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Astron. Astrophys. 339, 610-614 (1998)

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The sunspot as a self-excited dynamo

P. Lorrain 1, 2 and O. Koutchmy 3

1 Department of Earth and Planetary Sciences, McGill University, 3450 Rue Université, Montréal, Québec, H3A 2A7, Canada
2 Institut d'Astrophysique de Paris (CNRS), 98bis Boulevard Arago, F-75014 Paris, France
3 Analyse numérique (CNRS), Université Pierre et Marie Curie, Place Jussieu, F-75252 Paris, France

Received 23 March 1998 / Accepted 5 August 1998


Observations show that there is downflow in and around sunspots. Following Hénoux and Somov, Heyvaerts and Hagyard, Hirayama, and Lorrain and Koutchmy, we show that sunspots are self-excited magnetic-flux-tube dynamos run by the downflowing plasma. The azimuthal current results from an azimuthal [FORMULA] field generated by the radial inflow of the plasma. Comparison between various observations on sunspots with the fields of various coils seems to indicate that the azimuthal current is distributed nearly uniformly over the umbra and penumbra, with a slight excess near the axis. The resulting ohmic power density is negligible. The flux tube radius, at depth, is inversely proportional to the fourth root of the ambient pressure. The Standard Solar Model of Bahcall and Pinsonneau provides the pressure as a function of depth below -36 Mm.

Key words: MHD – Sun: magnetic fields – sunspots

Send offprint requests to: P. Lorrain

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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998