Models based on the Sun's polar magnetic fields performed best in simulating the solar cycle and predicting solar behavior. Solar activity level rises and falls every 11 years. The most recent maximum in solar activity level, the 24th since recording began in 1755, was the weakest in almost 100 years and peaked in early 2014.

Solar activity refers to dark regions on the surface of the Sun called sunspots, where the Sun's magnetic field has become tangled. They can produce sudden explosions of energy in the form of intense radiation and energetic particles.

Sometimes, these regions will even kick out a portion of the tangled magnetic field and send it hurdling into space in an event called a coronal mass ejection. When these bursts of radiation, particles, and magnetic field reach Earth, they cause geomagnetic storms, which can interfere with communication satellites and power grids on the ground. These events could also be harmful to astronauts traveling to Mars, exposing them to possibly deadly amounts of radiation.

For each solar cycle scientists publish their forecasts, using their own preferred methods, anticipating how large the peak of solar activity will be and when it will occur. Now that the peak of solar cycle 24 has passed, a new study by Pesnell compares all of the predictions to determine which forecasts were the most accurate.

The two most popular ways to model the solar cycle were (1) to use the historical record of the number of sunspots and (2) to couple the sunspot number to another measurement that varies with the solar cycle. Scientists who used the sunspot number alone argued that past trends can predict statistically what the next solar cycle will look like.

This method is particularly appealing because the historical record of sunspots goes back farther than any other space weather measurement. But it turns out that models using this number alone rarely made better predictions than just the average of previous solar maxima.

Scientists who coupled the sunspot number to another measurement predominantly chose either geomagnetic activity level or the strength of the magnetic fields at the Sun's poles.

For past solar cycles, models using geomagnetic activity level made more accurate predictions, but this accuracy may be due to the fact that solar polar magnetic field data were scarce or unreliable. This time, for solar cycle 24, the models using polar magnetic fields made the best predictions by far.

For solar cycle 25, slated to begin as early as 2020, the author predicts that scientists will need more information on the Sun's magnetic field to increase the accuracy of their models and points out that these predictions will be essential as our society's reliance on technology grows and we strive to become an interplanetary species. (Space Weather, doi:10.1002/2015SW001304, 2016)