Abstract
THE recurrence of certain terrestrial magnetic storms at intervals of 27 days (the sun's rotation period) without any disturbance being visible on the sun's disk has long been an outstanding puzzle of solar physics. Observations of the chromosphere in monochromatic light have established a close connexion between the chromospheric eruptions responsible for short-wave radio fade-outs and certain of the great sporadic magnetic storms, which tend to occur about a day after the visible flare; but the recurrent storms seem as unconnected with chromospheric activity as with photospheric. A recent observation by Waldmeier (Z. Astrophys., 21, 275 ; 1942) throws light on this subject. By coronagraph observations at Arosa he has found that certain regions of the solar corona (“C-regions”) exhibit temporary bursts of activity characterized by abnormally high intensity of the coronal line at 5303 A. The appearance of these areas at the sun's edge is often followed, or preceded, according as the region is being carried in front of or behind the disk by the solar rotation, by magnetic activity on the earth, the time interval being about 7 days (roughly a quarter period). The C-regions are very long-lived compared with chromospheric eruptions (at least 24 days as against about an hour). They are confined to the spot zones and seem to exhibit the 11-year periodicity in their frequency of occurrence. If further observations should confirm the connexion between magnetic storms and the central meridian passage of these active zones in the corona, it would seem that both C-regions and chromospheric eruptions must emit corpuscles, roughly radially and at speeds up to about 1,000 miles per second, throughout their life-times of the order of a month and an. hour respectively.
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The Solar Corona and Geomagnetism. Nature 152, 44 (1943). https://doi.org/10.1038/152044c0
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DOI: https://doi.org/10.1038/152044c0