Tuesday

In a plasma with high enough temperature or low enough density Coulomb collisions become vanishingly rare, and the regime of collisionless plasmas opens a vista of complex interactions between particles and fields. With no collisions there is a gamut of length scales depending on particle kinetics and related to gyromotion, inertia, etc. With no collisions the evolution of the particle phase space density can be as complex as the particle dynamics that controls it. With no collisions the particle distribution can contain free energy, with a zoo of linear instabilities depending on collective behaviour such as quasi-neutrality and particle resonances, each related to different length and time scales. Waves driven by these instabilities can themselves modify the particle distribution. Despite this, from experience with space plasmas, often collisionless plasmas behave in some ways as a fluid. In astrophysics this allows the fluid MHD description to be used over length scales many order of magnitudes away from the characteristic kinetic scale lengths. I will discuss some examples which may help to understand how the kinetic scales of collisionless plasmas can be related to astrophysical scales. Collisionless plasma shocks mediate energy flow from bulk energy to thermal energy and energetic particles, and although the controlling processes are fundamentally kinetic, their effects can be important over much larger scales. Cross-scale coupling is intrinsic to collisionless plasmas, and I will discuss examples of how fluid-like flows can create circumstances for instabilities over different coupled scales, that, in turn, affect the bulk properties of the plasma. In this way the kinetic scales of collisionless plasmas can play a crucial role in systems of astrophysical plasmas.