This article discusses the start of solar cycle 23 which is now coming to an end. We are currently
nearing the start of solar cycle 24. The general comments on sunspot latitudes from one cycle to the
next still apply. The implication of the last paragraph is that solar minimum is still 13-19 months away,
possibly during the first half of 2009. IPS has currently set solar minimum at around October 2008, so we may
need to slip out our cycle forecast by up to 6 months. You can see our cycle forecast
here.
This month we feature a more detailed version of the "Solar Butterfly Diagram" (see also August 1993 edition of
IPS Solar Geophysical Summary). The diagram is of interest now because it provides interesting
insights about the onset of a new solar cycle - Cycle 23.
The diagram below shows the solar latitude of sunspots observed since 1985 - a little earlier than the start of the present solar cycle
in September 1986. In both northern and southern hemispheres, the location of sunspots shows a steady drift from higher
latitudes early in the solar cycle towards lower, more equatorial, latitudes at the
end of the cycle. The combination of northern and southern hemispheres produces the look of the wings of a butterfly and hence the
name of the diagram.
At the left during 1985-86, there is a cluster of points at low latitudes belonging to the end of the butterfly wings for
the previous cycle, Cycle 21. During this period both cycles co-existed, with the higher latitude regions "belonging" to Cycle 22 and the equatorial
ones to the new Cycle 22.
Latitude is a good indicator of whether the spot belongs to the "old" cycle or to the "new" cycle. But it is not the only indicator because
detailed analysis of magnetic fields within the regions can confirm to which cycle they belong.
Over the last 9 years the latitude of sunspots has steadily decreased and we now see sunspots within about 15 degrees of the solar equator. But
we must soon reach the same situation as in 1985-86 with high latitude spots appearing as a first indicator of a surely
imminent Cycle 23.
However, as can be seen from the diagram, there have been no sunspots at higher latitudes
as yet. Solar scientists and forecasters are keenly awaiting their appearance
as a sign of the new cycle!
The arrival (or non-arrival) of new cycle sunspots can tell us about when the solar cycle will end - called
solar minimum. For past solar cycles, the first sunspots of Cycle 20 were seen 13 months prior to solar minimum; for Cycle
21 it was 19 months; and for Cycle 22 the first spots occurred 18 months before minimum. So, when finally we see the
first Cycle 23 region then we will know that minimum is 13-19 months away.
[ Our thanks to Peter Taylor (American Sunspot Number Program) for the
diagram based on sunspot locations from Space Environment Services
Center in Boulder, USA. ]
Educational