Mid-term quasi-periodicities in geomagnetic activity during the last 15 solar cycles: Connection to solar dynamo strength To the memory of Karolen I. Paularena (1957-2001)

Details Mid-term quasi-periodicities in geomagnetic activity during the last 15 solar cycles: Connection to solar dynamo strength To the memory of Karolen I. Paularena (1957-2001) Mid-term quasi-periodicities in geomagnetic activity during the last 15 solar cycles: Connection to solar dynamo strength To the memory of Karolen I. Paularena (1957-2001)

Jan 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003soph..212..201m&link_type=abstract

Solar Physics, v. 212, Issue 1, p. 201-207 (2003).

Physics

28

Scientific paper

Several recent studies have reported quasi-periodicities with a period between 1 and 2 years (to be called here `mid-term quasi-periodicities') in various heliospheric parameters, like solar wind speed, interplanetary magnetic field, cosmic rays, and geomagnetic activity. Here we study their long-term occurrence in geomagnetic activity using an extended aa index which covers the last 15 solar cycles. We confirm their intermittent occurrence and the alternation of their dominant period between a slightly shorter period of about 1.2 1.4 years and a slightly longer period of about 1.5 1.7 years. We find that the mid-term quasi-periodicities were strong during two intervals of high solar activity: in the mid-19th century and since 1930. Instead, contrary to earlier studies, we find that they were consistently weak during low solar activity from 1860s to 1920s. This implies a long-term connection between the amplitude of mid-term quasi-periodicities and the solar dynamo strength. Since the rotation speed at the bottom of the solar convection layer (tachocline) has recently been found to vary at a mid-term periodicity, this suggests that the stronger the solar dynamo is, the more variable the rotation rate of the tachocline is. We also note that the disappearance of mid-term periodicities may be used as a precursor for long intervals of very weak solar activity, like great minima.

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