Secular change in geomagnetic indices and the solar open magnetic flux during the first half of the twentieth century

Abstract

We examined several long-term geomagnetic indices (u, Ci, and Hm) to substantiate the secular increase in the aa index during the first half of the twentieth century. The long-term increase in aa and other geomagnetic indices was accompanied by a corresponding rise in the envelope of the sunspot number (∼130% increase of cycle averages). We used a correlation between solar cycle averages of sunspot number and solar open magnetic flux for recent cycles to infer a 140 ± 80% increase in the open flux between ∼1900 and ∼1950, comparable to the ∼130% increase in this parameter during the twentieth century deduced by Lockwood et al. [1999] from solar wind measurements during the space age. While the uncertainty in our result is large, our method, which is not based on aa, provides independent support for a substantial increase in the open magnetic flux during the last century. Copyright 2002 by the American Geophysical Union.