35th Scientific Assembly of COSPAR, Paris, France, 18-25 July 2004

Reconstructing the long-term aa index of solar activity

M.A. Clilverd1, E. Clarke2, Th. Ulich3, J. Linthe4, H. Rishbeth5

1Physical Sciences Division, British Antarctic Survey, Cambridge, U.K.
2Global Seismology and Geomagnetism Group, British Geological Survey, Edinburgh, U.K.
3Sodankylä Geophysical Observatory, Sodankylä, Finland
4Geoforschungszentrum, Potsdam, Germany
5School of Physics and Astronomy, University of Southampton, Southampton, U.K.


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 how variable solar radiative and particle output affects the EarthÕs atmosphere and 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. 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.