Harry watches as a sunspot group (almost) returns: AR 11029

November 19, 2009

ar11029 prom

Prominences on the Sun on 14 November 2009 (Australian time zones), drawn by Harry Roberts

Much of the fun in Sun watching is to monitor events unfolding at a given site over time. And we need to know where we are on the Sun for this to work. Helio freeware and a cross hair eye-piece is my method – there are others.

Sunspots are attached to the solar core by an umbilical cord of braided magnetic “ropes” that can often be seen (as faculae and plage) even before spots emerge. The strength of the field within these ropes is a good guide to a sunspot’s lifespan and how big it will grow.

Strong magnetic fields are not often found on Earth, outside the CERN device or a specialist lab, but they are common on the Sun. And fields grew quite strong in spot group AR 11029 in late October. From a scattered cluster of small spots it grew into an impressive penumbral group, closing with the Sun’s NW limb on the 30th. Many remarked on its large preceding (p) spot that at the time had striking penumbra and bright faculae. Fields inside this spot reached 2400G as logged by the Mt Wilson 150’ telescope; the strongest for any C24 spot group thus far.

As it passed behind the west limb we speculated whether it would survive to make a second transit of the Sun’s visible disc; well, it almost did! Spaceweather.com kept up a commentary as AR11029 transited the Sun’s far side – imaged by solar “seismology”, and speculated on a “grand reappearance” at the east limb around November 14.

“Helio” software gives solar coordinates for any point on the Sun, and on the 14th the longitude of the east limb was again 220 degrees, the longitude of the p spot of AR11029. On that day I searched the limb with no sign of the big spot from 30 October – no spots were visible at all, but prominences were active at the site suggesting the magnetic “ropes” were still strong there (Fig 1) as active region filaments.

ar11029 faculae

A region of bright faculae on the Sun, drawn by Harry Roberts

The 15th (the next day) showed an amazing sight: a vast region of bright faculae stretched across the site that had been AR11029 14 days earlier (Fig 2). “Helio” put the leading edge of this vast bright area at +16/224 (16ºN, longitude 224) and the trailing end at +22/216. Remembering that AR11029’s p spot was sited at +16/220 and f spots at +16/214, we see that this region of faculae lay right at the spot group’s earlier site.

And interestingly Mt Wilson’s 150’ ‘scope saw a single tiny spot on the 14th at 16:15 UT located at +14/217, centred within the faculae. It was not visible in Sydney only 6 hours later! No “grand return” for the most active spot group of C24 occurred; yet the vast spread of faculae was so impressive in WL that it almost made up for AR11029’s non-appearance.

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

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