A Great Prominence Ejects, Sketch and copyright Harry Roberts ©, all rights reserved.
Regular solar observer & correspondent Harry Roberts reports on the ejection of a large prominence from the Sun’s limb.
Sun-watchers have lately been noting the meager prominences seen above the solar limb, hoping for a rerun of some SC23 ‘greats’ perhaps – and current filaments too have been mostly faint. It’s known that global solar fields are currently (mid-SC24) well below SC23 levels.
Thus, it was a happy surprise to see a huge prominence on the Sun’s NE limb on 2015 March 27 (March 26, 23:13UT). “Helio” timings were ‘run’ across its main footpoints (FP), as well as heights, and some of its complex detail was logged; but much fine structure was not recorded. “Helio” © Peter Meadows, soon showed the prominence stretched from lat. 13°N to 34°N, some 21 degrees. It was 87Mm at the highest point, above (FP) 2, the latter sited at +23,287.
This height, well above Zirin’s “50Mm limit” for large ‘quiescents’, implied the prominence was already ejecting. Yet close study and repeat timings showed no increase in height over the next hour – apart from some rearrangement of material mainly between FPs 2 and 4.
Such prominences are, in reality, large disc filaments seen above the limb, and are termed quiet region filaments (QRF): that is, they arise within ‘plumes’ or ‘streaks’ of decayed field trailing behind (i.e. following) areas of active spot formation. The streaks develop over several solar rotations and fields within them are only <100G in strength; i.e. they are long-lived but low power features. Perhaps longevity is the reason QRF grow so large. As the streaks form, they drift BOTH east and polewards; i.e. they drift NE in our example (Fig1).
This means that in Fig1 the right-hand side (i.e.N) parts of the prominence are likely well behind the solar limb (at Ln289) – perhaps 20° behind the limb (~Ln270°); indeed the fine detail of the prominence and its decreasing height towards the N suggests this. The highest point on the prominence was above FP2, presumably right on the limb, and declined northward. There was no major change in the structure when the session ended at 00:0UT.
Ejection? The structure was again studied (Fig2) at 05:46 on Mar 27UT (5h50m after Fig1), when changes were seen: but bigger ones had been expected. It was brighter and the tallest part of the feature now lay between FPs 3 and 4, where it was 98Mm high, and the ‘arch’ between them was wider and higher: no doubt the ejection was slowly underway. Yet the setting Sun’s altitude was (at 06:35UT) just 15°: with wind and poor seeing the session ended.
Fig3 is an enhanced ©GONG Halpha image made at the Udiapur station. The robotic ‘scopes do not track ejecta, and have a field of view (FOV) just 100Mm above the solar limb. At 11:50UT (12h15m after Fig1) we see the ejection is well advanced, with the main arch lifted mostly above the FOV (broken arrow), though some around FPs 1 and 4 is still attached.
Questions. Filaments are known to regenerate quickly, as the ejection removes the accumulated material but not the ‘filament channel’ nor the progenitor ‘streaks’ of old sunspot polarity. Yet, since the event, there has been no sign of filament regeneration at the site; perhaps in time?
What of its earlier life? My logs show a large but faint filament nearing the west limb on March 10 UT, a possible precursor, but cloud prevented viewing around the 13th when a big prominence may have decorated the NW limb. The site should return to the west limb around April 5: what will we then see?
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.