  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.
