Proto-sunspot AR 11017, drawn by Harry Roberts
“The Sun is behaving in an unexpected and interesting way” said NASA’s Dean Pesnell in announcing yet another prediction for solar cycle 24 on 2009 May 29, adding “It turns out that none of our models were totally correct”. Whatever the prediction for the new cycle, sun-watching has never been this exciting; let me suggest why.
In old cycle 23 (C23) the sun “chose” to put much of its energy into solar flares – producing the strongest flares for thirty years, maybe for 100 years. Then, perhaps not surprisingly, it plunged into the deepest minimum for 100 years (i.e. since 1911).
And now, well into this deep minimum, we see some unusual activity on our star, activity perhaps last seen in 1911. In particular we have begun to see large faculae regions (FR) appear that are almost spotless. During normal solar activity large faculae regions evolve closely with major sunspot groups – with all authors noting one exception, the tiny spotless faculae at the sun’s poles during solar minima (the polar faculae).
NASA dubs the current large faculae regions “proto-sunspots” and speaks of them as “spots trying to emerge”. None of my texts (ranging from Bray to Zirin ) remark “proto-sunspots struggling to emerge” – and while much is known of faculae generally, these FR are perhaps “new” to solar science. Maybe they were common during that last deep minimum in 1911 – and have been forgotten in the modern literature?
They would be hard to interpret were it not for Livingston and Penn’s “Sunspots May Vanish” paper, first deemed “controversial” but now looking plausible. Is it possible that the FR are in fact normal C24 sunspots but their umbrae are too warm to be visible against the bright photosphere –as predicted by the above authors to occur in 2014 – but well ahead of schedule?
Many attributes of a normal sunspot region (AR) are present in these regions e.g. conspicuous bright faculae, with plage and filaments in H-alpha Flares erupt in them, but without strong spot fields the flares do not exceed GOES B-class, and the faculae seem to spread across wide areas. Yet only a few tiny spots or none at all emerge in the faculae regions (Fig1).
Several C24 “proto-sunspot” regions have occurred, I’ve counted six , the biggest and most recent being AR 11017. First seen May 11 as a very large patch of bright faculae 25º behind FR +22/218 (second rotation AR 11015), the new spotless FR was centred at +19/192 but covered 13º in both latitude and longitude – a faculae region about 5000 area units in size! Next day a single small spot arose at +18/194, with filaments and plage, and on the 13th a remarkable sight occurred. A grey smudge was in good seeing shown to be sixteen tiny spots in two tiny clusters about 5º apart, embedded in bright plage and tangled filaments. The clusters were seen next day, now 6º apart with 8 to 10 tiny spots – but hard to see.
The magnetic polarity of proto-sunspot AR 11017, drawn by Harry Roberts
Mt Wilson digital magnetogram for May 13 shows the extent of the faculae (with fields of ~10G) and with concentrated stronger field coincident with the two spot clusters of about 150G. In fact the magnetogram gave the strong impression that the group was a large bipolar sunspot pair with umbrae about 6º apart (Fig2). Mt Wilson’s Babcock magnetograph showed umbral fields in the two clusters of 1700G – just 200G above invisibility at 1500G.
I suspect this record (of 13th) resolved individual magnetic flux “ropes” each about 1000km diameter separating at the sun’s surface (photosphere) –only some having fields strong enough to appear dark. More normall umbral fields (~2700G say) would have caused large dark p and f umbrae to form instead the tiny spots. This was a memorable observation, and I suspect we are watching sunspot slowly umbrae fading from view as the “Spots May Vanish” authors suggest –events that are unfolding ahead of schedule, for unknown reasons.
By email I suggested to Livingston that these regions (FR) are like Alice’s Cheshire Cat, with only the “smile” (the faculae) remaining once the “cat” (sunspot umbrae) has faded. He liked the analogy and advised he had a week coming on the McMath-Pierce solar telescope shortly. May he have some sunspots to measure.
Magnetic field strength for solar cycle 24 sunspots, drawn by Harry Roberts
Mt Wilson has reported umbral fields for 14 of the 17 C24 groups that have so far emerged. Plotting these fields (Fig 3) shows very weak levels in the first 4 groups (2008), rising to a modest peak of ~2300G in AR11008 2008 November. From there (2009) fields have declined with each new C24 spot group – in line with the Livingston-Penn prediction – and of the six C24 faculae regions seen so far three have been spotless. Currently umbral fields lie just above invisibility (at ~1500G). Note the authors’ published graphs show a steep zig-zag in measured field – with a steady decline only when averaged, so perhaps we are in a short-lived “trough” and values may partly recover to fall again. – but current events seem to confirm, amazingly, that sunspots ARE in fact vanishing.
Harry Roberts (Sun and Moon observer and member of the Sydney City Skywatchers)
July 2009 night sky map
