1,064 research outputs found
Driven waves in a two-fluid plasma
We study the physics of wave propagation in a weakly ionised plasma, as it
applies to the formation of multifluid, MHD shock waves. We model the plasma as
separate charged and neutral fluids which are coupled by ion-neutral friction.
At times much less than the ion-neutral drag time, the fluids are decoupled and
so evolve independently. At later times, the evolution is determined by the
large inertial mismatch between the charged and neutral particles. The neutral
flow continues to evolve independently; the charged flow is driven by and
slaved to the neutral flow by friction. We calculate this driven flow
analytically by considering the special but realistic case where the charged
fluid obeys linearized equations of motion. We carry out an extensive analysis
of linear, driven, MHD waves. The physics of driven MHD waves is embodied in
certain Green functions which describe wave propagation on short time scales,
ambipolar diffusion on long time scales, and transitional behavior at
intermediate times. By way of illustration, we give an approximate solution for
the formation of a multifluid shock during the collision of two identical
interstellar clouds. The collision produces forward- and reverse J shocks in
the neutral fluid and a transient in the charged fluid. The latter rapidly
evolves into a pair of magnetic precursors on the J shocks, wherein the ions
undergo force free motion and the magnetic field grows monotonically with time.
The flow appears to be self similar at the time when linear analysis ceases to
be valid.Comment: 18 pages including 24 figures, accepted by MNRA
An adiabatic approximation for grain alignment theory
The alignment of interstellar dust grains is described by the joint
distribution function for certain ``internal'' and ``external'' variables,
where the former describe the orientation of a grain's axes with respect to its
angular momentum, J, and the latter describe the orientation of J relative to
the interstellar magnetic field. I show how the large disparity between the
dynamical timescales of the internal and external variables--- which is
typically 2--3 orders of magnitude--- can be exploited to greatly simplify
calculations of the required distribution. The method is based on an
``adiabatic approximation'' which closely resembles the Born-Oppenheimer
approximation in quantum mechanics. The adiabatic approximation prescribes an
analytic distribution function for the ``fast'' dynamical variables and a
simplified Fokker-Planck equation for the ``slow'' variables which can be
solved straightforwardly using various techniques. These solutions are accurate
to order epsilon, where epsilon is the ratio of the fast and slow dynamical
timescales. As a simple illustration of the method, I derive an analytic
solution for the joint distribution established when Barnett relaxation acts in
concert with gas damping. The statistics of the analytic solution agree with
the results of laborious numerical calculations which do not exploit the
adiabatic approximation.Comment: 22 pages (LaTeX+4 eps figs); accepted by MNRAS 6/30/9
Alien Registration- Roberge, Louis G. (Lewiston, Androscoggin County)
https://digitalmaine.com/alien_docs/27713/thumbnail.jp
Theory of grain alignment in molecular clouds
Research accomplishments are presented and include the following: (1) mathematical theory of grain alignment; (2) super-paramagnetic alignment of molecular cloud grains; and (3) theory of grain alignment by ambipolar diffusion
Cosmic rays and grain alignment
The recent detection of interstellar polarization in the solid CO feature
near 4.67 micron shows that CO-mantled grains can be aligned in cold molecular
clouds. These observations conflict with a theory of grain alignment which
attributes the polarization in molecular clouds to the effects of cosmic rays:
according to this theory, oblate spheroidal grains with H_2O and CO_2-dominated
ice mantles are spun up to suprathermal energies by molecular evaporation from
cosmic ray impact sites but spin up does not occur for CO-mantled grains.
Motivated by this conflict, we reexamine the effects of cosmic rays on the
alignment of icy grains. We show that the systematic torques produced by cosmic
rays are insufficient to cause suprathermal spin. In principle, the random
torques due to cosmic rays can enhance the efficiency of Davis-Greenstein
alignment by raising the grain rotational temperature. However, a significant
enhancement would require cosmic ray fluxes 6--7 orders of magnitude larger
than the flux in a typical cold cloud.Comment: 14 pages, 1 figure. Accepted to MNRA
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