Is stratification of a rare gas plasma a kinetic or fluid phenomenon?

The formation of self-organized luminous structures or striations in the positive column of rare gases is linked to the development of ionization waves. In this study, we examine a neon plasma column at low current, under conditions where metastable ionization is negligible. Recently, a Particle-In-Cell Monte Carlo Collisions method successfully simulated plasma instabilities in a rare gas positive column, realistically reproducing the formation of striations. To elucidate the mechanisms responsible for stratification, a fluid model was linearized around the uniform state of a positive column. The linear analysis revealed a positive growth rate, indicating instability, when the density gradient coefficient in the electron energy flux (thermoelectric transport coefficient or Dufour density gradient coefficient) was negative, a common characteristic in rare gases. The validity of the fluid approach in the conditions of stratification is, however, questionable. In the present paper, we use a kinetic approach, based on Monte Carlo simulations in the presence of a given perturbation of the electric field, to investigate the mechanisms leading to stratification. Our findings confirm that while the fluid model fails to capture the intricate physics of stratification, the kinetic approach underscores the crucial role of the thermoelectric transport coefficient in the emergence of striations in rare gases. Additionally, our results corroborate previous studies that identified resonances in the electron response to the perturbation. However, unlike earlier work, we find that these resonances produce minima rather than maxima in the growth rate and are not responsible for stratification.