There is still some debate concerning the validity of the long-term trend apparent in the aa geomagnetic index. This debate links into discussions on trends in solar activity and potential effects on Earth's climate.

To test the robustness of the trend in aa we have reconstructed the index using two independent long-running European stations to provide data for the northern component of the index i.e., Sodankyla and Neimegk. The two reconstructions were scaled to the official aa-north values at the start of their data series by comparing the first full solar cycle of data. Both of the fully reconstructed aa series, based on Sodankyla and Neimegk data in combination with the southern hemisphere data, confirm the increasing trend in the index. We have also reconstructed the aa index just using data from the long running UK station, Eskdalemuir, following a technique known as inter-hourly variation (IHV) proposed recently by Svalgaard and Cliver. This series is designed to be primarily sensitive to solar wind conditions. The reconstructed aa_IHV index also shows an increasing trend with time, and high consistency with the official aa index. These results provide confirmation of the long-term trend in the aa index, and allow additional comparison to be made with other long-running data series such as the radiocarbon index in order to explore future trend possibilities.