The<i>aa</i>indices: A 100-year series characterizing the magnetic activity

Abstract

After the 100-year series of the aa indices is described, two results are set forth. The general trend of the activity variation permits us to forecast a very-low level of intensity (3–4 in Ap units for the yearly values) during the next solar minimums (this finding would be of interest in magnetospheric studies), and the annual variation of the activity is definitely proved.