Long-term change in geomagnetic activity

Geomagnetic-activity indices are among the longest directly-measured time series recording solar-terrestrial interaction. Derived from magnetometer data produced at mid-latitude ground-based observatories, the K index records the maximum variational range of magnetic-field disturbance over 3-hr durations of time at each individual observatory. The global-activity aa index is derived from K values from a pair of observatories, one in the northern hemisphere (Britain) and one in the southern hemisphere (Australia). Since the aa time series extends from 1868 to the present, it is often used for studies of long-term trends. In this study we examine the source K values used for constructing aa, and we compare them with K values from Germany (1890-present). While we identify numerous problems and biases in aa, we also find that it does provide a qualitative measure of long-term change in global-scale geomagnetic activity, which, in addition to showing an 11-year solar-cycle modulation, has generally been increasing over the past 140 years. More definitive conclusions can be drawn from consistent patterns in the (independently measured) K indices from different observatories. The well-known tendency for magnetic storms to occur during the declining phase of a solar cycle was not clearly seen until about 1902, with the commencement of solar cycle 14; or, at least, it was not obviously present in solar cycles 11-13. And, since about 1954, with the commencement of solar cycle 19, magnetic quiescence has diminished, something that is not well correlated with sunspot number. These observations have implications for solar physics, geophysics, and possibly for space-weather hazard mitigation. They also highlight the value of both continuous long-term monitoring and preserving historical data in their original and unaltered numerical form. For the future, we advocate long-term, space-based monitoring in coordination with ground-based monitoring.