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.
Educational