The dark filament at the same site where the sunspot group AR11429 had been seen during the previous rotation of the Sun. A small part was ejected on 3 April 2012. Sketches and copyright Harry Roberts ©, all rights reserved

Sunspot group AR11429 first appeared at the sun’s east limb on March 3 (2012) as a compact “Island Delta” group that, much evolved, finally passed behind the western limb on the 15th – after strong flaring. At the time we noted that “Island Delta” groups were the most active sunspots – but were also short-lived ones. Would it return at the east limb in late March?

During its two weeks behind the sun several CME’s erupted at the spot‘s location – suggesting it might reappear. March 30 showed bright faculae at the east limb – but no spots. March 31 however, showed more faculae at the site (now 30º onto the disc) and two dark filaments stretching N-S through it. Finally, with high magnification, two tiny spots were detected (Fig 1, LHS). AR 11429 had survived a full rotation of the sun – but only just! NOAA promptly dubbed the returnee AR 11451.

Filaments: While the tiny spots were gone by April 3, the filaments had grown into the most striking feature on the disc (Fig1, RHS), compared to the other small groups present (not shown).

For amateurs, filaments are only visible in H-alpha (and in satellite EUV bands). They are magnetic “channels” between surface fields of opposite polarity where cooler material collects; they come in two kinds: active region filaments and quiet region filaments. This filament was a mixture of both!

Helio freeware sited the darkest part of the big filament between +20º, 302 and +11º, 311 with a “tail” northwards. These points were very close (~5º) to the inversion line that bisected spot group AR11429 a month before: the stage was set for spotless flares (Zirin, H. “Astrophysics of the Sun” PP 221 and 333).

The Mt Wilson daily magnetogram on 4 April 2012. Copyright Regents of the University of California ©

AR11429 had been (briefly) an “Island Delta” class with mixed and reversed polarities in one penumbra, but when it evolved into a larger simpler group it remained fully reversed. Mt Wilson’s daily magnetogram (Fig 2) now showed that regions of reversed polarity persisted at the old sunspot site, little changed from the previous rotation (Fig2. Black line is the filament site). And the low power fields still showed concentrations of stronger field where the main spots had been during the previous transit.

Although now spotless, the old AR11429 site did have more flares – but not big ones, mostly along the filament channel that a month earlier had separated the spot group’s opposite polarities.

The magnetogram also showed that the whole active area now stretched across 45º of longitude with three attached active regions; new “normal polarity” groups AR 11450 and AR 11452 (both small), with reversed AR11451 (old 11429) in between. Some unusual coronal links between new and old spots were seen in SDO EUV images of the region.

Filament Ejection: A small ejection at the south end of the big filament erupted on April 3rd. While stable from 21:48 to 22:56, at 23:02 a small globule ejected that was only seen ‘off central band’ H-alpha (Fig1, detail). This was tracked as it moved westwards– always ‘off band’, and invisible ‘on band’. This was likely part of the large filament that, while showing small lateral motion (6º in total, and dividing at b, 23:16UT), was ejecting along the line-of-sight at high velocity. The filter’s tuning range is stated to be ~10Å; assuming a -5Å shift the ejection velocity was 250km/s in approach. Meanwhile, the big filament remained stable. Since it seems to be the product of active and quiet region fields the potential is for a big ejection of this filament – spectacularly at the limb perhaps. Here’s hoping!

AR11429 (aka 11451) was by now only a ghostly manifestation of its younger self – with no spots, just the wraith-like filament and its recurrent flares – yet there in the magnetogram were signs of the once majestic spot group: the largest reversed group yet of SC24.

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers