Solar Physics

, Volume 131, Issue 1, pp 187–209 | Cite as

A two-component solar cycle

  • J. P. Legrand
  • P. A. Simon


From a previous analysis of a long series of geomagnetic data, we came to the conclusion that, during 91.5% of the time, geomagnetic activity is controlled by the solar wind flow at the Earth's orbit.

In this paper, we consider the flow of the solar wind plasma in a coronal field whose source is a dipole. The temporal evolution of the dipole source as well as any small scale evolution occurring in the associated coronal field topology can be closely monitored from the latitudinal distribution of the wind velocity.

In the geomagnetic data series, the index Aa is closely linked to the wind velocity at the power 2.25. From this data set, we can reconstruct the behavior of the solar dipole field from 1868 onward.

The main results of our analysis are as follows. The solar cycle has two distinct components, dipole and toroidal, of which the respective cycles are out of phase. The toroidal component is strongly linked, with a 5–6 yr delay, to the preceding dipole component. This finding is in contradistinction to the view that the dipole field is a result of the poleward migration of the decaying toroidal field. This result should contribute to improve our understanding of the Sun's cyclical behaviour.


Solar Wind Wind Velocity Solar Cycle Geomagnetic Activity Dipole Field 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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Copyright information

© Kluwer Academic Publishers 1991

Authors and Affiliations

  • J. P. Legrand
    • 1
  • P. A. Simon
    • 2
  1. 1.CNRS-INSUSaint-Maur CedexFrance
  2. 2.Observatoire de Meudon, DASOPMeudon Principal CedexFrance

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