Harry spies normally invisible events as sunspot AR11726 goes out of view
Four views of rapidly changing post flare loops and other phenomena associated with sunspot group AR11726 at the edge of the Sun. Sketch and copyright Harry Roberts ©, all rights reserved
At the Sun’s limb we see events ‘side on’ and silhouetted against the blackness of space. At the limb we can measure the relative heights of H-alpha features, on the assumption that they are ‘normal’ to our line of sight.
When a large and complex spot group crosses the limb we may see ‘things’ erupt that are very hard to detect when the same group is viewed on the bright disc.
In late April AR11726, a large and magnetically active spot group, passed behind the Sun’s western limb and at the same time hosted a moderate flare; the resulting fireworks were impressive.
The figure shows the group on April 26 (UT, local 27th) when most of the spot group had gone behind the limb – only a large following (f) spot remained. In white light it was conspicuous near the limb, when Helio freeware, © Peter Meadows, and transit timings gave the spot’s latitude and longitude: +13,320, in good agreement with earlier data. As well, “Helio” gave the coordinates for the adjacent limb: +14,328, showing that the (f) spot was still some 8 degrees of longitude from the limb and that the preceding (p) spots at long. 331 were no longer visible.
The first H-alpha session showed little activity at the limb near AR11726, some small surges only, and after logging other H-alpha features on the disc, the ‘white’ or integrated light session began, to record spot groups in detail and their positional data. While this was happening the GOES flux logged a C5.7 flare at 22:25 that went unnoticed!
View 1: Resuming H-alpha at 23:03UT showed a small bright post flare loop, PFL, at the AR11726 site (23:07). Other types of H-alpha transient were also active, some flare-related. The post flare loops were 42Mm high, less than those of a great flare (~100Mm), but eye-catching nonetheless.
The loops are field transition arches (FTA) that emerge above sunspots and are usually invisible – but after a flare, coronal material ‘condenses’ along the arches and drains down to the solar disc – making the loops briefly visible. These small arches showed two brighter ‘droplets’ involved.
Two tiny flare loops (FL) close to the remaining sunspot are most likely the flare itself, seen side-on – and are ~10Mm high. Also several bright surges of various shapes rose above the limb. In the south one surge seemed to be involved with the large PFL (view 1, below).
View 2: Seven minutes later at 23:14 the large loop was fainter with bright knots and some gaps. The flare loops had evolved, but were still small and bright. Two large surges now dominated the view, a degree or so south of the (f) sunspot – one showed how surges can bend through a right angle.
View 3: the PFL’s, now twinned, were slowly fading – and the flare loops have gone – the flare probably over. The surges however are still bright and active.
View 4: The big surges had retracted, but a pair of small ones is erupting close to the penumbra of the (p) sunspot – the usual site of surges. The two ‘counter’ curves crossing the original post flare loops are surge ejecta – temporarily ‘frozen’ and soon to recoil back into their ejection site.
These dynamic transients are all typical of a complex spot group – and we may ask why they are not so often seen when the group is viewed while on the Sun’s disc? One reason is contrast – the transients, when viewed against the bright chromosphere, are faint or invisible. Another reason is Doppler shift: surges eject at ~50-100km/s and have a Doppler shift of 1-2 angstroms, making them invisible in most H-alpha filters.
When a big spot group rounds either limb it is the time to search for rare H-alpha transients – particularly if the group is hosting regular flares. And you need a little bit of luck too!
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.