45 research outputs found
Dynamics of Alfv\'en waves in partially ionized astrophysical plasmas
We develop a two dimensional, self-consistent, compressible fluid model to
study evolution of Alfvenic modes in partially ionized astrophysical and space
plasmas. The partially ionized plasma consists mainly of electrons, ions and
significant neutral atoms. The nonlinear interactions amongst these species
take place predominantly through direct collision or charge exchange processes.
Our model uniquely describe the interaction processes between two distinctly
evolving fluids. In our model, the electrons and ions are described by a single
fluid compressible magnetohydrodynamic (MHD) model and are coupled
self-consistently to the neutral fluid via compressible hydrodynamic equations.
Both plasma and neutral fluids are treated with different energy equations that
adequately enable us to monitor non adiabatic and thermal energy exchange
processes between these two distinct fluids. Based on our self-consistent
model, we find that the propagation speed of Alfvenic modes in space and
astrophysical plasma is slowed down because these waves are damped
predominantly due to direct collisions with the neutral atoms. Consequently,
energy transfer takes place between plasma and neutral fluids. We describe the
mode coupling processes that lead to the energy transfer between the plasma and
neutral and corresponding spectral features.Comment: To appear in Journal of Plasma Physic
An analytic model of plasma-neutral coupling in the heliosphere plasma
We have developed an analytic model to describe coupling of plasma and
neutral fluids in the partially ionized heliosphere plasma medium. The sources
employed in our analytic model are based on a -distribution as opposed
to the Maxwellian distribution function. Our model uses the
-distribution to analytically model the energetic neutral atoms that
result in the heliosphere partially ionized plasma from charge exchange with
the protons and subsequently produce a long tail which is otherwise not
describable by the Maxwellian distribution. We present our analytic formulation
and describe major differences in the sources emerging from these two distinct
distributions.Comment: This paper has been accepted in Journal of Plasma Physics. It is in
pres
Whistler Wave Turbulence in Solar Wind Plasma
Whistler waves are present in solar wind plasma. These waves possess
characteristic turbulent fluctuations that are characterized typically by the
frequency and length scales that are respectively bigger than ion gyro
frequency and smaller than ion gyro radius. The electron inertial length is an
intrinsic length scale in whistler wave turbulence that distinguishably divides
the high frequency solar wind turbulent spectra into scales smaller and bigger
than the electron inertial length. We present nonlinear three dimensional, time
dependent, fluid simulations of whistler wave turbulence to investigate their
role in solar wind plasma. Our simulations find that the dispersive whistler
modes evolve entirely differently in the two regimes. While the dispersive
whistler wave effects are stronger in the large scale regime, they do not
influence the spectral cascades which are describable by a Kolmogorov-like
spectrum. By contrast, the small scale turbulent fluctuations
exhibit a Navier-Stokes like evolution where characteristic turbulent eddies
exhibit a typical hydrodynamic turbulent spectrum. By virtue of
equipartition between the wave velocity and magnetic fields, we quantify the
role of whistler waves in the solar wind plasma fluctuations.Comment: To appear in the Proceedings of Solar Wind 1
Self-consistent Simulations of Plasma-Neutral in a Partially Ionized Astrophysical Turbulent Plasma
A local turbulence model is developed to study energy cascades in the
heliosheath and outer heliosphere (OH) based on self-consistent two-dimensional
fluid simulations. The model describes a partially ionized magnetofluid OH that
couples a neutral hydrogen fluid with a plasma primarily through
charge-exchange interactions. Charge-exchange interactions are ubiquitous in
warm heliospheric plasma, and the strength of the interaction depends largely
on the relative speed between the plasma and the neutral fluid. Unlike
small-length scale linear collisional dissipation in a single fluid,
charge-exchange processes introduce channels that can be effective on a variety
of length scales that depend on the neutral and plasma densities, temperature,
relative velocities, charge-exchange cross section, and the characteristic
length scales. We find, from scaling arguments and nonlinear coupled fluid
simulations, that charge-exchange interactions modify spectral transfer
associated with large-scale energy-containing eddies. Consequently, the
turbulent cascade rate prolongs spectral transfer among inertial range
turbulent modes. Turbulent spectra associated with the neutral and plasma
fluids are therefore steeper than those predicted by Kolmogorov's
phenomenology. Our work is important in the context of the global heliospheric
interaction, the energization and transport of cosmic rays, gamma-ray bursts,
interstellar density spectra, etc. Furthermore, the plasma-neutral coupling is
crucial in understanding the energy dissipation mechanism in molecular clouds
and star formation processes.Comment: To appear in the Proceedings of Solar Wind 1
Theory and Simulations of Whistler Wave Propagation
A linear theory of whistler wave is developed wihtin the paradigm of a two
dimensional incompressible electron magnetohydrodynamics model. Exact analytic
wave solutions are obtained for a small amplitude whistler wave that exhibit
magnetic field topological structures consistent with the observations and our
simulations in linear regime. In agreement with experiment, we find that the
parallel group velocity of the wave is large compared to its perpendicular
counterpart. Numerical simulations of collisional interactions demonstrate that
the wave magnetic field either coalesces or repels depending upon the polarity
of the associated current. In the nonlinear regime, our simulations demonstrate
that the evolution of wave magnetic field is governed essentially by the
nonlinear Hall force
Inhomogeneous whistler turbulence in low beta space plasmas
A nonlinear two dimensional fluid model of whistler turbulence is developed
that nonlinearly couples wave magnetic field with electron density
perturbations. This coupling leads essentially to finite compressibility
effects in whistler turbulence model. Interestingly it is found from our
simulations that despite strong compressibility effects, the density
fluctuations couple only weakly to the wave magnetic field fluctuations. In a
characteristic regime where large scale whistlers are predominant, the weakly
coupled density fluctuations do not modify inertial range energy cascade
processes. Consequently, the turbulent energy is dominated by the large scale
(compared to electron inertial length) eddies and it follows a Kolmogorov-like
spectrum, where is a characteristic wavenumber. The weak
coupling of the density fluctuations is explained on the basis of a whistler
wave parameter that quantifies the contribution of density perturbations in the
wave magnetic field.Comment: This paper is accepted for publication in Monthly Notices of the
Royal Astronomical Societ