[en] We propose a mechanism for the fast dissipation of magnetic fields which is effective in a stratified medium where ion motions can be neglected. In such a medium, the field is frozen into the electrons, and Hall currents prevail. Although Hall currents conserve magnetic energy, in the presence of density gradients they are able to create current sheets which can be sites for efficient dissipation of magnetic fields. We recover the frequency ω_MH for Hall oscillations modified by the presence of density gradients. We show that these oscillations can lead to an exchange of energy between different components of the field. We calculate the time evolution, and show that magnetic fields can dissipate on a time scale of order 1/ω_MH. This mechanism can play an important role in magnetic dissipation in systems with very steep density gradients, where the ions are static such as those found in the solid crust of neutron stars. (c) 2000 The American Physical Society

[en] The growth and nonlinear evolution of the modified Simon-Hoh instability (MSHI) is observed in a weak electron beam-produced collisionless cylindrical plasma, in which electrons are strongly magnetized and the ions are essentially unmagnetized. The evolution of this instability occurs through a sequence of sideband instabilities, thought to be induced by trapped ions, and a period doubling sequence. Transient study of the MSHI reveals that the growth rate of the MSHI is extremely rapid; of the order the instability frequency. (c) 2000 American Institute of Physics

[en] A simple two-fluid theory of the drift kink instability is developed. The validity of the theory is restricted to the regime where the ion gyroradius is small in comparison with the sheet thickness ρ_i<< L and wavelength ρ_ik_y<<1. Analytic expressions are derived for the real frequency and growth rate. For the limited region of parameter space where two-fluid theory is applicable, the analytic expressions are in excellent agreement with a full Vlasov solution [Daughton, 1999]. (c) 2000 American Geophysical Union

[en] An analytical dispersion relation is derived which shows that, in toroidal plasmas, zonal flows can be spontaneously excited via modulations in the radial envelope of a single-n coherent drift wave, with n the toroidal mode number. Predicted instability features are verified by three-dimensional global gyrokinetic simulations of the ion-temperature-gradient mode. Nonlinear equations for mode amplitudes demonstrate saturation of the linearly unstable pump wave and nonlinear oscillations of the drift-wave intensity and zonal flows, with a parameter-dependent period doubling route to chaos. (c) 2000 American Institute of Physics