On long-term variations of simple geomagnetic indices and slow changes in magnetospheric currents: The emergence of anthropogenic global warming after 1990?

We introduce very simple indices based on hourly mean values of geomagnetic components measured in observatories which we show can characterise many aspects of magnetic activity. When data from observatories with very long records (close to a century) are averaged over, say 11 yr (the solar cycle), these indices display remarkably similar time variations, allowing us to define an “overall magnetic trend” with smooth, decade long, variations separated by rather sharp extrema at 1956, 1968 and 1990. All 4 indices we propose (one for each component and a more complete one for the total vector variation) display essentially the same “overall magnetic trend” for all components and all observatories. The curves are however not strictly affine, and ratios of similar indices from two observatories vary, again with the same “overall magnetic trend”, with amplitude variations up to 20%. Variations in our magnetic indices also correlate remarkably well with variations in classical, computationally more complex indices such as the magnetic index aa, with the sunspot (Wolf) number W, and solar irradiance S. This implies that both electromagnetic radiations and corpuscular flux in the vicinity of the Earth (whose original source lies in the Sun) are subject to the same variations; the current systems responsible respectively for the solar daily (regular) variation and irregular activity react roughly proportionally to the solar wind, due to corresponding changes in geometry of the magnetosphere. The “overall magnetic trend” bears some puzzling resemblances to long-term internal geomagnetic secular variations, with its discrete jerks. Finally, our indices and solar irradiance co-vary with long-term averages in global temperature of the lower atmosphere until approximately 1990, when the temperature curve sharply diverges upward. This lends support to the proposal that an anthropogenic component to the change in climate may not have been the dominant effect until the last decade of the XXth century.