## 2. DC electric current generationIn a non axially symmetrical geometry, electric currents can be generated in separatrices or separators of the coronal magnetic field, for example, by shear motions either parallel or perpendicular to the photospheric neutral line. For axially symmetrical magnetic fields, steady azimuthal motions can generate currents. In this case, two models of current generation are distinguished. The first one assumes the plasma is fully ionized with infinite
conductivity. Hence, the magnetic field is frozen and moves with the
plasma. Consequently, the generation of an azimuthal component of the
field, and subsequently of a current along the flux tube, requires a
variation of the angular velocity in the
azimuthal plasma velocity along In the second model presented in this paper, where currents are generated by azimuthal motions in a partially ionized atmosphere, the azimuthal velocities may be constant along the loop length. Since it is the relative azimuthal velocity between the magnetic field lines and the partially ionized atmosphere that generates currents, these currents can result either from azimuthal motions of the gas around a fixed magnetic field or from the rotation around the flux tube axis of the magnetic field imbedded in a static partially ionized atmosphere. Assuming steady state, the azimuthal and radial components of the current density can be derived from the steady equation of fluid dynamics The corresponding equation for the horizontal components of the forces and velocities, in a thin flux tube of constant cross section, leads to the following expressions for the azimuthal and radial current densities where the contribution of the terms of inertia is neglected, and Eq. (3) shows that radial current generation requires an influx of matter and angular momentum inside the flux tube. The radial current density cannot be computed without knowing the plasma radial velocity . © European Southern Observatory (ESO) 1997 Online publication: July 3, 1998 |