Dark spots on the Sun in H-alpha.
Criteria for observing are:
--There must be no sunspot present (proved by professional observations);
--The dark spot must have been seen by professionals [in Ha] to exclude the possibility of its being dust on the CCD or optics;
--Ideally the time around sunspot minimum would be best though not necessarily so.
To meet the criteria listed, it is clear that such conditions will be very rare but just a few observations would prove the reality of the feature. Such a spot was seen on 2009 January 20. Professional findings confirm that there was no sunspot on this date, indicating that the spot was likely to be a filament seen in line of sight. An observation made on 2009 May 21 at 08:55 UT shows a normal filament with a small spot close by.
Correspondence with Alan Heath revealed that he too had seen such a feature, at a time when there were some sunspots but not where the dark spot was seen. The dates given by Heath are 2005 Aug 27, 07:40 UT; 2005 Sept 24, 07:35 UT and 2005 Oct 15, 10:45 UT. He also saw a group of three close spots on 2007 May 03, 12:15 UT.
A filament in line of sight is the most likely explanation and it is hoped that others will look for such a feature in the future, providing it is confirmed that an ordinary sunspot was not located in that position.
My thanks are extended to Alan Heath who assisted in the preparation of this letter.
7 Tower Way, Woolton Park, Liverpool, L25 6EB
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|Publication:||Journal of the British Astronomical Association|
|Article Type:||Letter to the editor|
|Date:||Dec 1, 2010|
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