7 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
Solar Wind Electron Acceleration via Langmuir Turbulence
The solar wind electrons observed at 1 AU are characterized by velocity distribution functions (VDF) that deviate from the Maxwellian form in a high energy regime. Such a feature is often modeled by a kappa distribution. In the present paper a self-consistent theory of quiet-time solar wind electrons that contain a power-law tail component, f â v-ÂŁ\ is discussed. These electrons are assumed to be in dynamic equilibrium with enhanced electrostatic fluctuations with peak frequency near the plasma frequency (i.e., the Langmuir turbulence). In order to verify the theoretical prediction, the solar wind electrons in the high-energy range known as the super-halo distribution detected by WIND and STEREO spacecraft are compared against the theoretical model where it was found that the theoretical power-law index is intermittent with regard to the observed range of indices, thus indicating that the turbulent equilibrium model of suprathermal solar wind electrons may be valid
Characterization of Monkey Enteropathogenic Escherichia coli (EPEC) and Human Typical and Atypical EPEC Serotype Isolates from Neotropical Nonhuman Primates
Enteropathogenic Escherichia coli (EPEC) has been associated with infantile diarrhea and mortality in humans in developing countries. While diarrhea is also a major problem among primates kept in captivity, the role of E. coli is unclear. This study was designed to characterize diarrheagenic E. coli recovered from the feces of 56 New World nonhuman primates, primarily marmosets (Callithrix spp.). Seventeen of the 56 primates had signs of diarrhea and/or enteritis. E. coli recovered from feces from these animals was tested by PCR for genes encoding virulence factors of diarrheagenic E. coli and for patterns of adherence to HeLa cells. In addition, isolates were characterized by the fluorescence actin staining test and by their ability to induce attaching and effacing lesions. PCR for the eae gene was positive in 10 of the 39 (27%) apparently healthy animals and in 8 of the 17 (47%) animals with diarrhea and/or enteritis. Colonies of eae(+) E. coli were serotyped and examined by PCR for genes encoding EPEC virulence markers. The eae(+) E. coli isolates recovered from both healthy and sick nonhuman primates demonstrated virulence-associated attributes similar to those of EPEC strains implicated in human disease and are designated monkey EPEC. The results presented here indicate that EPEC may be a significant pathogen for nonhuman primates, deserving further investigation. The similarities between the affected animals investigated in this study and human EPEC infections suggest that marmosets may represent an important model for EPEC in humans
Generation of harmonic Langmuir mode by beam-plasma instability
In this article, numerical solutions of the generalized weak turbulence equation [P. H. Yoon, Phys. Plasmas 7, 4858 (2000)] are carried out. In the generalized weak turbulence theory, the generation of the 2vpe-harmonic Langmuir mode is treated as a fundamental process in turbulent beam-plasma interaction process, in addition to, and concomitant to, the well-known nonlinear processes such as Langmuir and ion-sound mode coupling and wave-particle interactions. The present numerical analysis shows that the harmonic mode, which is a solution to a nonlinear dispersion equation, hence a âânonlinearââ eigenmode, grows primarily due to an induced emission process, which is a ââlinearââ wave-particle interaction process. The harmonic Langmuir mode generation has been observed since the late 1960s in laboratory experiments, simulations, and in space. However, adequate and quantitative theoretical explanation has not been forthcoming. The present work represents a step toward an understanding of such a phenomenon