Why Is Solar Minimum So Hard To Pick?
Why is confirmation of solar minima sometimes so delayed? For example, it took more than a year to ensure the minimum in May 1996 had taken place.
Part of the answer is that the minimum is defined from the 12 month smoothed sunspot number. This means that six extra months of sunspot data are needed before the smoothed sunspot number can be calculated. Hence, the May 1996 smoothed sunspot number could not be calculated until the end of November 1996.
Then it takes some time to be sure that the smoothed sunspot number is indeed rising and will not return to a lower value. This is the main problem when there is an extended minimum as in 2008 or the smoothed sunspot number shows no definite trend.
The graph shows how much the smoothed sunspot number rose from solar minimum in the first six months of historical solar cycles plotted against the ultimate amplitude (largest smoothed sunspot number) of the cycle. There is a weak trend for larger cycles to rise more quickly. Cycle 22 was the fastest rising cycle on record (the point furthest to the right on the graph).
Another complicating factor can be the appearance of "new cycle" regions. These are regions that have characteristics of the new cycle rather than the old cycle - they have a different magnetic structure and they appear at higher latitudes on the Sun. New cycle regions can therefore act as a sign post to the start of the new cycle but can begin to occur over a year before minimum.
Sometimes regions with new cycle magnetic structure occur at lower latitudes than expected. This was the case at the end of Cycle 22. In May 1995, regions that were of the correct magnetic structure to be of the new cycle appeared at intermediate latitudes on the solar surface - too low for the regions to definitely belong to the new cycle and yet higher than the old cycle regions then appearing on the Sun. There was, and still is, debate about such regions. Are they "new" or "old" cycle regions? The first regions on the Sun of the correct magnetic structure and latitude for Cycle 23 appeared in May 1996, the very month of solar minimum. It seems somewhat difficult to believe that the first new cycle regions appeared as Cycle 23 began.
Picking solar minimum will continue to be difficult with each solar cycle as the Sun's behaviour varies.
Material prepared by Richard Thompson