Effects of Weak Collisions on the Ion Acoustic Instability in the Linear and Nonlinear Regimes
Carrie Black (UNH)
Monday 17th May 2010, 12:00pm
Pratt conference room, 60 Garden Street
Small scale kinetic effects have long been of interest to those studying space and laboratory plasmas. For instance, kinetic plasma instabilities are widely believed to be responsible for the generation of anomalous resistivity in reconnection layers, providing a possible mechanism for fast reconnection in space plasmas. The concept of Landau damping is fundamental to such wave kinetic instabilities, and is treated typically within the framework of the collisionless Vlasov equation. It has become clear in recent theoretical and experimental work that weak collisions are a singular perturbation on the collisionless theory, and qualitatively alter the results of the collisionless theory. In particular, it has been demonstrated by C. S. Ng, A. Bhattacharjee, and F. Skiff [C. S. Ng, A. Bhattacharjee, and F. Skiff, Phys. Rev. Lett. 83, 1974 (1999)] that the continuous spectrum of Case-Van Kampen (the underlying eigenmodes of the collisionless system), are completely eliminated and replaced by a discrete spectrum (hereafter referred to as the NBS spectrum). The NBS spectrum includes Landau-damped roots as exact eigenmodes, but is significantly broader, including a larger spectrum of discrete roots. Many types of waves in space plasmas such as the hybrid waves and acoustic waves can develop instabilities under the right conditions. We discuss the ion acoustic instability and its associated anomalous resistivity in light of these new findings. A new Vlasov code which includes the Lenard-Bernstein collision operator is introduced. We compare our numerical findings with previous analytical and numerical work on the subject, to quantify the effect of weak collisions.