2 research outputs found
Generation of Suprathermal Electrons by Collective Processes in Collisional Plasma
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/aa956
Parker Solar Probe Observations of Near-f Ce Harmonic Emissions in the Near-Sun Solar Wind and Their Dependence on the Magnetic Field Direction
Wave emissions at frequencies near electron gyrofrequency harmonics are observed at small heliocentric distances below about 40 R ☉ and are known to occur in regions with quiescent magnetic fields. We show the close connection of these waves to the large-scale properties of the magnetic field. Near electron gyrofrequency harmonic emissions occur only when the ambient magnetic field points to a narrow range of directions bounded by polar and azimuthal angular ranges in the RTN coordinate system of correspondingly 80° ≲ θB ≲ 100° and 10° ≲ ϕB ≲ 30°. We show that the amplitudes of wave emissions are highest when both angles are close to the center of their respective angular interval favorable to wave emissions. The intensity of wave emissions correlates with the magnetic field angular changes at both large and small timescales. Wave emissions intervals correlate with intervals of decreases in the amplitudes of broadband magnetic fluctuations at low frequencies of 10–100 Hz. We discuss possible generation mechanisms of the waves.QC 20220831</p