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Current Sunspots

Nature volume 142, page 610 (01 October 1938) | Download Citation



A FAIRLY large single sunspot of area about 700 millionths of the sun's hemisphere crossed the sun's central meridian on September 25- 6 in latitude 10° S. This Was followed by a larger group of stream type which is crossing the disk between September 21 and October 4 in latitude 12° S., the time of central meridian passage of the group centre being September 27-7. The area of this big group on September 23 was 1,800 millionths of the sun's hemisphere. Spot groups as large as this are more often than not associated with terrestrial magnetic storms, the average time of commencement being about one day after central meridian passage. Two striking examples of the activity of this region of the sun's chromosphere Were witnessed at Greenwich on September 20 and 22 with the Hale spectro-helioscope working in Hx. On September 20 at 14h 38m U.T., a highly eruptive prominence was observed to spring from this region then passing into view at the sun's east limb. The prominence was unusually brilliant, a photometric measure at 14h 41m giving a central intensity of 66 per cent (where continuous spectrum 10 A. from the centre of Hen at the centre of the disk=100). There were big differences of measured radial velocity amounting to 225 km./sec. between adjacent filaments at 14h 54m. The prominence rose from its point of origin to a height of some 41 (175,000 km.) within 15 minutes. On September 22, an extensive absorption marking on the disk, representing the projected image of a prominence, was observed at 8h 50m with a velocity of ascent exceeding 260 km./sec. This marking, with both ascending and descending filaments, extended as seen in projection over some 150,000 km. of the chromosphere and partly encircled the leader sun-spot. Activity persisted for at least 3½ hours.

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