Why Was Solar Minimum So Hard To Pick?
It is now (June 1997) apparent that the minimum of the solar cycle was reached in May 1996 and that we are rising to the next peak of the cycle expected in the year 2000. Why was confirmation of the minimum so delayed? It took more than a year for us to sure that it had in fact 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 are needed before the sunspot number can be defined. Hence, we did not have the May 1996 smoothed sunspot number until the end of November 1996.
Then it is takes sometime to be sure that the smoothed sunspot number is indeed rising and will not return to a lower value. This was the main problem in picking the current minimum because the smoothed number did in fact rise at first after May, then dropped back a little, before starting a more rapid rise in October and November 1996.
The graph shows how much the smoothed sunspot number rose in the first six months of historical solar cycles plotted against the ultimate amplitude of the cycle (chosen because the larger cycles tend to rise more quickly). The vertical dashed line indicates how much Cycle 23 has risen in its first six months. It is evident that part of the problem for Cycle 23 has been that it has risen very slowly. Of the 23 cycles shown in the graph, Cycle 23 has so far risen more slowly than all but five, in stark contrast to Cycle 22 which was the fastest rising cycle on record (the point to the furthest right on our graph).
Another complicating factor was 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.
The sign post for Cycle 23 did not work well as the "new cycle" regions created only confusion. In May 1995, we saw regions that were of the correct magnetic structure to be of the new cycle but they 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 these regions. Were they "new" or "old" cycle regions?
The first regions on the sun of the correct magnetic structure and latitude for the new cycle appeared in May 1996, the very month of solar minimum. It seems somewhat difficult to believe that the first new cycle regions appeared as the the new cycle began.
So the new cycle has already its first mystery. Without doubt, there will be many more things to learn as the cycle progresses.
Material prepared by Richard Thompson