THE 155-DAY SOLAR PERIOD IN THE 16TH-CENTURY AND LATER

RECENT interest in short solar periods (<1 yr) has been stimulated by the discovery of a period of ∼150-160 days in very energetic solar flares1 and hard-X-ray flares2. The intensity of the peak seems to be very variable over extended intervals, and it is unclear whether it is a permanent feature of solar activity. Here I use auroral data to show the presence of this peak in data from 1570 to 1573 and in some other time periods, and to show its absence, or at least lack of prominence, at other times in the nineteenth and early twentieth centuries. © 1990 Nature Publishing Group.