Fig shows the group as logged in WL – with Mt Wilson polarity from earlier added, showing large umbrae of preceding (i.e. p) ‘sign’ (violet) amid the following (f) spots: the Delta configuration. The polarity boundary or ‘inversion line’ is dotted blue based on SDO HMI magnetogram – it matches Mt Wilson data – but with a hint of a smaller one below the big (p) spot. Note the flares will develop along this configuration.
Obscured umbrae. Some authors note that precursors to ‘big flare’ events include the ‘obscuration of spot umbrae by H-alpha emissions’. Often logged in the past, the writer noted that the intricate light bridges (LB) in the big (p) spot umbra (seen in WL), were obscured in H-alpha by emissions of plage brightness (Fig) – and unchanged during the ensuing flares.
Flare 1(Fig 1). This began to brighten at 01:20 and by 01:30 reached GOES M1 level – slender ribbons stretched from the (f) spot at +14,297 to a fiducial point at ~+8,304, south of the big (p) spot. By 01:42 M2 level was reached (Fig 2): now the lesser ribbons were gone, yet those of the (f) spot remained bright – in fact grew brighter. Note the changes to filaments at points X and Y.
Flare 2. By 01:55 the flare had faded to 1F(visual) and the session ended at 02:05. The ‘scopes were stowed and calculation of coordinates began. A glance at the GOES Flux about 02:40 showed another strong flare in progress: perhaps in AR12367 – the only other group on the disc? In haste, the scope was reassembled to reveal a second flare in AR12371! It was 02:47 before the ‘new’ flare could be logged – when clearly past its peak (Fig 3).
Twin Peaks. The earlier M2 had peaked at 01:40 and the M2.7 Flare2 peaked at 02:34 – almost an hour apart –unusual in the writer’s experience. Flares mostly discharge their energy source (entangled strong field and helicity, perhaps) in one event – and some time is needed, usually hours, before a repeat flare.
Flare2 was unusual in retaining much of the geometry of earlier event – in fact the first flare never fell below M1 before the rise to M2.7. The second flare involved new ribbons extending from the remnants of Flare1 across to the big (p) spot along paths similar to the earlier event.
Flare2 was rated visual 2F at 02:47 and when the session ended at 02:52 was still at GOES M1.7 level. Both events remained at peak brightness for extended periods – Event2 for ~30mins: most unusual for current flares – and reminiscent of ‘great’ flares of previous solar cycles.
Fig 4 (below) shows the group the next day: changes include the re-shaping of the (p) spot – to become rounder; and the (f) cluster has split along a line ~ the inversion line, though violet field is still mixed with red as shown.
The hectic session of 20/21 reminded me of the importance of Delta groups in the account of flaring in SC24: a cycle remarkable for the weakness of its X-class flares.
AR12371: Delta Groups and Flares of Solar Cycle 24 – the day after. Sketch and copyright Harry Roberts ©, all rights reserved.
Harry also provided the following plot showing the X-ray flux as measured by the GOES spacecraft.
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.