Harry sees a rare spot on the Sun and it is a large one – AR 11029

Sunspot AR 11029 on the surface of the Sun on the morning of Thursday 29 October 2009 (AEDT), image courtesy SOHO/MDI

Sunspot AR 11029 on the surface of the Sun on the morning of Thursday 29 October 2009 (AEDT), image courtesy SOHO/MDI

The Astronomical Society of NSW (ASNSW) field day at Epping (Oct 24) was successful partly because there was something to see on the Sun. Members of the public could watch an emerging string of spots (AR11029) and when told they stretched across 50,000 km were suitably impressed. As well the clouds had more gaps as the day wore on. Half a dozen ASNSW members showed the Sun with a variety of methods, from image projection to narrow-band filters.


Sunspot AR 1109 (fig 1), drawing Harry Roberts

AR 11029 showed a tight cluster of small dark spots at the following (f) or east end, with a hint of two isolated spots well to the west (p) of the cluster. Timings showed the group covered about six degrees of longitude (Fig 1). In H-alpha some plage and a dark filament was recorded.


Sunspot AR 1109 (fig 2), drawing Harry Roberts

Clouds and heavy rain prevented further views until Oct 28 – when big changes in the group were obvious (compare Figs 1 and 2). During our cloudy days the professional websites showed rapid development in AR11029 and a big rise in GOES flaring, with several C class flares and dozens of lesser events. Fig 3 shows a B3 flare and associated ejection of dark material with detectable Doppler shift – a presumed filament ejection.

The writer keeps watch on the daily Mt Wilson umbral field magnetographs recording field strengths inside the sunspots themselves (hand-drawn). As you know current research suggests the sunspot fields have fallen to historic lows – and further decline is predicted. Figs 1 and 2 show the group in WL and H-alpha combined with the magnetograph closest in time on the right side of the Fig. (Note these are reversed project images).

AR11029 emerged (23rd Oct) with slightly stronger fields (2100G) in the (f) spot cluster and only 2000G in the isolated (p) spots. But things quickly changed. Even during the field day the isolated (p) spots became more distinct as they grew rapidly, although none seemed to have penumbra at the time. The magnetographs show that over the next few days stronger field emerged in the (p) spots while the (f) spots faded and disappeared – the tendency was for the group to shorten and concentrate magnetic flux at the western (p) end.
Fields there reached the strongest level yet recorded for a C24 spot of 2400G on the 27th. It’s easy to overstate the field strength argument. Several penumbral spot groups belonging to C24 have appeared since the cycle commenced in January 2008. The strongest fields recorded prior to the current group were 2300G in AR11024 (Jul 09) and 2300G in AR11008 (Nov 08). AR11019 had 2200G in May 09. So while 2400G is the strongest C24 field yet seen it represents only a small increase (100G) on the previous records – perhaps close to the equipment’s detection limits. Remember spots disappear around 1800G, and note the many at this level shown in Figs 1 and 2.


Sunspot AR 1109 (fig 3), drawing Harry Roberts

Through much of the 20th century sunspot fields were typically 3000G, yet the McMath-Pierce team find average fields currently at only 2200G. Do the 2400G detections suggest a turn-around in solar core fields? Clearly it’s too early to know. The present shortage of large prominences, disc filaments and penumbral sunspots confirms the weak core fields – and a turn-around may be a long way off. Still it was good to see an almost “normal” looking sunspot (Fig 2 and 3) with penumbra, visible filaments and some modest flaring.

Harry Roberts, Sun and Moon observer and member of the Sydney City Skywatchers

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