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APPENDIX VIII The annual variation of magnetic space weather index (aa) 1844 - 2006 and the sunspots

Disturbances in space weather follow the 11-year cycles of sunspots. Furthermore, they also have shorter variation. It's typical that there are two peaks on each side of the sunspot maximum. The latter maximum during the diminishing period of the cycle is often higher. During the sunspot maximum between the two peaks disturbance can get low onto the level of sunspot minimum as it happened in 1979 – 1980. The black diagram shows the long term, statistic variation of magnetic disturbance. See the diagram below.

Magnetic disturbance has increased during the whole 20th century when the maximum values have gone higher. In fact, the annual aa-index is now at its highest since the 19th century. The average solar activity is now exceptionally high. In the autumn of 2003 the average level of disturbance was higher than ever during the 160 years of recorded observations. The greatest sunspot numbers occurred in the late 1950's when there were more magnetic storms than on average and they were exceptionally strong.

Source: Nevanlinna Heikki. 2006. Avaruussää – Auringosta tuulee. Ursa.