OBSERVATIONAL ASPECTS OF THE SUNSPOT-GEOMAGNETIC STORM RELATIONSHIPS

Abstract

Summary With the use of international magnetic character figures, the statistical rise of geomagnetic activity is investigated for the solar disk passage of four area groupings of sunspots, 1914-1944. This is the reverse process of earlier Greenwich work in which the comparison was made from geomagnetic storms to sunspots. For sunspots exceeding 1500 millionths of the Sun's hemisphere there is a very definite increase of geomagnetic activity centred at about two days after the date of central meridian passages of the sunspots. But this relationship rapidly degenerates, and from the area criterion alone ceases to be significant below the area group 1000-750 millionths. Other sunspot groupings based on solar flare incidence greatly improve the correlation. and for the minor naked-eye sunspots, around 500 millionths, a peak of geomagnetic activity now appears. Twenty-seven-day recurrence tendencies of the geomagnetic activity associated with the various sunspot groupings are examined, and the pseudocharacter of what is found is contrasted with the strong and more precise twenty-seven-day recurrence tendency of the smaller storms, especially those without SC onset and not apparently dependent upon sunspot or solar flare incidence. The statistical non-recurrence of great magnetic storms is confirmed from 1907-1944 data.