Astronomical Society of Coonabarabran

Sunspots: The many rotations of Active Region 918

by Harry Roberts

"How many rotations of the Sun can one sunspot make?"

Putting this another way we might ask: how long can a solar active region (i.e. a sunspot) endure on the Sun, before the magnetic "flux rope" that sustains it fades away, and the spot disappears?

Sun-watchers are asking themselves this question as they watch a very long-lived sunspot go round, and around for over five months! Also, what does this long-lived magnetic structure tell us about the fields deep down at the solar tachycline where the flux-ropes are generated, and about solar activity cycles in general? (There are no easy questions where the Sun is concerned!) Here, fellow Sun-watchers, are the facts.

AR 918 (aka 10,918) was briefly seen on the Sun's bright disc on 2006 Oct. 23 at 20:40 UT (Fig 1). It arose just west of the large group AR917, and as the gap between 917 and 918 seemed to be less than 5º of longitude, many perhaps deemed it to be just more flux emerging as part of 917. But magnetograms clearly showed that 918 had its own bi-polar structure, and was not part of group 917. AR 918 was located at heliocentric latitude -4º, longitude (ln) 359º. Its Hale class was Beta, and it soon went behind the west limb.

Two weeks later on the Sun's east limb H-alpha post flare loops heralded the approach of a new, large, active region (AR). The writer first saw the new AR on 2006 Nov. 8 at 01:50 UT; a large, simple spot, now numbered AR923, located at -5º ln4º. It reached a maximum size of ~640 units on Nov. 14, the Hale class was Beta. AR 923 became an easy (suitably filtered) naked eye object. By Nov. 21 it was right on the west limb, embedded in bright faculae (Fig 2).

Go forward to Dec. 5th and there was general amazement at a GOES X9 flare on the east limb at 10:30 UT (local night time). X9 is a great flare, and one of the biggest for solar cycle 23! Group 923 had returned, renamed AR 930, and it unleashed five X-class flares, becoming one of the most active sunspots for C23. Its position was -6º ln8º (Fig 3); Hale class Beta-Gamma-Delta (the most active class). Its area grew again to 680 units. (Why does it grow, then shrink, to grow again?)

On 2007 Jan. 3, 00 UT a now smaller spot AR 935 (930 returned again) is seen at the east limb. It is at -7ºln9º, Hale class Beta. It is still embedded in bright faculae, but the great flares are over. AR 935 went behind the west limb around the 14th (not seen by the writer.)

Yet there it was again on the 28th at 20:35 UT! Renumbered AR 941, and located at -7ºln.7º, it was now quiet at Hale class Alpha.

This was the fifth rotation of group AR 918, a very long life for a sunspot group. The active region survived to commence its sixth rotation on February 27, located at latitude -6º, longitude 7º, but it was now reduced to a single small spot (AR 945) that finally vanished near the centre of the solar disc around March 2nd.

Presumably AR 918 was one of the longest-lived spot groups of Cycle 23. And I ask if this is typical for large groups late in the solar cycle? Also it appears that its companion group AR 940/944, (located 36º west of 941) has survived for at least three rotations. This longevity may arise from the stability of solar magnetic fields, yet much power has to be expended to support a spot group over long periods.
Keep watch on the Sun; strange things are happening!

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