The dynamics of current carriers in standing Alfven waves: Parallel electric fields in the auroral acceleration region

[1] The acceleration of current carriers in an Alfven wave current system is considered. The model incorporates a dipole magnetic field geometry, and we present an analytical solution of the two-fluid equations by successive approximations. The leading solution corresponds to the familiar single-fluid toroidal oscillations. The next order describes the nonlinear dynamics of electrons responsible for carrying a few muAm(-2) field aligned current into the ionosphere. The solution shows how most of the electron acceleration in the magnetosphere occurs within 1 R-E of the ionosphere, and that a parallel electric field of the order of 1 mV m(-1) is responsible for energising the electrons to 1 keV. The limitations of the electron fluid approximation are considered, and a qualitative solution including electron beams and a modified E-parallel to is developed in accord with observations. We find that the electron acceleration can be nonlinear, (nu(eparallel to)del(parallel to))nu(eparallel to) > omeganu(eparallel to), as a result of our nonuniform equilibrium field geometry even when nu(eparallel to) is less than the Alfven speed. Our calculation also elucidates the processes through which E-parallel to is generated and supported.