Harry looks for the return of sunspot group AR11057 while the Solar Dynamics Observatory returns breathtaking images

A still from a movie showing an explosive prominence at the edge of the Sun on 30 March 2010. One of the group of breathtaking images from NASA’s Solar Dynamics Observatory that have recently been released. Credit: SDO/AIA

The story of any sunspot group is not complete until we’ve asked, “did the spot group return?”

The sun’s rapid rotation (~27d) means groups developing on “our” side often disappear behind the western limb, where further changes occur unseen. It’s always useful to examine the site of an earlier spot group when it returns at the eastern limb for any clues as to what may have happened during those “missing” weeks. And the biggest groups can make several rotations of the solar disc before fading to only faculae and filaments.

Most surface features of the sun are shaped and maintained by magnetic fields with a range of strengths (in gauss, G): faculae and filaments at ~100G, bright H-alpha plages 1000G, and tiny sunspots around 1800G. Penumbra appears around spots at 2200G.

A still from a movie recorded by the Solar Dynamics Observatory showing a close up of the sunspot group AR11057 on 29 March 2010. Credit NASA/Goddard Space Flight Center Scientific Visualization Studio

AR11057 during its time on the sun’s nearside developed a large penumbra as umbral fields reached 2600G in the (f) spot and 2400G in the (p) spot. The long-lived (f) component was last seen at the west limb April 4th as a complex single penumbra with three umbrae and bright faculae, and a large filament.

On April 20 23:00UT (in white light) a large mass of bright faculae was seen spread over 16º of longitude, well onto the disc, in the sun’s NE region (see diagram below). In H-alpha a large faint prominence detached from the disc hung above the faculae – and a tangle of dark filaments lay near the faculae site – these features were logged using Helio software and their coordinates found.

Two days later the region was examined again. The faculae was no longer visible (WL) but dark filaments near the area, seen in H-alpha, had their coordinates calculated. These coordinates as well as those of the earlier faculae and prominence were all close to the earlier spot group AR11057, suggesting the site had returned, but without any spots (Monty Leventhal had a single small spot at the site on 21st UT that was coincidentally also seen by Nick at Melbourne Observatory a few hours later – a post on the visit to Melbourne Observatory is coming).

A sketch of the area where sunspot group AR11057 had been one solar rotation earlier. Sketch by Harry Roberts

The data for the two days was combined in a single image (see diagram above) showing the region at April 22nd but with the facula and prominence data from the 20th added. As Helio calculates heliocentric latitude and longitude, data from different dates can be related to any given point on the sun’s rotating surface.

The result left no doubt that the site of AR11057 had returned – but the strong sunspot fields (and spots) had gone – the earlier (p) and (f) sunspots are shown as + signs at the sites recorded three weeks earlier (with the spots outlines dotted). Interestingly, the dark filaments seen on 22nd, when plotted, closely match the active region filaments (arf) of the (f) spot mapped earlier (the Fig shows the latter as broken lines with block arrows). While the newer filaments would now be termed quiet region filaments (qrf) we can see they were once active region filaments leading north from the (f) spot – an interesting evolution.

The large faint prominence (of the 20th) closely matches these filaments in location and is assumed to show their true form when seen side-on. The features now at the site of AR11057 are all phenomena of weaker surface fields – the strong spot fields having faded– but they leave no doubt that a large spot group was once located at the site. It also means that one of the largest spot groups of cycle 24 (so far) and with the strongest fields, failed to survive for more than one rotation of the sun’s disc, though its bright faculae may persist for several rotations.

Harry Roberts, an avid solar observer, a frequent contributor to this blog and a member of the Sydney City Skywatchers