Long-term trends in geomagnetic daily variation

Long-term changes in the magnetic environment of the Earth are of interest to those studying climate change. To this end we examine long-term changes in daily variation as derived from hourly mean values from 14 geomagnetic observatories around the world. Their time series date back to the beginning of the 20th century. We find that there are similar features in all the records, with extrema in the amplitudes of the daily variation occurring in the 1950s, 1960s and 1980s, and a small upward trend of about 7% since the start of the century. The extrema coincide with those seen in proxy solar irradiance data, in particular the F10.7 flux density dataset. The most likely cause of the long-term increase is also related to the Sun but the possibility of processes inside the Earth contributing to this observation cannot be discounted. This work demonstrates the possibility of using long-term geomagnetic data as a proxy for processes in the upper atmosphere.