As Cycle 24 has not yet achieved a Solar Maximum, it may seem a little early to begin forecasting Cycle 25. But several forecasts have been made. A recent posting in WattsUpWithThat notes such forecasts by Penn and Livingston and by David Hathaway.
You remember from previous postings on this site, that Penn and Livingston have been measuring Sunspot magnetic field strength and the temperature and luminosity of the umbra. They began this study in 1990 and as of 2010 they have analyzed some 17,000 spots. Plotted on the chart below are data from their paper LONG-TERM EVOLUTION OF SUNSPOT MAGNETIC FIELDS through 2010 and additional readings since:
Focusing on the bottom chart, sunspots are plotted against magnetic field strength and time. The individual dots are representative of sunspots. The larger blue dot represents the normalized sunspot number for each year. The black line is the trend line for the umbral magnetic field of the sunspots. The horizontal blue line indexes a magnetic field strength of ca. 1500 Gauss. Note that the sunspots extend vertically above the trend line, and below the trend line but not below the 1500 Gauss line. The two scientists speculate that sunspots do not form when the magnetic field strength is less than 1500 Gauss. If the trend line continues on this same slope, somewhere around the year 2025+/- at least half of the sunspots will disappear.
Using a linear decrease of 65 Gauss per year and a cycle duration of 11 years, they computed the magnetic probability distribution function for Cycles 24 and 25. Using this, a sunspot number is forecast. Cycles 24 and 25 are shown along with actual data from Cycle 23 in the chart below from their paper:
The contrast of Cycle 24 and specifically Cycle 25 from the completed Cycle 23 is quite dramatic. The Cycle 24 forecast, so far, seems to be reasonably in tune with actual data. At a Cycle 25 sunspot number of 7, David Archibald says it would be the lowest sunspot number for a Cycle in 300 years!!!!
Livingston and Penn say that if the linear decrease were 50 Gauss per year rather than 65, the Cycle 25 sunspot number would be 20 which is still a very low number.
Livingston and Penn caution that it is always risky to extrapolate linear trends.
Next posting on this topic will be an examination of David Hathaway’s 2006 forecast of both Cycle 24 and Cycle 25. It will also discuss one of the underlying theories for the decrease in sunspots.