Long-term trends in geomagnetic daily variations

Long-term changes in the magnetic environment of the Earth are of interest to those studying space weather and climate change. To this end we examine changes in daily variation as derived from hourly mean values from 15 geomagnetic observatories around the world with records extending back to 1900. For the period after 1947 we, not surprisingly, find correlations with the F10.7 flux density, a solar irradiance proxy. We here examine a more robust technique of removing the solar cycle signal than simply applying 11-year running means. This involves a procedure developed by Lockwood and Fröhlich (2007) to account for the variations in the solar cycle length. We find a long-term increase in the amplitude of the daily variation since 1900, confirming earlier results using fewer data from fewer observatories and 11-year running means (Macmillan and Droujinina, 2007). This work demonstrates the possibility of using long-term geomagnetic data as a proxy for processes in the upper atmosphere.