Giant Sunspot and Geomagnetic Storm

Abstract

A VERY large group of sunspots of 'bipolar' type crossed the spot disk during January 16-29. At central merdian passage on January 22·7 U.T., the middle of the group passed 28° above the centre of the disk. Two big spots were the chief components, will their centres separated 12° in longitude, or approximately 85,000 miles. The aggregate area of the spots for several days was about 2,300 millionths of the sun's hemisphere, or about 55 times the cross-section area of the earth. Greenwich data show that sunspots of this order of size are very likely to be associated with geomagnetic storms beginning within the time-limits of two days before and four days after central meridian passage of the spot group. A severe geomagnetic storm did, in fact, occur, commencing abruptly on January 24 at 18h. 28m. U.T. The disturbance continued with even increased vigour to the night of January 25-26. Displays of the aurora were widely seen on both nights in Great Britain. Provisional overall ranges recorded at Abinger of the earth's magnetic elements (kindly communicated by the Astronomer Royal) are as follow: D, 1° 29' ; H, 660 γ ; V, 490 γ. Long-distance radio-communication was seriously affected during the period of the geomagnetic storm.