Abstract

This chapter focuses on the small-scale plasma waves and fluctuations directly conditioned by the velocity distributions of plasma particles. The dynamics of collision-poor plasmas from space is governed by wave-particle interactions, which trigger important kinetic phenomena, such as wave instabilities or emissions induced by the free energy of particles, as well as the energization of particles by waves. In-situ measurements in the solar wind reveal multiple non-thermal features of particle distributions, combining beaming populations with anisotropic temperatures, and the ubiquitous suprathermal populations well reproduced by the Kappa power-laws. These sources of free energy can excite various waves and fluctuations with different dispersive characteristics (frequencies, wave-numbers, growth rates). In turn, the enhanced fluctuations can interact with plasma particles shaping their distributions and constraining their anisotropies. We review recent linear and quasi-linear (QL) theories that have adopted realistic Kappa approaches and interpretations, and managed to describe a series of plasma waves and instabilities of interest in heliospheric plasmas. In order to unveil how these wave spectra change in the presence of suprathermal populations, it is also performed a direct comparative analysis with previous results, largely relying on idealized Maxwellian models of distributions. A number of contradictory results from the literature are also explained, being based on modified and less relevant Kappa approaches.