490 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
Mass-loading and parallel magnetized shocks
Recent observations at comets Giacobini-Zinner and Halley suggest that simple non-reacting gas dynamics or MHD is an inappropriate description for the bow shock. The thickness of the observed (sub)shock implies that mass-loading is an important dynamical process within the shock itself, thereby requiring that the Rankine-Hugoniot conditions possess source terms. This leads to shocks with properties similar to those of combustion shocks. We consider parallel magnetized shocks subjected to mass-loading, describe some properties which distinguish them from classical MHD parallel shocks, and establish the existence of a new kind of MHD compound shock. These results will be of importance both to observations and numerical simulations of the comet-solar wind interaction
Properties of mass-loading shocks, 2. Magnetohydrodynamics
The one-dimensional magnetohydrodynamics of shocked flows subjected to significant mass loading are considered. Recent observations at comets Giacobini-Zinner and Halley suggest that simple nonreacting MHD is an inappropriate description for active cometary bow shocks. The thickness of the observed cometary shock implies that mass loading represents an important dynamical process within the shock itself, thereby requiring that the Rankine-Hugoniot condition for the mass flux possess a source term. In a formal sense, this renders mass-loading shocks qualitatively similar to combustion shocks, except that mass loading induces the shocked flow to shear. Nevertheless, a large class of stable shocks exist, identified by means of the Lax conditions appropriate to MHD. Thus mass-loading shocks represent a new and interesting class of shocks, which, although found frequently in the solar system, both at the head of comets and, under suitable conditions, upsteam of weakly magnetized and nonmagnetized planets, has not been discussed in any detail. Owing to the shearing of the flow, mass-loading shocks can behave like switch-on shocks regardless of the magnitude of the plasma beta. Thus the behavior of the magnetic field in mass-loading shocks is significantly different from that occurring in nonreacting classical MHD shocks. It is demonstrated that there exist two types of mass-loading fronts for which no classical MHD analogue exists, these being the fast and slow compound mass-loading shocks. These shocks are characterized by an initial deceleration of the fluid flow to either the fast or the slow magnetosonic speed followed by an isentropic expansion to the final decelerated downstream state. Thus these transitions take the flow from a supersonic to a supersonic, although decelerated, downstream state, unlike shocks which occur in classical MHD or gasdynamics. It is possible that such structures have been observed during the Giotto-Halley encounter, and a brief discussion of the appropriate Halley parameters is therefore given, together with a short discussion of the determination of the shock normal from observations. A further interesting new form of mass-loading shock is the āslow-intermediateā shock, a stable shock which possesses many of the properties of intermediate MHD shocks yet which propagates like a slow mode MHD shock. An important property of mass-loading shocks is the large parameter regime (compared with classical MHD) which does not admit simple or stable transitions from a given upstream to a downstream state. This suggests that it is often necessary to construct compound structures consisting of shocks, slip waves, rarefactions, and fast and slow compound waves in order to connect given upstream and downstream states. Thus the Riemann problem is significantly different from that of classical MHD
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