Harry dissects the giant sunspot AR11476 to reveal its anatomy
Two views of AR11476 sunspot group at different times on 5 May 2012 (UT). Sketches and copyright Harry Roberts ©, all rights reserved
A big spot group emerging on the sun can host a wide range of amazing phenomena when viewed in hydrogen-alpha. While attention is mostly on any big flares there is a lot more to record that, over a two-hour session say, can be almost overwhelming. To show this let’s review the logs for May 5 and May 9, 2012.
May 5 summary: First views of AR11476 (476 for short) showed remarkable surges erupting from the group (Fig1). These arose as narrow jets near 476’s central spots and bent almost horizontal to travel north for 60Mm. There they turned upwards for a further 40Mm – in a flattened ‘S’ shape 100Mm in length! New surges joined the earlier ones and engaged in an “out there and back again” display of plasma physics. Several records of these were made and two are shown. Some surges were dark (in absorption) against the disc, while a smaller surge at the site was bright against the disc; some were both bright and dark against the disc: a rare display!
Flare M1.3: At 23:02 the surface (i.e. chromosphere) between the large preceding (p) spot (+9, 189) and the intermediate spots was lit-up in a scatter of brilliant points – a flare that, I later found, was a short-lived M1.3 that peaked just sixty seconds earlier as I switched from WL back to H-alpha (!).
White light: This showed the group stretched across 14 degrees of longitude from a large double (p) spot sited at +9, 189 (twenty degrees onto the disc) to a single following (f) spot at +10, 175, just six degrees from the sun’s limb. Smaller spots lay between the two. This was a very big group and contained ten spot umbrae. Helio freeware gave an aggregate area of 500 units – it was another northern ‘supergroup’ arising (cf 11429 in March)
Magnetic class: at this stage 476 was a relatively simple Beta-preceding group, with a well-defined separation of violet spots in front and red in the rear.
White light image of the whole Sun on 10 May 2012 at 0:13 UT. Photo and copyright Nick Lomb, all rights reserved
May 9 summary: From the fifth to the ninth the logs shows a slow increase in magnetic complexity. On the 7th (not shown) a single red polarity spot arose in the large “violet” (p) spot on the south side, promoting the entire group to Hale class Gamma-Delta (i.e. opposite polarities in a single penumbra: the most complex type).
Sunspot group AR11476 on 9 May 2012 (UT). Sketches and copyright Harry Roberts ©, all rights reserved
By the 9th this was well advanced (Fig3) with the huge and complex “violet” (p) spot sprouting “red” spots on its south side. As expected the group produced a burst of GOES Class M flares that peaked with three on the 9th.
The fit between the WL spots and the Mt Wilson magnetograph on 9 May 2012. © Regents of University of California
Fig 4 suggests the fit between the WL spots and Mt Wilson’s magnetograph of the ninth of May. The possible inversion line is marked in black with triangle arrows.
Flaring: No large flares were logged on the 9th during the two-hour session but three small one were. These are coloured in the figure; the brightest, a C1.5 at 22:45, is orange. All were small with only the latter being ‘bright’, and they arose near the “inversion line” cited – the boundary between opposite polarities in the group (outlined, Fig4).
Surges: as on the fifth surges were very active. The largest (Fig3, partly shown upper left) was 60Mm long and showed Doppler ‘blue-shift’ in approach at 21:30, presumably during retraction. On the west side of the (p) spot are several smaller surges (arrows in Fig) that emerged and retracted during the session. Some faint active region filaments were also present, captioned ‘arf’, but such filaments were unusually faint in Group 476. Why?
Summary: These two records of AR11476 are not meant to be a comprehensive history of the group. Developments in the huge preceding spot, as well as the fantastic surges of the fifth, will need to be treated in more detail (Ed willing!)
At 80 magnification the sun’s disc is a little larger than the eye-piece FOV, and the image is full of detail that changes moment by moment. Recording everything is at times impossible – but it sure is fun trying. Keep your h-alpha ‘scopes at the ready!.
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers