NDLI: 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)

Content Provider	Springer Nature Link
Author	Mursula, K. Zieger, B. Vilppola, J.H.
Copyright Year	2003
Abstract	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.
Starting Page	201
Ending Page	207
Page Count	7
File Format	PDF
ISSN	00380938
Journal	Solar Physics
Volume Number	212
Issue Number	1
e-ISSN	1573093X
Language	English
Publisher	Kluwer Academic Publishers
Publisher Date	2003-01-01
Publisher Place	Dordrecht
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Astronomy, Astrophysics and Cosmology Astronomy Extraterrestrial Physics, Space Sciences
Content Type	Text
Resource Type	Article
Subject	Astronomy and Astrophysics Space and Planetary Science

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