Abstract. Predicting the maximum intensity of geomagnetic activity for an
upcoming solar cycle is important in space weather service and for
planning future space missions. This study analyzed the
highest and lowest 3-hourly aa index (aaH∕aaL)
in a 3 d interval, smoothed by 363 d to analyze their variation
with the 11-year solar cycle. It is found that the maximum of
aaH (aaHmax) is well correlated with the
preceding minimum of either aaH (aaHmin,
r=0.85) or aaL (aaLmin, r=0.89) for
the solar cycle. Based on these relationships, the intensity of
aaHmax for solar cycle 25 is estimated to be
aaHmax(25)=83.7±6.9 (nT), about 29 % stronger than
that of solar cycle 24. This value is equivalent to the ap index
of apmax(25)=47.4±4.4 (nT) if employing the high
correlation between ap and aa (r=0.93). The maximum of
aaL (aaLmax) is also well correlated with
the preceding aaHmin (r=0.80). The maximum amplitude
of the sunspot cycle (Rm) is much better correlated
with high geomagnetic activity (aaHmax, r=0.79)
than with low geomagnetic activity (aaLmax, r=0.37). The rise
time from aaHmin to aaHmax is weakly
anti-correlated to the following aaHmax (r=-0.42).
Similar correlations are also found for the 13-month smoothed
monthly mean aa index. These results are expected to be useful
in understanding the geomagnetic activity intensity of solar cycle 25.
Study of Cosmic-Ray Modulation During the Recent Unusual Minimum and Mini-Maximum of Solar Cycle 24