  Long-term geomagnetic activity presented by the aa index has been used to
show that the heliospheric magnetic field has more than doubled during the last
100 years. However, serious concern has been raised on the long-term
consistency of the aa index and on the centennial rise of the solar magnetic
field. Here we reanalyze geomagnetic activity during the last 100 years by
calculating the recently suggested IHV (Inter-Hour Variability) index as a
measure of local geomagnetic activity for seven stations. We find that local
geomagnetic activity at all stations follows the same qualitative long-term
pattern: an increase from early 1900s to 1960, a dramatic dropout in 1960s and
a (mostly weaker) increase thereafter. Moreover, at all stations, the activity
at the end of the 20th century has a higher average level than at the beginning
of the century. This agrees with the result based on the aa index that global
geomagnetic activity, and thereby, the open solar magnetic field has indeed
increased during the last 100 years. However, quantitatively, the estimated
centennial increase varies greatly from one station to another. We find that
the relative increase is higher at the high-latitude stations and lower at the
low and mid-latitude stations. These differences may indicate that the fraction
of solar wind disturbances leading to only moderate geomagnetic activity has
increased during the studied time interval. We also show that the IHV index
needs to be corrected for the long-term change of the daily curve, and
calculate the corrected IHV values. Most dramatically, we find the centennial
increase in global geomagnetic activity was considerably smaller, only about
one half of that depicted by the aa index.
