The Most Active Solar Regions in the Last 20 Years

Activity on the sun during June, 1991 was very impressive with a number of particularly energetic solar flares being observed. These major flares arose from one sunspot region - a large one which appeared on the eastern edge of the sun on June 01. This region, numbered 6659 by Space Environment Services Center (SESC) in Boulder Colorado, rotated across the apparent disk of the sun leaving the western edge on June 17. During its time on the visible disk of the sun, Region 6659 produced some outstanding solar flares (see list on page 2), including 5 flares which were large enough (more energetic than X12) to saturate the X-ray detectors on US GOES satellites. The region also produced an X10 flare, not far from this limit. Whilst previous solar regions have produced flares which have saturated these detectors, no other region has produced more than one such flare. In producing 5 such flares, Region 6659 can, with justification, claim to be the most important solar region in the last twenty years.

How can we compare this impressive solar region with other regions observed over the past twenty years? One method of comparison is to calculate the total output of X-ray flares by the region. This index, known as the X-ray flare index (XRI), adds together all the M and X class flares produced by the region with M1 flares counting as 0.1; M2 flares as 0.2; X1 flares as 1.0 and so on.

Patrick McIntosh, a solar physicist with the NOAA Space Environment Laboratory in Boulder Colorado, has compared the solar regions observed since August, 1972. The following table is based on his data and includes his estimate for the XRI of Region 6659.

This comparison indicates that Region 6659 in June 1991 easily heads the list of solar regions, surpassing even the March 1989 region. The present solar cycle (Number 22) is well represented with 6 out of the 15 flares in the list, including the top two flares!

Material prepared by Richard Thompson