Nongyrotropic particle distributions in space plasmas
- 1German Aerospace Center (DLR), Institute for Planetary Exploration, D-12484 Berlin, Germany
- 2Now at Institute for Theoretical Physics, Technical University, D-38106 Braunschweig, Germany
- E-mail: U.Motschmann@tu-bs.de
- 3Institute for Geophysics and Meteorology, Technical University, D-38106 Braunschweig, Germany
- 4Centro de Fisica de Plasmas, Instituto Superior Tecnico, P-1096 Lisboa Codex, Portugal
Abstract. In nonstationary, strong inhomogeneous or open plasmas particle orbits are rather complicated. If the nonstationary time scale is smaller than the gyration period, if the inhomogeneity scale is smaller than the gyration radius, i.e. at magnetic plasma boundaries, or if the plasma has sources and sinks in phase space, then nongyrotropic distribution functions occur. The stability of such plasma configurations is studied in the framework of linear dispersion theory. In an open plasma nongyrotropy drives unstable waves parallel and perpendicular to the background magnetic field, whereas in the gyrotropic limit the plasma is stable. In nonstationary plasmas nongyrotropy drives perpendicular unstable waves only. Temporal modulation couples a seed mode with its side lobes and thus it renders unstable wave growth more difficult. As an example of an inhomogeneous plasma a magnetic halfspace is discussed. In a layer with thickness of the thermal proton gyroradius a nongyrotropic distribution is formed which may excite unstable parallel and perpendicular propagating waves.
Key words. Interplanetary physics (plasma waves and turbulence) · Ionosphere (plasma waves and instabilities) · Magnetospheric physics (plasma waves and instabilities)