There was a recent mention in the K7RA Solar Update (and the Contest Update) of a NASA web page from a couple of years ago showing how different latitudes on the sun rotate at slightly different speeds. It was an interesting thought exercise but most of the earth-directed solar badness seems to come around on a fairly reliable schedule every 28 days. Equally as important, if not more so, is that exceptionally quiet solar conditions recur on the same schedule.
After finally working 100 DXCC entities on 80m last winter, this season I turned my attention to 160m. So far, this has been a great season for "top band" propagation into the polar region. Since the end of September I've been working a steady trickle of new ones on 160m CW and FT8. One date, however, stood out. Conditions were amazing on October 22nd! I worked a total of FOUR new ones on that day and I immediately marked my calendar for November 18, exactly 28 days later.
The rest of October and early November were pretty good for 160m from the Arctic. I typically spend an hour in the morning before work and an hour or two (or three...) at the radio in the evenings. Sometimes a new one, sometimes just North American stations, sometimes no propagation at all. Then, just like clockwork, that quiet spot on the sun finished rotating around and faced the earth again. The solar wind and other numbers didn't really seem to be much different from the days before but, sure enough, band conditions were stellar. I worked FOUR more new ones on November 18th!
The dates in this table are the UTC dates which roll over in the evening here.