29 research outputs found
Effect of a weak ion collisionality on the dynamics of kinetic electrostatic shocks
In strictly collisionless electrostatic shocks, the ion distribution function
can develop discontinuities along phase-space separatrices, due to partial
reflection of the ion population. In this paper, we depart from the strictly
collisionless regime and present a semi-analytical model for weakly collisional
kinetic shocks. The model is used to study the effect of small but finite
collisionalities on electrostatic shocks, and they are found to smooth out
these discontinuities into growing boundary layers. More importantly, ions
diffuse into and accumulate in the previously empty regions of phase space,
and, by upsetting the charge balance, lead to growing downstream oscillations
of the electrostatic potential. We find that the collisional age of the shock
is the more relevant measure of the collisional effects than the
collisionality, where the former can become significant during the lifetime of
the shock, even for weak collisionalities.Comment: Published in J. Plasma Phy
Electron Energization in Reconnection: Eulerian versus Lagrangian Perspectives
Particle energization due to magnetic reconnection is an important unsolved
problem for myriad space and astrophysical plasmas. Electron energization in
magnetic reconnection has traditionally been examined from a particle, or
Lagrangian, perspective using particle-in-cell (PIC) simulations.
Guiding-center analyses of ensembles of PIC particles have suggested that Fermi
(curvature drift) acceleration and direct acceleration via the reconnection
electric field are the primary electron energization mechanisms. However, both
PIC guiding-center ensemble analyses and spacecraft observations are performed
in an Eulerian frame. For this work, we employ the continuum Vlasov-Maxwell
solver within the Gkeyll simulation framework to re-examine electron
energization from a kinetic continuum, Eulerian, perspective. We separately
examine the contribution of each drift energization component to determine the
dominant electron energization mechanisms in a moderate guide-field Gkeyll
reconnection simulation. In the Eulerian perspective, we find that the
diamagnetic and agyrotropic drifts are the primary electron energization
mechanisms away from the reconnection x-point, where direct acceleration
dominates. We compare the Eulerian (Vlasov Gkeyll) results with the wisdom
gained from Lagrangian (PIC) analyses.Comment: 10 pages, 7 figure
Reduced convergence and the local smoothness parameter: bridging two different descriptions of weak lensing amplification
Weak gravitational lensing due to the inhomogeneous matter distribution in
the universe is an important systematic uncertainty in the use of standard
candles in cosmology. There are two different descriptions of weak lensing
amplification, one uses a local smoothness parameter , the
other uses reduced convergence (where
is convergence). The description involves Dyer-Roeder distance
( corresponds to a smooth universe);
it is simple and convenient, and has been used by the community to illustrate
the effect of weak lensing on point sources such as type Ia supernovae. Wang
(1999) has shown that the description can be made realistic by
allowing to be a local variable, the local smoothness
parameter. The description has been used by Wang, Holz, & Munshi (2002)
to derive a universal probability distribution (UPDF) for weak lensing
amplification. In this paper, we bridge the two different descriptions of weak
lensing amplification by relating the reduced convergence and the local
smoothness parameter . We give the variance of
in terms of the matter power spectrum, thus providing a quantitative guidance
to the use of Dyer-Roeder distances in illustrating the effect of weak lensing.
The by-products of this work include a corrected definition of the reduced
convergence, and simple and accurate analytical expressions for
. Our results should be very useful in studying the weak
lensing of standard candles.Comment: Revised and expanded version. ApJ accepte
Low Mach-number collisionless electrostatic shocks and associated ion acceleration
The existence and properties of low Mach-number () electrostatic
collisionless shocks are investigated with a semi-analytical solution for the
shock structure. We show that the properties of the shock obtained in the
semi-analytical model can be well reproduced in fully kinetic Eulerian
Vlasov-Poisson simulations, where the shock is generated by the decay of an
initial density discontinuity. Using this semi-analytical model, we study the
effect of electron-to-ion temperature ratio and presence of impurities on both
the maximum shock potential and Mach number. We find that even a small amount
of impurities can influence the shock properties significantly, including the
reflected light ion fraction, which can change several orders of magnitude.
Electrostatic shocks in heavy ion plasmas reflect most of the hydrogen impurity
ions.Comment: In Plasma Physics and Controlled Fusio
Collisionless energy transfer in kinetic turbulence: field-particle correlations in Fourier space
Turbulence is ubiquitously observed in nearly collisionless heliospheric
plasmas, including the solar wind and corona and the Earth's magnetosphere.
Understanding the collisionless mechanisms responsible for the energy transfer
from the turbulent fluctuations to the particles is a frontier in kinetic
turbulence research. Collisionless energy transfer from the turbulence to the
particles can take place reversibly, resulting in non-thermal energy in the
particle velocity distribution functions (VDFs) before eventual collisional
thermalization is realized. Exploiting the information contained in the
fluctuations in the VDFs is valuable. Here we apply a recently developed method
based on VDFs, the field-particle correlation technique, to a ,
solar-wind-like, low-frequency Alfv\'enic turbulence simulation with well
resolved phase space to identify the field-particle energy transfer in velocity
space. The field-particle correlations reveal that the energy transfer,
mediated by the parallel electric field, results in significant structuring of
the ion and electron VDFs in the direction parallel to the magnetic field.
Fourier modes representing the length scales between the ion and electron
gyroradii show that energy transfer is resonant in nature, localized in
velocity space to the Landau resonances for each Fourier mode. The energy
transfer closely follows the Landau resonant velocities with varying
perpendicular wavenumber and plasma . This resonant signature,
consistent with Landau damping, is observed in all diagnosed Fourier modes that
cover the dissipation range of the simulation.Comment: 31 pages, accepted by JPP, minor improvements compared to v