Sketches recording the ejection of a filament from the south-east edge of the Sun on New Year’s Eve (Universal Time). Image and copyright Harry Roberts ©, all rights reserved
Both hydrogen-alpha and white light bands are logged with a portable 8-inch telescope – stopped to100mm for white light (the continuum), and 75 to 55mm for H-alpha; both bands at 2000mm focal length. Various methods are used to control solar heating of the optical tube, the mount and the observer (!) and observing sessions typically last two hours.
Generally, the H-alpha band is logged first, as transients in this band may occur suddenly – and only after the sun has been searched for signs of these is the narrow band gear replaced with the WL filter. Since H-alpha transients can be rapid, the practice means that fewer transients are missed while WL data is being recorded. Events of 2011 Dec 31 show the limitations of the method.
H-alpha began at 20:23UT, when surface details and prominences were logged. Each activity centre was tested for precursors of ejections or flares – such as darkening in filaments (on or off-band) or brightening of plages. Groups that recently flared get extra attention. Brightening in stable prominences, or surges at the limb also hint at impending activity.
On this date there were no such signs and the H-alpha log was completed. Inter alia, a fine set of prominences on the SE limb was mapped; they showed some signs of detaching, but as such events are slow and there were no other hints, when narrow-band ended (21:30UT), the filters were switched and WL viewing began (21:32).
According, while sunspots and faculae were logged, a transient erupted, unexpectedly. Only when WL ended (22:02) and H-alpha restarted (22:04) was the high-speed ejection revealed (Fig, panel 2).
The figure (panel 1) shows the large quiescent prominence, stretching some 30 degrees around the SE limb, yet it was not this prominence that erupted. The bright ejecta seen against the prominence was, perhaps, a filament from behind the limb. Timings showed its ejection velocity was high, ~90km/s, suggesting the event began soon after WL started – and those initial phases must be guessed at!
However, any record is better than none, and timings continued in H-alpha until the ejecta faded to invisibility, around 200Mm above the limb.
The big quiescent (a quiet region filament) remained mostly unaffected by the ejection, except that the footpoints of the northern component continued slowly to detach; even the delicate tendril connecting the parts at 20ºS with those at 35ºS survived the nearby ejection. It was felt that the big quiescent too would soon eject, but this was not observed.
The above velocity depends on the ejecta’s direction of movement, which seemed to be W to E, but if we are seeing an arch rising above the limb, then the heights may need correction by a factor of ~0.7. In any event, the bright points timed were visibly moving, supporting the higher value. The 90km/s suggests an active region event – yet no sunspot group has so far appeared at the site. As is often the case, we must “wait and see” – and the bright ejection made a nice finale to 2011 activity, just one hour short of (astronomical) 2012.
Best wishes for the New Year to all readers!
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers