Solar Cycle Precursors and the Outlook for Cycle 25.

Sunspot Cycle 25 over 3 years past the cycle minimum of December 2019. At this point, curve‐fitting becomes reliable and consistently indicates a maximum sunspot number of 135 ± 10—slightly larger than Cycle 24's maximum of 116.4, but well below the Cycles 1–24 average of 179 (ranging from 81 for Cycle 6 to 285 for Cycle 19). A geomagnetic precursor, the minimum in the aa‐index, and the Sun's magnetic precursors, the polar field strength and axial dipole moment at the time of minimum, are often used to predict the amplitude of the cycle at (or before) the onset of the cycle. We examine Cycle 25 predictions produced by these precursors. The geomagnetic precursor indicated a Cycle 25 slightly stronger than Cycle 24, with a maximum of 132 ± 8. The Sun's magnetic precursors indicated that Cycle 25 would be similar to Cycle 24, with a maximum sunspot number of 120 ± 10 or 114 ± 15. Combining the curve‐fitting results with the precursor predictions, we conclude that Cycle 25 will have a maximum smoothed sunspot number of 134 ± 8 with maximum occurring late in the fall of 2024. Models for predicting the Sun's magnetic field ahead of minimum, were generally successful at predicting the polar precursors years in advance. The fact that Sun's magnetic precursors at cycle minimum were successfully predicted years before minimum and that the precursors are consistent with the size of Cycle 25 suggests that we can now more reliably predict the solar cycle. Plain Language Summary: Now that over 3 years have passed since the start of Cycle 25, we can determine the size of the cycle and look back at previous predictions. For the last eleven months we have consistently found that Cycle 25 is following the behavior of a smaller than average sunspot cycle, just slightly larger than the last cycle. The strength of the Sun's magnetic field at the start of a sunspot cycle has become recognized as the best predictor for the ultimate strength of that cycle. This follows from solar magnetic dynamo models in which the magnetic field at minimum gets stretched and strengthened to produce the magnetic sunspots and explosive magnetic activity of cycle maximum. Three different measurements of the strength of the Sun's magnetic field in late 2019 and early 2020 (at the start of the current sunspot cycle) indicated that this cycle would be slightly stronger than the previous cycle, but still weaker than average. Models can be used to estimate two of these measurements well before cycle minimum, thus providing a reliable prediction years before the start of a sunspot cycle. Key Points: Solar Cycle 25 has revealed itself as a small cycle with an expected sunspot number maximum of about 134Three magnetic precursors from the cycle minimum in 2019 predicted a similar Cycle 25 sunspot number maximumTwo of these magnetic precursors were accurately predicted years before cycle minimum using surface flux transport [ABSTRACT FROM AUTHOR]