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What will be the monthly average of sunspots around Solar Cycle 25’s minimum?



The 11-year Solar Cycle has been intensely monitored and recorded since the 18th century. The ebb and flow of solar activity constitutes an interesting astrophysical problem and it is also for increasing importance here on Earth. In particular, the timing and the scaling of the Solar Cycle’s peaks and troughs is of great importance to operators of both Earth-orbiting satellites and power transmission grids.

A key measure of solar activity is the number of sunspots, whose presence is correlated with the occurrence of solar flares. Wolf’s number (also called the International Sunspot Number or the Relative Sunspot Number) is expressed by the relation R = k(10g +s), where s is the number of individual spots, g is the number of groups of sunspots, and k is a factor that varies with location and instrumentation (also called observatory factor, or personal reduction coefficient). By convention, Wolf’s number is used as a count of the daily number of sunspots.

In addition to the extensive historical record of sunspots, there now exists a trove of data from modern solar observations, obtained both from the ground and by a flotilla of spacecraft. Moreover, astrophysical models of the Sun and its processes have reached a high degree of refinement. By combining all of this information, and by adopting a variety of forecasting methods, predictions of the intensity of upcoming solar cycles can be generated.

Accurate knowledge of the relative strength of the next cycle would have profound implications for satellite operations, radio communication, and power transmission. For example, if Cycle 25 generates a high sunspot number at solar maximum, it will be associated with increased upper-atmospheric drag. On the one hand, atmospheric drag can be beneficial, as drag removes dangerous space junk from orbit. With a strong maximum, however, satellite operators face increased risk of solar flares destroying satellite electronics and damaging solar panels. High levels of solar activity enhance long-range radio communication, but they are associated with coronal mass ejections (which generate geomagnetic storms) that can wreak havoc with terrestrial power grids.

At present (late Summer 2020) Solar Cycle 24 has reached its end, and solar activity is close to a minimum. Forecasts for the upcoming Solar Cycle 25, however, differ wildly. We thus ask:

What will be the monthly average of sunspots around Solar Cycle 25’s minimum?

For context, the minimum of Cycle 24 occurred in December of 2008 with a total of 2.2 sunspots as a monthly average, and the maximum occurred in April of 2014 with a monthly average of 116.4. Data for this can be found here.

Resolution Criteria

Resolution will be carried out with the Sunspot Index and Long-term Solar Observations (SILSO) site. SILSO records the number of sunspots at minimum and maximum points in the solar cycle and will report on Cycle 25’s minimum. The monthly mean numbers are also recorded as part of the 13 month moving sunspot number series.

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