INERTIAL CURRENTS IN ISOTROPIC PLASMA

The magnetospheric convection electric field contributes to Birkeland currents. The effects of the field are to polarize the plasma by displ acing the bounce paths of the ions from those of electrons, to redistr ibute the pressure so that it is not constant along magnetic field lin es, and to enhance the pressure gradient by the gradient of the bulk s peed. Changes in the polarization charge during the convection of the plasma are neutralized by electrons in the form of field-aligned curre nts that close through the ionosphere. The pressure drives field-align ed currents through its gradient in the same manner as in quasi-static plasma, but with modifications that are important if the bulk speed i s of the order of the ion thermal speed; the variations in the pressur e along field lines are maintained by a weak parallel potential drop. These effects are described in terms of the field-aligned currents in steady state, isotropic, MHD plasma. Solutions are developed by taking the MHD limit of two-fluid solutions and illustrated in the special c ase of Maxwellian plasma for which the temperature is constant along m agnetic field lines. The expression for the Birkeland current density is a generalization of Vasyliunas' expression for the field-aligned cu rrent density in quasi-static plasma and provides a unifying expressio n when both pressure gradients and ion inertia operate simultaneously as sources of field-aligned currents. It contains a full account of di fferent aspects of the ion flow (parallel and perpendicular velocity a nd vorticity) that contribute to the currents. Contributions of ion in ertia to field-aligned currents will occur in regions of strong veloci ty shear, electric field reversal, or large gradients in the parallel velocity or number density, and may be important in the low-latitude b oundary layer, plasma sheet boundary layer, and the inner edge region of the plasma sheet.