Forecasters are already starting to make predictions for what might be in store as our sun winds-down its current sunspot cycle (Number 24) in a few years. Are we in for a very intense cycle of solar activity, or the beginning of a century-long absence of sunspots and a rise in colder climates?
Figure showing the sunspot counts for the past few cycles. (Credit:www.solen.info)
Ever since Samuel Schwabe discovered the 11-year ebb and flow of sunspots on the sun in 1843, predicting when the next sunspot cycle will appear, and how strong it will be, has been a cottage industry among scientists and non-scientists alike. For solar physicists, the sunspot cycle is a major indicator of how the sun’s magnetic field is generated, and the evolution of various patterns of plasma circulation near the solar surface and interior. Getting these forecasts bang-on would be proof that we indeed have a ‘deep’ understanding of how the sun works that is a major step beyond just knowing it is a massive sphere of plasma heated by thermonuclear fusion in its core.
So how are we doing?
For over a century, scientists have scrutinized the shapes of dozens of individual sunspot cycles to glean features that could be used for predicting the circumstances of the next one. Basically, we know that 11-years is an average and some cycles are as short as 9 years or as long as 14. The number of sunspots during the peak year, called sunspot maximum, can vary from as few as 50 to as many as 260. The speed with which sunspot numbers rise to a maximum can be as long as 80 months for weaker sunspot cycles, and as short as 40 months for the stronger cycles. All of these features, and many other statistical rules-of-thumb, lead to predictive schemes of one kind or another, but they generally fail to produce accurate and detailed forecasts of the ‘next’ sunspot cycle.
Prior to the current sunspot cycle (Number 24), which spans the years 2008-2019, NASA astronomer Dean Pesnell collected 105 forecasts for Cycle 24 . For something as simple as how many sunspots would be present during the peak year, the predictions varied from as few as 40 to as many as 175 with an average of 106 +/-31. The actual number at the 2014 peak was 116. Most of the predictions were based on little more than extrapolating statistical patterns in older data. What we really want are forecasts that are based upon the actual physics of sunspot formation, not statistics. The most promising physics-based models we have today actually follow magnetic processes on the surface of the sun and below and are called Flux Transport Dynamo models.
Solar polar magnetic field trends (Credit: Wilcox Solar Observatory)
The sun’s magnetic field is much more fluid than the magnetic field of a toy bar magnet. Thanks to the revolutionary work by helioseismologists using the SOHO spacecraft and the ground-based GONG program, we can now see below the turbulent surface of the sun. There are vast rivers of plasma wider than a dozen Earths, which wrap around the sun from east to west. There is also a flow pattern that runs north and south from the equator to each pole. This meridional current is caused by giant convection cells below the solar surface and acts like a conveyor belt for the surface magnetic fields in each hemisphere. The sun’s north and south magnetic fields can be thought of as waves of magnetism that flow at about 60 feet/second from the equator at sunspot maximum to the poles at sunspot minimum, and back again to the equator at the base of the convection cell. At sunspot minimum they are equal and opposite in intensity at the poles, but at sunspot maximum they vanish at the poles and combine and cancel at the sun’s equator. The difference in the polar waves during sunspot minimum seems to predict how strong the next sunspot maximum will be about 6 years later as the current returns the field to the equator at the peak of the next cycle. V.V Zharkova at Northumbria University in the UK uses this to predict that Cycle 25 might continue the declining trend of polar field decrease seen in the last three sunspot cycles, and be even weaker than Cycle 24 with far fewer than 100 spots. However, a recent paper by NASA solar physicists David Hathaway and Lisa Upton re-assessed the trends in the polar fields and predict that the average strength of the polar fields near the end of Cycle 24 will be similar to that measured near the end of Cycle 23, indicating that Cycle 25 will be similar in strength to the current cycle.
But some studies such as those by Matthew Penn and William Livingston at the National Solar Observatory seem to suggest that sunspot magnetic field strengths have been declining since about 2000 and are already close to the minimum needed to sustain sunspots on the solar surface. By Cycle 25 or 26, magnetic fields may be too weak to punch through the solar surface and form recognizable sunspots at all, spelling the end of the sunspot cycle phenomenon, and the start of another Maunder Minimum cooling period perhaps lasting until 2100. A quick GOOGLE search will turn up a variety of pages claiming that a new ‘Maunder Minimum’ and mini-Ice Age are just around the corner! An interesting on-the-spot assessment of these disturbing predictions was offered back in 2011 by NASA solar physicist C. Alex Young, concluding from the published evidence that these conclusions were probably ‘Much Ado about Nothing’.
What can we bank on?
The weight of history is a compelling guide, which teaches us that tomorrow will be very much like yesterday. Statistically speaking, the current Cycle 24 is scheduled to draw to a close about 11 years after the previous sunspot minimum in January 2008, which means sometime in 2019. You can eyeball the figure at the top of this blog and see that that is about right. We entered the Cycle 24 sunspot minimum period in 2016 because in February and June, we already had two spot-free days. As the number of spot-free days continues to increase in 2017-2018, we will start seeing the new sunspots of Cycle 25 appear sometime in late-2019. Sunspot maximum is likely to occur in 2024, with most forecasts predicting about half as many sunspots as in Cycle 24.
None of the current forecasts suggest Cycle 25 will be entirely absent. A few forecasts even hold out some hope that a sunspot maximum equal to or greater than Cycle 24 which was near 140 is possible, while others place the peak closer to 60 in 2025.
It seems to be a pretty sure bet that there will be yet-another sunspot cycle to follow the current one. If you are an aurora watcher, 2022-2027 would be the best years to go hunting for them. If you are a satellite operator or astronaut, this next cycle may be even less hostile than Cycle 24 was, or at least no worse!
In any event, solar cycle prediction will be a rising challenge in the next few years as scientists pursue the Holy Grail of creating a reliable theory of why the sun even has such cycles in the first place!
Check back here on Friday, February 17 for my next blog!