Harry observes the weak sunspots of May 2010 and wonders if the Sun is “Hotting Up” or not

Four views of Sunspot Group AR11069 in May 2010. Sketch by Harry Roberts

May 2010 was greeted with shouts of “Callooh! Callay!” as no less than six new spot groups appeared on the sun at the one time. (Morris dancing broke out in solar observatories world-wide!)

But, oh dear me, something was wrong! The baby spots failed to thrive – and in a short time all but one had vanished! What had gone wrong? Readers, here is the melancholy tale.

Last year NASA confessed to the world that its attempts to model solar activity had completely failed, blaming the sun for “behaving unpredictably”. (Perhaps one of many reasons the new Administration has clipped their “wings”). Presumably the Universe (including the sun) always behaves unpredictably – we only ‘know’ about 5% of it – and that rather poorly!

NASA and most other modellers predicted that Cycle 24 (C24) would rapidly rise to a powerful maximum around 2010 after a short minimum – a much stronger maximum than that of C23. What happened in fact was that C24 was delayed by 18 months, and only in 2009 did a shaky start occur.

What went wrong with the nine new groups that arose in May? (There were nine groups on the disc in May, AR11063 to 11071 inclusive. Only 11069 was seen by the writer to have obvious spots. AR11067 and 11071 were seen briefly to have a few spots.) Basically it was the fact that their umbral fields failed to rise above 1800G, increasing field strength (i.e. energy) being crucial for the growth of sunspots (just as it is with humans). While the writer knew the small groups were on the sun’s disc, repeated scrutiny of the disc failed to show them, except for AR11069 and some faculae, plage and associated filaments at the various sites.

Last year I asked veteran Mt Wilson observer Tom Cragg how weak a field the 150’ ‘scope’s Babcock Magnetograph could detect, and was surprised to hear that values as low as 1000G were sometimes logged. The texts generally agree that sunspots become invisible at umbral fields below 1800G.

As stated above only one of the six new groups developed a field much above the 1800G threshold, this was AR11069, while the others quickly faded to lower values and vanished. AR11069 had a peak field of 2200G in the preceding (p) spot on May 5 (23:40UT), with 1900G in the following (f) spots. At these values only small penumbrae developed around some spots, since penumbra needs fields over 2200G to grow to any size. Accordingly, this group consisted of a chain of small spots stretching over ~9º of longitude at the high solar latitude of 41ºN where spots are seldom seen – the only example yet for C24. Moderate to strong flares occurred in this group, with a GOES M1 (or C9, authorities differ) on May 5 at 17:20 UT – but the writer recorded none. Also on the 5th the group had opposite polarities within the one (central) penumbra – making it Hale class Beta Gamma Delta, the reason for the flares perhaps. The group was last seen at the west limb on the 7th when only the central complex spot remained, with large filaments stretching northwards from the site, and bright faculae over the whole region.

Of the other small groups one of the more interesting was AR11063 that first arose on April 28 but disappeared on the 30th. You may think it would stay ‘gone’ – but it reappeared four days later on May 4 and remained visible for a few more days (in big ‘scopes, not mine). Presumably its umbral fields fluctuated around 1800G causing it to fade to invisibility, then reappear some days later. What do these weak and ephemeral sunspots indicate?

At least for the present they support “Ol’ Bill” Livingston’s thesis that “Sunspots may vanish”. Livingston you will recall is a respected solar astrophysicist, the one whose “Sunspots May Vanish” paper caused much controversy a few years ago. At least for the present the evidence supports his thesis – and perhaps, just perhaps, sunspots will become invisible by 2014.

Sun watchers, gather ye sunspots while ye may!

Harry Roberts, a regular contributor to this blog and a member of the Sydney City Skywatchers.