Butterflies and the New Solar Cycle
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. See also our current solar cycle forecast.
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.
Material prepared by Richard Thompson. Our thanks to Peter Taylor (American Sunspot Number Program) for the diagram based on sunspot locations from Space Environment Services Center in Boulder, USA.