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Astron. Astrophys. 318, 947-956 (1997)

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The photospheric dynamo

I. Physics of thin magnetic flux tubes

J.C. Hénoux 1 and B.V. Somov 2

1 Observatoire de Paris, DASOP-LPSH, URA2080, F-92195 Meudon Principal Cedex, France (henoux@mesiob.obspm.fr)
2 Astronomical Institute of the Moscow State University, Universitetskii Prospekt 13, Moscow B-234, 119899, Russia

Received 8 January 1996 / Accepted 19 March 1996


In a previous paper (Hénoux and Somov, 1991) it has been shown that, in an initially weak magnetic field, a radial inflow of neutrals can generate azimuthal DC currents, and that an azimuthal velocity field can create radial DC currents leading to the circulation of vertical currents. The effects of such azimuthal velocity field on the intensity and topology of electric currents flowing in thin magnetic flux tubes is now investigated in detail in this paper. Two systems of currents flowing in opposite direction are created connected at photospheric level by transverse currents. The electromagnetic forces produced by these currents play a significant rôle in the structure and dynamics of flux tubes. Even for moderate values of the azimuthal photospheric velocities, the currents created are strong enough to prevent by the pinch effect an opening of the flux tube with height; despite the decrease of the ambient gas pressure with height, the thin flux tube extends into the solar atmosphere above the temperature minimum region.

In the internal current shell, the rise from the photosphere of a partially ionized gas is found to have two main effects: (a) the upflow of this gas associated to a leak of neutrals across magnetic field lines leads to an increase of the ionization degree with altitude typical for the chromosphere, and brings above the temperature minimum region an energy flux comparable to the flux required for chromospheric heating, (b) the outflow of neutrals that takes place at the chromospheric level across magnetic field lines, and leads to ion-neutral separation, may explain the observed abundance anomalies in the corona by enhancing in the upper part of the tube the abundances of elements of low ionization potential.

Upward motions are also present between the two current systems outside the internal cylindrical current. Their velocity is high enough to lift the matter to an altitude characteristic of spicules.

Key words: Sun: magnetic fields; chromosphere

Send offprint requests to: J.C. Hénoux

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998