Solar minimum

The activity of the Sun increases and decreases in a cycle that lasts approximately 11 years. When the cycle reaches a maximum there are a larger number of sunspots and an increase in solar radiation and charged particles reaching our planet. This can cause the atmosphere of the Earth to expand slightly. A satellite in low Earth orbit experiences a small amount of atmospheric drag despite the low density of air. The exact amount depends on how active the Sun is. Each cycle varies somewhat in duration. A cycle can be as short as 9 years or as long as 14. The average is about 10.7 years. These variations complicate the process of making predictions of future activity which are important for estimating the orbital decay of a satellite.

Long term solar activity
Long term observations of sunspot number showing two dozen cycles of solar activity. The 11 year cycle is fairly consistent but there is large variability in peak height.

The magnitude and shape of the peaks in activity is also variable. The maximums in the early 1800s were very small, a time period known as the Dalton minimum. The next figure is a closer look at the Sun’s activity during the space age. The maximum in the late 1950s when Sputnik launched was the largest ever observed. During the Apollo missions the maximum was lower than other recent peaks but greater than the most recent maximum.

Recent solar activity
Solar activity during the space age. Also shown is the 2015 prediction which was based on observations made during the first few years after the last minimum.

Predictions of future activity by astronomers are fairly reliable once the solar activity begins to increase at the start of a solar cycle. Accurate predictions can typically be made two or three years after minimum. The figure above shows how well these predictions matched the observed activity. This prediction was made in 2015. The last maximum occurred in 2014. It was not only the lowest maximum during the space age but also the weakest seen in a century. We are currently approaching a minimum which is expected in the next year or so. Predicting the characteristics of the next solar cycle before a minimum is challenging.

Recent solar activity
A comparison of the 2015 prediction and the most recent forecast with observed activity. Note that the current forecast includes a regression toward the mean of the 2015 prediction.

The 2015 prediction of recent activity is remarkably close to what has been observed. The actual measurements are at the low end of the predicted range. But they are within the confidence interval. The current forecast cautiously predicts a reversion to the mean of the 2015 prediction but activity could continue to be below that level. This method observing the Sun has been used since the 17th century and contemporary measurements are calibrated to be compatible.

Another method of measuring activity is a radio telescope tuned to 2.8 GHz. This is called the F10.7 cm radio flux. It has been consistently observed since 1947 and it is a good measure of overall activity. This along with measurements of planetary geomagnetic activity is used to predict atmosphere drag on satellites near the Earth. The most recent measurements of sunspot number, F10.7 radio flux, and the Ap geomagnetic index are available from the NOAA Space Weather Prediction Center.

Solar radio flux
Observations of the 10.7 cm radio emissions through early October with the most recent forecast of predicted activity.

Solar activity has been very low since 2017. This increases the lifetime of satellite in low Earth orbit. Notice in the figure above that the minimum near 1996 was shorter than the one in 2009. The exact date of the lowest point in the upcoming minimum can not yet be accurately determined. We also don’t know precisely how long or deep the minimum will be. But the activity will remain low for the next year or so regardless of how broad the current minimum turns out to be. Here is a closer look at recent activity and what to expect in the near future.

Solar radio flux predictions
Forecast of predicted solar radio flux through 2019 and an estimate used for satellite orbit decay calculations beyond the forecast.

There were some intense solar flares and coronal mass ejections in Sept. 2017 that caused a spike in radio flux. Other than that the measurements are at or somewhat below the mean prediction. Activity continues to decline at a slow rate. The dotted lines show the prediction and range expected through Dec. 2019. Accurate forecasts are not available after that date.

The dashed lines show values used by STK software to predict the lifetime of a satellite orbit. The STK values through 2019 are near the high end of the Space Weather Center prediction andĀ  just below the 95% confidence interval. This choice give a worst-case scenario that assumes somewhat higher than average predicted activity. This is a reasonable method but the lifetime estimate could be shorter than the actual outcome.

Long range estimate of activity
The long range estimate used by STK software for orbital decay calculations. The next two solar cycles are a smoothed and scaled repetition of the last cycle.

The STKĀ  values after 2020 are simply a repeat of the last solar cycle and should not be interpreted as a scientific prediction. Given the lack of a detailed long range forecast this is about the only option available. Overall the Sun will likely follow these trends trough 2020. But the average amount of activity could be lower with an occasional burst of increased activity. Projections beyond 2020 are highly speculative. The timing could be off by a few years and the magnitude of the peaks will likely have even larger errors.

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