Long-term occurrence of mid-term periodicities in solar wind speed and geomagnetic activity

Several recent studies have reported quasi-periodicities with a period between I and 2 years (to be called here "mid-term periodicities") in various heliospheric parameters, like solar wind speed, interplanetary magnetic field, cosmic rays, and geomagnetic activity. Here we discuss the long-term occurrence of mid-term periodicities in geomagnetic activity using an extended aa index which covers the last 14 solar cycles since 1844. We find that the mid-term periodicities were strong during tile last 6-7 high-amplitude solar cycles. Contrary to earlier studies, we find that the mid-term periodicities nearly vanished during the weal, solar cycles from 1860s to 1920s. However, using the extended aa index we find that mid-term periodicities were fairly strong even in the middle of the 19th century in 1840s-1850s when solar activity was almost as high as recently. We also verify and quantify the intermittent nature of the mid-term periodicities and tile alternation of the dominant period between a slightly shorter period of about 1.3 years during the latest even solar cycles (SC 18, 20, 22) and a slightly longer period of about 1.5-1.7 years inbetween (Mursula, and Zieger, 2000). The implied roughly 22-year oscillation of the power in the two frequency bands is demonstrated.