Guest essay by David Archibald

The most accurate method for predicting the amplitude of the next solar cycle is to use the strength of the solar polar magnetic fields at solar minimum. But with solar minimum likely to be three years away, who can wait that long?

The strength of the solar polar magnetic fields at solar minimum is a very accurate indicator of the maximum amplitude of the following solar cycle, as per this graph from Dr David Hathaway of NASA:

Up until late 2014, solar polar magnetic field strength was still quite weak as shown in this graph of the history of that parameter by solar cycle from the last four solar minima:

Over the last year the solar polar magnetic fields of Solar Cycle 24 have strengthened to almost the level of Solar Cycle 23 at the same stage. But for the previous two cycles, they also weakened a bit from this point. All things considered, the amplitude might be around 40 at solar minimum, which may be three years away. That in turn corresponds to a maximum amplitude for Solar Cycle 25 of about 55.

There is another methodology that derives a similar result. The following graph plots up sunspot area for the northern and southern hemispheres of the Sun from 1874:

What is apparent is that the two hemispheres have different drivers. The northern hemisphere had a consistent rate of increase in amplitude from Solar Cycle 12 to Solar Cycle 15. The southern hemisphere had the opposite trend but with a step-like change. During the 8,000 year peak of solar activity in the mid-20th century, the hemispheres had very similar amplitudes. They started diverging again from Solar Cycle 22. The northern hemisphere is weakening faster than the southern hemisphere. If they both maintain their established trends for one more cycle, then we can predict the maximum amplitude for Solar Cycle 25 (assuming we get the year correct). By this method an maximm amplitude of 56 is derived if the hemispheric peaks are aligned. As this method relies upon the disciplined decline evident over Solar Cycles 22, 23 and 24, that is shown in more detail in the following graph:

The original data is available here. If this relationship is true, then the fall in maximum amplitude from Solar Cycle 23 to Solar Cycle 24 should be the same as the fall from Solar Cycle 22 to Solar Cycle 23. The respective falls are 39 and 38. A fall of 39 from the maximum amplitude of Solar Cycle 24 gives us a result of 43.

In 2014, Dr Javaraiah Javaraiah of the Indian Institute for Astrophysics in Bangalore published a paper which has a similar prediction for the maximum amplitude of Solar Cycle 25 of 50 (on page 15).

In summary, a few methodologies are givings us predictions for Solar Cycle 25 which straddle the amplitudes of the Solar Cycles 5 and 6 of the Dalton Minimum which had maximum amplitudes of 49.2 and 48.7 respectively.

Expectations of climate can be adjusted accordingly.

David Archibald’s most recent book is Australia’s Defence.

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