Observations

AR 953 : “Strange days, indeed!”

Sunspot 953 on 25 April 2007 drawn by Harry Roberts

Sunspot 953 on 25 April 2007, drawn by Harry Roberts

Moon and Sun watcher extraordinaire and member of the Sydney City Skywatchers, Harry Roberts, sends the following article on a recent and surprising sunspot:

Sun watchers continue to be amazed by our star’s unpredictable antics. Many had predicted that solar activity had bottomed and we were now in solar minimum. But a recent compilation of the predictions for Solar Cycle 24, by forty or so workers in the field, revealed two interesting things. The first was that the group split evenly between a higher maximum than C23 or a lower maximum than C23. The second issue, that they mostly agreed upon, was that the expected minimum of the current cycle (C23) would not occur until 2008!

This sun watcher had been expecting minimum any time now, and was surprised by these predictions. As if to prove them right the Sun produced a large active region (AR) with the fairly high southern latitude of 10º.

As a solar cycle progresses new sunspots appear steadily closer to the solar equator. But over recent years Dr David Hathaway has warned that new southern spot groups seem to be “stuck” at latitudes around 10º, and had almost stopped moving equator-wards.

Hathaway’s point is proved by the steady emergence of southern spots at that latitude, of which AR 953 was to be another example. On 2007 April 25th, after almost three weeks without any spots, a large group appeared at the east limb, AR 953. It was big, at over four hundred units, and yes, as with so many groups before, it was located ten degrees south of the equator. As the sketch shows it consisted of one large penumbral spot with two outlying smaller spots, all immersed in very bright faculae that usually means high activity. GOES X-ray Flux data showed rapid fluctuations of low amplitude (~class A5).

Sunspot 953 on 28 April 2007_Harry Roberts
Sunspot 953 on 28 April 2007, drawn by Harry Roberts

My next view of AR 953 on 2007 April 28 21:30 UT showed a big spot with a many-branched umbra within a round-ish penumbra, that was much divided on the eastern side with several small spots and a scatter of detached spots. In H-alpha an arch of bright plage with a dark active region filament (ARF) curving SE from the main spot could be seen. Several associated ARF’s lay east and west of the group. The Hale class was Beta Gamma (Delta), which implied flare potential. Small surges (ejecta) were active along the path of the dark ARF curving SE, and a fan shaped surge emerged north of the main spot at 04:28.

Sunspot 953 on 31 April & 4 May 2007_Harry Roberts
Sunspot 953 on 31 April & 4 May 2007, drawn by Harry Roberts

Viewed on April 31st (in white light) AR 953 showed minor evolution from the 28th, with some evidence of counter-clockwise rotation. By 2007 May 4 03:00 UT it had changed shape dramatically, and in H-alpha the SE dark filament had grown to a great length. Over two hours repeated surges erupted along the line of the ARF, mostly visible only in H-alpha blue wing (red arrows in fig). The events looked like the cracking of a whip. The “Spaceweather” website (for May 4) has an H-alpha “movie” of this “whip-crack” in action: amazing to see!

AR 953 produced a few large flares, the first being a long-lived C9 at 23:00 UT on May 2. Thereafter a C4 on May 5 and I think another low C-class or two occurred before the group passed behind the west limb. Work or cloud meant I recorded none of these, but Monty Leventhal made a good series of images of the C9 event. AR 953 had been a great spot group, proving that rumour of Cycle 23’s demise were exaggerated, and the prediction of minimum a year from now may well be right.

2 responses to “AR 953 : “Strange days, indeed!”

  • Adolfo, your question is most thought provoking. A recent paper on sunspot field strengths over the last 100 years “Livingston et al “Sunspots with the stongest fields” in Journal of Astrophysics I think showed sunspot fields were at their strongest in 1992, and flaring seems to have reached an historic peak in 2003(Only yesterday!)

    Strange things are happening on the Sun.

    Harry.

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