  A hybrid kinetic-fluid model is used to study plasma stratification in
alternating current (AC) discharges in noble gases at low plasma densities.
Self-consistent coupled solutions of a non-local kinetic equation for
electrons, a drift-diffusion equation of ions, and a Poisson equation for the
electric field are obtained for a positive column with periodic boundary
conditions and for the entire discharge to study near-electrode effects within
a 1d model with radial ambipolar loss. A simplified two-level
excitation-ionization model neglects the nonlinear effects due to stepwise
ionization, gas heating, and Coulomb interactions among electrons. Standing
striations are obtained for the reduced values of electric fields corresponding
to the inelastic energy balance of electrons in a range of driving frequencies.
An analog of Novak law (striation length proportional to the excitation
threshold of atoms and inversely proportional to the mean square root of the
electric field) is observed in simulations, indicating the nonlocal nature of
standing striations in AC discharges at low plasma densities. Stratified plasma
operates in a dynamic regime for a wide range of driving frequencies because
heat conduction occurs much faster than the ambipolar diffusion controlled by
slow ion motion. We demonstrate that standing striations in symmetric AC
discharges previously observed in experiments are similar to moving striations
in DC discharges and are both due to nonlocal kinetic effects at low discharge
currents.
