EarthRef.org Reference Database (ERR) -- Cliver & Ling 2002

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. 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.