Studies on Counterstreaming Plasma Expansion

Abstract

Counterstreaming plasma expansion occurs in a variety of physical situations ranging from laboratory devices to plasmas in space. In such situations, the counterstreaming plasma streams consisting of accelerated ions, collide with each other. Upon this collision, the interaction of the ion streams can lead to a variety of plasma physical phenomena such as the excitation of waves and instabilities and formations of shocks. In order to study these phenomena, several types of numerical simulations were carried out. One-dimensional Vlasov simulations with Boltzmann electrons showed that the H+ streams produced by the counterstreaming expansion of an O+ - H+ ion plasma in which H+ is minor, do not couple with each other. The H+ streams pass through each other without exciting any instability which turns out to be in agreement with the linear stability of the plasma. Thus, the streams do not thermalize in the expansion region and penetrate the source plasmas, where they slow down and interact with the ambient plasmas exciting relatively strong oscillations. In order to remove the limitations on the above type of simulations, such as the plasma being one dimensional and the electrons obeying the Boltzmann law, two-dimensional particle simulations, in which electrons and ions were treated like particles, were carried out. Despite the limitations imposed by computer resources, the 2-D simulations brought out some interesting results as follows. The ion streams were seen to mix together via the ion-ion instability when Te/Ti ≳ 3, where Te and Ti are the electron and ion temperatures in the source plasmas. For Te/Ti ≳ 5, the collisions of the ion streams produced a pair of electrostatic shocks propagating in opposite directions. On the other hand, for smaller temperature ratio (5 < Te/Ti ≲ 3), the instability resulted into short wavelength waves, which evolved into highly structured nearly stationary waveforms giving spiky electric fields. Since the small scale Vlasov or particle simulations are limited in their spatial extent, their applicability to large scale counterstreaming plasma expansions in space cannot be easily appreciated. This led to the studies on counterstreaming ionospheric plasma expansion along the closed geomagnetic flux tubes in the outer region of the terrestrial plasmasphere. In such studies the hydrodynamic equations were solved. The formation of a shock pair, upon the collision of the ion streams, was always seen, irrespective of the electron-ion temperature ratio, as expected from hydrodynamic calculations. The refilling of the flux tube occurs by the downward propagation of the shocks. The refilling rates derived from the hydrodynamic calculations are found to be in fair agreement with satellite observations. The kinetic aspects of the shock formations as derived from the small-scale simulations are discussed.