Guest post by David Archibald

The heliospheric current sheet has flattened meaning that Solar Cycle 24 is over and we are now in Solar Cycle 25.

Figure 1: Heliospheric current sheet tilt angle 1976 -2020

The solar cycle isn’t over until the heliospheric current sheet has flattened. The data is provided by the Wilcox Solar Observatory at Stanford University. There were no observations from about 19 December to 5 February; so the values in between have been interpolated from the rotations before and after.

Figure 2: Heliospheric current sheet tilt angle aligned on solar cycle minimum

As measured in Carrington rotations, averaging 27.23 days, Solar Cycle 24 was of average length.

Figure 3: F10.7 flux aligned on solar minimum

On the basis of the F10.7 flux, Solar Cycle 24 was 11.1 years long – which is the average solar cycle length.

Figure 4: Ap Index 1932 – 2020

What this figure shows is that there has been a permanent shift down in the Sun’s magnetic flux into the New Cold Period. The breakover was most probably in 2005 when the Ap Index suddenly dropped to solar-minimum type levels just above the Modern Warm Period activity floor. Then there was another sudden break down in 2008 with the absolute low in 2009.

Figure 5: F10.7 Flux and Ap Index 1964 – 2020

More than anything else, it is the variation in the Sun’s magnetic flux that changes the Earth’s climate on decadal time scales. Higher magnetic flux pushes cosmic rays away from the inner planets of the solar system. The Ap Index averaged 13.6 from 1964 to the end of Solar Cycle 24. It averaged 8.1 over Solar Cycle 24. If the 11 year lag in terrestrial response to solar input holds, then the much-anticipated solar-driven cooling should start now.

Figure 6: Hemispheric aunspot area and F10.7 flux 1985 – 2020

As Figure 6 show, the F10.7 flux equates to the sunspot area of the Sun.

Figure 7: Sunspot Area 1874 – 2020

The change in character from the Little Ice Age to the Modern Warm Period is quite evident in the hemispheric sunspot area data. Apart from the much lower amplitude, there was far more asymmetry between the northern and southern solar hemispheres in the Little Ice Age than in the Modern Warm Period. Solar Cycle 24 returned to the sunspot area amplitudes of the late Little Ice Age and has north-south asymmetry similar to that of Solar Cycle 12 from 1878 to 1890.

Figure 8: Oulu Neutron Count 1964 – 2020

Now that the solar minimum is in, the peak in neutron count should be one year from now. The amplitude of the peak is likely to be close to the amplitude of the 23/24 minimum.

Figure 9: F10.7 flux and Oulu neutron count

As with Figure 5, this figure shows the result of the sudden step down of the Sun’s magnetic activity in 2005. The Sun has entered a new regime of activity with the New Cold Period.

Figure 10: Solar polar field strength 1975 – 2020

This figure is from the Wilcox Solar Observatory. The solar polar field strength at solar minimum is the most accurate predictor of the amplitude of the following solar cycle. Figure 10 indicates that the amplitude of Solar Cycle 25 will be close to that of Solar Cycle 24. The New Cold Period continues.

David Archibald is the author of The Anticancer Garden in Australia