We are well into the Solar Cycle 24/25 minimum but 24 may not have ended yet. A solar cycle isn’t over until the heliospheric current sheet has flattened. And that could be as late as March 2021. Solar cycle amplitude does matter with respect to climate and the amplitude of Solar Cycle 25, from projecting trends from the last three cycles, looks like being about 80 in 2027.
Figure 1: F10.7 flux 2014 to 2019
The F10.7 flux has been flat for a couple of years now. What is interesting is that a low of 63.4 was recorded on 21st October, which may be a low for the instrumental record. Since then the F10.7 has been in a narrow range from 68 to 70.
Figure 2: Heliospheric current sheet tilt angle 1976 to 2019
The solar cycle isn’t over until the heliospheric current sheet tilt angle has flattened and as at Carrington rotation 2224, the tilt angle was still a few degrees from having flattened. If the date of flattening is constrained by the slope of the decline from the cycle peak then the latest date for flattening is March 2021.
Figure 3: Ap Index 1980 – 2019
This figure shows the break in the Ap Index in 2006 at the end of the Modern Warm Period to the New Cold Period. It also shows the relationship between month of heliospheric current sheet flattening (red arrows) and the month of minimum as determined by the low in sunspot activity (green arrows) which is far more subjective.
Figure 4: Aligned heliospheric current sheet tilt angle by month of minimum
Solar Cycle 23 was stronger for longer while Solar Cycle 24 is largely tracking 21 and 22, suggesting the end may be soon.
Figure 5: Solar hemispheric sunspot area and F10.7 flux 1985 – 2019
This figure is included to show that solar activity, as measured by the F10.7 flux, is directly proportional to the sum of the sunspot areas of the solar northern and southern hemispheres. Sunspot number is proportional to F10.7 flux but is a less precise measure.
Figure 6: Sunspot area by solar hemisphere 1985 to 2019
In this figure the hemispheric sunspot areas are plotted individually instead of cumulatively. What is readily apparent is that each hemisphere has been driven by its own trend in activity – the peaks of both cycles all line up. Which begs the question of what will happen if those trends in activity continue? In Solar Cycle 24 the peak of northern hemispheric activity occurred three years before that of the southern hemisphere. If that repeats in Solar Cycle 25 with the amplitude of each hemisphere constrained by the blue trend lines, then the northern hemisphere would peak in 2024 with an amplitude of 200 millionths of the solar hemispheric area and the southern hemisphere would peak in 2027 with an amplitude of 600 millionths of the solar hemispheric area. For the whole solar cycle, the peak amplitude would be in 2027 with a smoothed sunspot number of 80, down a third from Solar Cycle 24.
Figure 7: Sunspot area by hemisphere 1874 to 1924
This figure is included to show that trends in hemispheric sunspot activity can hold for nearly four solar cycles as shown by the southern solar hemisphere (red line) from the late 19th century.
Figure 8: Oulu Neutron Count 1964 to 2019
This is the main part of where the rubber meets the road in terms of the effect of solar activity on climate. Weaker solar activity, as is predicted for Solar Cycle 25, means that more galactic cosmic rays make it into the inner planets of the solar system instead of being pushed away by the Sun’s magnetic flux carried on the solar wind. The shower of neutrons in the lower atmosphere increases and provides more nucleation sites for cloud droplets. The increased cloud cover reflects more sunlight and the Earth cools. The peak in neutron flux may be as late as 2022.
Figure 9: North Dakota December 3, 2019
The economic consequences of a cooling Earth are shown in this figure of part of a satellitephoto of a rural area of North Dakota taken on December 3, 2019. White is snow and the brown rectangles are unharvested corn. In NASA’s words:
“a wet fall, combined with corn plants that contained too much moisture, provoked famers to leave the corn in the fields this year. Snow on corn stocks can clog harvesting equipment. But the bigger concern is the moisture content. It is more economically prudent for a farmer to wait and let the corn dry on the stalks—harvesting it in February or early March—than it is to harvest it now and have to dry it in storage facilities.”