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We study the sunspot activity in relation to spotless days (SLDs) during thedescending phase of solar cycle $11$--$24$ to predict the amplitude of sunspotcycle $25$. For this purpose, in addition to SLD, we also use the geomagneticactivity (aa index) during the descending phase of a given cycle. A very strongcorrelation of the SLD (R=$0.68$) and aa index (R=$0.86$) during the descendingphase of a given cycle with the maximum amplitude of next solar cycle has beenestimated.The empirical relationship led us to deduce the amplitude of cycle$25$ to be 99.13$\pm$ 14.97 and 104.23$\pm$ 17.35 using SLD and aa index,respectively as predictors.Both the predictors provide comparable amplitude forsolar cycle $25$ and reveal that the solar cycle $25$ will be weaker than cycle$24$. Further we derive that the maximum of cycle $25$ is likely to occurbetween February and March 2024. While the aa index has been used extensivelyin the past, this work establishes SLDs as another potential candidate forpredicting the characteristics of the next cycle.

Spotless days and geomagnetic index as the predictors of solar cycle 25