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Keywords:

  • long-term trends;
  • solar activity;
  • geomagnetic index

[1] The robustness of the aa geomagnetic index is of critical importance to the debate about the previously reported doubling of the solar coronal magnetic field in the last 100 years, inferred from an increasing trend in this index. To test the trend in aa, we have reconstructed the aa index using two long-running European stations (Sodankylä from 1914 and Niemegk from 1890) to provide data for the northern component of the index that are independent of data from the UK observatories used in the “official” aa index. Both the fully “reconstructed” aa series, based on Sodankylä (67°N, L = 5.2 RE) and Niemegk (52°N, L = 2.3 RE) data in combination with the official aa Southern Hemisphere data, confirm the increasing trend in the index. The Niemegk-based index shows little solar cycle variation in its deviation from the official index, probably because of the midlatitude location of the station. The high-latitude station, Sodankylä, is more affected by active geomagnetic conditions during solar maximum because of the proximity of the auroral oval to the station. Nevertheless, its index also clearly confirms the increasing trend in the aa index and hence supports the idea of a long-term increase in solar coronal magnetic field strength. As an added test, we reconstructed the aa index from a single site using data from two long-running UK stations, Eskdalemuir and Lerwick, applying a technique known as interhourly variation (IHV) proposed by Svalgaard et al. (2004). The resulting series is designed to be primarily sensitive to solar wind conditions. Both the reconstructed aaIHV also showed an increasing trend with time and high consistency with the official aa index. Overall, we conclude that the robustness of the trend in the aa index supports the idea of a long-term increase in solar coronal magnetic field strength.