Observations

Harry reports on the Delta Group that didn’t flare as strongly as expected

Regular solar observer & correspondent Harry Roberts reports on a Delta group of sunspots, AR12443, that didn’t flare strongly.

AR12443: Delta group ‘that didn’t’

Of the four Hale magnetic classes of sunspots, Delta is known to be the most active for solar flaring.  Weather permitting, Delta’s are watched closely for such events: most put on an impressive show; but there are exceptions.

AR12443 was first seen at the Sun’s east limb on Oct29, a pair of large round penumbral spots, separated by just three degrees of solar longitude, at 7ºN latitude. It was a new group, although the writer first thought it was a return of AR12414 (a southern group at the same longitude). The two big spots were shown by Mt Wilson to both have violet polarity – and had likely arisen earlier as a single large spot that split apart. Next day showed a scatter of tiny spots of opposite ‘sign’ following (f) the large pair some 8º to the east. In H-alpha several active filaments linked the (p) and (f) spots and the GOES X-ray flux began to log minor flares: all suggesting that a highly active group approached.

On Oct30, in white light, an amazing sight was seen. Overnight a large amount of new flux had emerged following (i.e. easterward of) the two big spots: a dense ‘chain’ of new spots now stretched across some 15º of solar longitude – reminiscent of AR 11429 in 2012. “Helio” freeware now gave 12443 an area about 600 units. While recording it in detail a small but bright C5 flare erupted (Fig1, in red).

2015Nov11ar12433
The Delta group AR12443 that did’t flare as strongly as expected. Sketch and copyright Harry Roberts ©, all rights reserved.

Delta class. Mt Wilson’s log for Oct31 showed a mix of polarities in the following chain of spots (Fig1, underlined): the Delta configuration.

Delta groups are fairly rare: with ~40 out of 1400 groups thus far in SC24 – or about 3%. Yet all the strong flares of SC24 have occurred in Delta groups.

Inversion line. Both Mt Wilson and SDO HMI logs showed an irregular separation or ‘inversion line’ between the densely ‘packed’ preceding (p) and (f) polarities (Fig1, in blue). The C5 flare likely arose along that ‘line’ and spread out from there.

Flaring. To erupt, strong flares seem to need at least three things: the Delta mix of fields in one penumbra, as well as strong umbral fields (>2400G say) – and a mysterious factor termed ‘helicity’, i.e. the degree of twist in the emerging flux. Despite its promising ‘looks’, AR12443 apparently lacked some of these: its strongest flare seems to have been an M3.7 on Nov5 (at 00:43UT) – a paltry effort!

Solar Cycle 24. AR12443, a northern hemisphere group, at 7º north latitude –was fairly close to the solar equator. Meanwhile, current southern spots tend to be further from the equator, suggesting that the northern hemisphere cycle of spot generation (by subsurface magnetic ‘currents’) is now much more advanced than is its southern counterpart. In fact, the two ‘currents’ are now known to be some years ‘out of synch’.  Statistics shows that we are now, in late 2015, well past the overall SC24 sunspot activity peak, which was reached in 2014 (Fig2).

2015Nov11hemssn_aug2015
Hemispheric sunspot number for solar cycles 23 and 24. Bold curves are smoothed data, fine curves are monthly numbers. Total (grey), northern (blue) and southern (red) numbers are plotted.

But remember, in SC23, flare activity peaked some years after the peak of sunspot count and SC24 may do likewise. So keep the solar ‘scopes at the ready: anything can happen!

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.

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