646 research outputs found
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
Properties of mass-loading shocks: 1. Hydrodynamic considerations
The one-dimensional hydrodynamics of flows subjected to mass loading are considered anew, with particular emphasis placed on determining the properties of mass-loading shocks. This work has been motivated by recent observations of the outbound Halley bow shock (Neubauer et al., 1990), which cannot be understood in terms of simple hydrodynamical or magnetohydrodynamical descriptions. By including mass injection at the shock, we have investigated the properties of the Rankine-Hugoniot conditions on the basis of a geometric formulation of the entropy condition. Such a condition, which is more powerful than the usual thermodynamical formulation, serves to determine those solutions to the Rankine-Hugoniot conditions which correspond to a physically realizable downstream state. On this basis a concise theoretical description of hydrodynamic mass-loading shocks is obtained. We show that mass-loading shocks have more in common with combustion shocks than with ordinary nonreacting gas dynamical shocks. It is shown that for decelerated solutions to the Rankine-Hugoniot conditions to exist, the upstream flow speed u0 must satisfy u0 > ucrit > cs, where cs is the sound speed. Besides the usual supersonic-subsonic transition, mass-loading fronts can also admit a decelerating supersonic-supersonic transition, the structure of which consists of a sharp decrease in the flow velocity preceding a recovery and an increase in the final downstream flow speed. We suggest the possibility that such structures may describe the inbound Halley bow shock (Coates et al., 1987a). Both parallel and oblique shocks are considered, the primary difference being that oblique shocks are subjected to a shearing stress due to mass loading. It is conjectured that such a shearing may destabilize the shock
Instabilities in decelerating supersonic flows with applications to cosmic ray shocks
The nature of instabilities in cosmic ray shocks is investigated by using two distinct models for the shock wave. For wavelengths which are short relative to the thickness of the shock wave, the shock is treated as a smoothly decelerating low, and an appropriate JWKB type expansion is used to describe the perturbations to the flow. In this, the short wavelength regime, the presence of squeezing and an effective g renders strong cosmic ray shocks unstable in a way which is similar to instabilities in other supersonic flows, such as in de Laval nozzle flow or a heat conduction dominated shock wave. In the long wavelength limit, where the shock is treated as a discontinuous transition, a stability function is derived which, if negative, corresponds to unstable disturbances growing exponentially in time. In this case, it was found that if the cosmic ray fluid is relativistic (gamma sub c = 4/3) and the background plasma ideal (gamma = 5/3), then strong shocks are unstable
Phenomenology of hydromagnetic turbulence in a uniformly expanding medium
A simple phenomenology is developed for the decay and transport of turbulence in a constant-speed, uniformly expanding medium. The fluctuations are assumed to be locally incompressible, and either of the hydrodynamic or non-Alfvénic magnetohydrodynamic (MHD) type. In order to represent local effects of nonlinearities, a simple model of the Kaármá-Dryden type for locally homogeneous turbulent decay is adopted. A detailed discussion of the parameters of this familiar one-point hydrodynamic closure is given, which has been shown recently to be applicable to non-Alfvénic MHD as well. The effects of the large-scale flow and expansion are incorporated using a two-scale approach, in which assumptions of particular turbulence symmetries provide simplifications. The derived model is tractable and provides a basis for understanding turbulence in the outer heliosphere, as well as in other astrophysical applications
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