Solar Cycle Precursors and the Outlook for Cycle 25

Abstract 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.