Fundamental issues on kappa-distributions in space plasmas and interplanetary proton distributions

Numerous in situ observations indicate clearly the presence of nonthermal electron and ion structures as ubiquitous and persistent feature in a variety of astrophysical plasma environments. In particular, the detected suprathermal particle populations are accurately represented by the family of kappa-distributions, a power-law in particle speed. After clarifying the characteristics of high-energy tail distributions under various space plasma conditions, different generation mechanisms of energetic particles are introduced where numerical simulations of wave-particle interaction based on a Fokker-Planck approach demonstrate how Landau interaction ultimately leads to kappa-like distributions. Because of lack of theoretical justification, the use of the analytical form of kappa-functions was frequently criticized. It is shown that these distributions turn out as consequence of an entropy generalization favored by nonextensive thermo-statistics, thus providing the missing link for powerlaw models of suprathermal tails from fundamental physics, along with a physical interpretation of the structure parameter kappa. Moreover, with regard to the full nonextensive formalism, compatible also with negative values Of kappa, it is demonstrated that core-halo distribution structures, as observed for instance under typical interplanetary plasma conditions, are a natural content of the pseudo-additive entropy concept. The significance of the complete kappa-distribution family with regard to observed core-halo electron and double-humped ion velocity space characteristics is illuminated, where the observed peak separation scale of interplanetary proton distributions is compatible with a maximum entropy condition. (C) 2004 American Institute Of Physics.