The causes of long-term change in the aa index

The aa index provides the longest geomagnetic data set that can be used in the analysis of magnetospheric and ionospheric phenomena. All phases of the solar cycle show increases in storm activity since the end of cycle 14 in 1915. The activity increase does not appear to be strongly associated with any instrumental, ionospheric or magnetospheric effects. Small effects have been confirmed in the long-term change in ionospheric Pedersen and Hall conductivities due to the changing dipole moment of the Earth but not due to increasing greenhouse gases. Three instrumental effects have been identified where significant changes in quiet time conditions can be seen, that is, 1938, 1980, and 1997. These do not account for the majority of the increase in aa. Noise levels for the aa index are now close to those seen at the beginning of the data set. The prime cause of the increase in storm activity is an increase in solar activity. The average aa in cycle 23 should be about 1 nT less than that predicted from previous cycles due to the reduction in baseline noise levels at the start of the cycle (1997).

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