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

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

Mar 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002esasp.477..467m&link_type=abstract

In: Proceedings of the Second Solar Cycle and Space Weather Euroconference, 24 - 29 September 2001, Vico Equense, Italy. Editor:

Physics

1

Geomagnetic Activity, Solar Wind

Scientific paper

Several recent studies have reported quasi-periodicities with a period between 1 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 the last 6 - 7 high-amplitude solar cycles. Contrary to earlier studies, we find that the mid-term periodicities nearly vanished during the weak 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 the 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 in between (Mursula and Zieger, 2000). The implied roughly 22-year oscillation of the power in the two frequency bands in demonstrated.

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