Abstract. An increasing number of high-resolution spacecraft observations identify magnetic fluctuations in various space plasma environments as mirror mode structures. In a collisionless plasma the excitation of mirror modes has to be analyzed in a kinetic approach, where details of the velocity space distributions are of considerable significance. Nonthermal, high-energy tail distributions, commonly detected in planetary magnetospheres and the interplanetary medium, are known to be accurately modeled by the family of kappa distributions. It is demonstrated that the conventional fluid instability criterion is inadequate since suprathermal particle populations provide a strong source for the generation of mirror wave mode activity even in the low-/3 limit and small temperature anisotropy. The specific structure of velocity space distributions at high energies dominates as regulating mechanism for the mirror instability threshold. IntroductionIn a plasma of sufficiently large velocity space anisotropy two

Abstract. Suprathermal particle populations and loss-cone structures are the most common characteristics of ion and electron velocity distributions observed in space plasmas. We introduce a significant generalization of the family of kappa distributions to a suprathermal loss-cone distribution, applicable to a variety of space plasma modeling. On the basis of this concept a general mirror instability threshold criterion is derived from an energy principle for collisionless plasmas, covering the full range from two-temperature Maxwellians to complex loss-cone conditions occupied by high-energy ion species. We demonstrate that nonthermal plasma characteristics are the source of significant mirror instability threshold reduction where the specific structure of the velocity-space distribution dominates as a regulating mechanism for wave mode excitation.

Abstract. Magnetic fluctuations are recognized in a large variety of space plasmas by increasingly high resolution, in situ observations as mirror wave mode structures. A typical requirement for the excitation of mirror modes is a dominant perpendicular pressure in a high-beta plasma environment. Contrary, we demonstrate from a realistic kinetic analysis how details of the velocity space distributions are of considerable significance for the instability threshold. Introducing the most common characteristics of observed ion and electron distributions by a mixed suprathermal-loss-cone, we derive a universal mirror instability criterion from an energy principle for collisionless plasmas. As a result, the transition from two temperature Maxwellians to realistic non-thermal features provides a strong source for the generation of mirror wave mode activity, reducing drastically the instability threshold. In particular, a number of space-related examples illuminate how the specific structure of the velocity space distribution dominates as a regulating excitation mechanism over the effects related to changes in the plasma parameters.

This paper investigates the effect of ion-and electron-induced electron emission from a material wall on the voltage drop across the adjacent plasma sheath ('plasma sheath voltage' (PSV)). For this purpose, a new model involving a collisionless kinetic sheath consistently coupled to a fluid presheath is developed. The underlying analysis is valid for plasmas (both magnetized and unmagnetized) in which the Debye length is much smaller than the relevant characteristic presheath length ('asymptotic two-scale limit'). Material boundaries of particular interest are first walls and divertor target plates bounding magnetically confined fusion plasmas. Majority and impurity ions accelerated from the bulk plasma towards the material boundary release electrons flowing back into the plasma, thus giving rise to a lower PSV than without electron emission. In addition, sufficiently fast electrons from the plasma impinging on the bounding wall produce secondary electrons and are also partially reflected. The present work represents a first step in which the unmagnetized case is considered and electron reflection at the wall is still neglected. Considering typical boundary-plasma conditions and characteristic particle-induced electron emission (PIEE) data (i.e. electron yields and energy distributions), the PSV is self-consistently calculated by means of the new sheath model, showing appreciable effects of PIEE.

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