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The ubiquity of high-energy tails in the charged particle velocity distribution functions observed in space plasmas suggests the existence of an underlying process responsible for taking a fraction of the charged particle population out of thermal equilibrium and redistributing it to suprathermal velocity and energy ranges. The present Letter focuses on a new and fundamental physical explanation for the origin of suprathermal electron distribution function in a highly collisional plasma. This process involves a newly discovered electrostatic bremsstrahlung emission that is effective in a plasma in which binary collisions are present. The steady-state electron velocity distribution function dictated by such a process corresponds to a Maxwellian core plus a quasi-inverse power-law tail, which is a feature commonly observed in many space plasma environment. In order to demonstrate this, the system of self-consistent particle- and wave- kinetic equations are numerically solved with an initially Maxwellian electron velocity distribution and Langmuir wave spectral intensity, which is a state that does not reflect the presence of electrostatic bremsstrahlung process, and hence not in force balance. The electrostatic bremsstrahlung term subsequently drives the system to a new force-balanced steady state. After a long integration period it is demonstrated the initial Langmuir fluctuation spectrum is modified, which in turn distorts the initial Maxwellian electron distribution into a velocity distribution that resembles the said core-suprathermal velocity distribution. Such a mechanism may thus be operative at the coronal source region, which is characterized by high collisionality., Comment: 7 pages, 2 figures. Published at: The Astrophysical Journal Letters, Volume 849, Number 2, L30. url: https://doi.org/10.3847/2041-8213/aa956f

We study the dispersion relation for parallel propagating ion-cyclotron (IC) waves in a dusty plasma, considering situations where the velocity dispersion along perpendicular direction is greater than along the parallel direction, and considering the use of product-bi-kappa (PBK) velocity distributions for the plasma particles. The results obtained by numerical solution of the dispersion relation, in a case with isotropic Maxwellian distributions for electrons and PBK distribution for ions, show the occurrence of the electromagnetic ion-cyclotron instability (EMIC), and show that the decrease in the kappa indexes of the PBK ion distribution leads to significant increase in the magnitude of the growth rates and in the range of wavenumber for which the instability occurs. On the other hand, for anisotropic Maxwellian distribution for ions and PBK distribution for electrons, the decrease of the kappa index in the PBK electron distribution contributes to reduce the growth rate of the EMIC instability, but the reduction effect is less pronounced than the increase obtained with ion PBK distribution with the same $\kappa$ index. The results obtained also show that, as a general rule, the presence of a dust population contributes to reduce the instability in magnitude of the growth rates and range, but that in the case of PBK ion distribution with small kappa indexes the instability may continue to occur for dust populations which would eliminate completely the instability in the case of bi-Maxwellian ion distributions. It has also been seen that the anisotropy due to the kappa indexes in the ion PBK distribution is not so efficient in producing the EMIC instability as the ratio of perpendicular and parallel ion temperatures, for equivalent value of the effective temperature., Comment: Accepted for publication in Astrophysics and Space Science