Long-term rise in geomagnetic activity - A close connection between quiet days and storms

Geomagnetic quiet days and magnetic storms are naturally believed to be due to very different solar wind conditions. In this study we however demonstrate that the long‐term variation of geomagnetic quiet and disturbed days are surprisingly similar. By the use of daily averages of the geomagnetic index aa we examine the activity level of the most quiet and most disturbed days separately through 130 years from 1868 to 1998. We find that the linear correlation between the average level of activity at the 5 most quiet days pr year and the yearly average is 0.8. For the 5 most disturbed days it is 0.7. The results indicate that the long‐term increase is due to an increase in the background solar wind parameters, rather than in the number of solar wind disturbances.