282 research outputs found
Collisionless plasma expansion in the presence of a dipole magnetic field
The collisionless interaction of an expanding high-energy plasma cloud with a
magnetized background plasma in the presence of a dipole magnetic field is
examined in the framework of a 2D3V hybrid (kinetic ions and massless fluid
electrons) model. The retardation of the plasma cloud and the dynamics of the
perturbed electromagnetic fields and the background plasma are studied for high
Alfven-Mach numbers using the particle-in-cell method. It is shown that the
plasma cloud expands excluding the ambient magnetic field and the background
plasma to form a diamagnetic cavity which is accompanied by the generation of a
collisionless shock wave. The energy exchange between the plasma cloud and the
background plasma is also studied and qualitative agreement with the analytical
model suggested previously is obtained.Comment: 10 pages, 4 figure
Energy dissipation and ion heating at the heliospheric termination shock
The Los Alamos hybrid simulation code is used to examine heating and the partition of dissipation energy at the perpendicular heliospheric termination shock in the presence of pickup ions. The simulations are one-dimensional in space but three-dimensional in field and velocity components, and are carried out for a range of values of pickup ion relative density. Results from the simulations show that because the solar wind ions are relatively cold upstream, the temperature of these ions is raised by a relatively larger factor than the temperature of the pickup ions. An analytic model for energy partition is developed on the basis of the Rankine-Hugoniot relations and a polytropic energy equation. The polytropic index gamma used in the Rankine-Hugoniot relations is varied to improve agreement between the model and the simulations concerning the fraction of downstream heating in the pickup ions as well as the compression ratio at the shock. When the pickup ion density is less than 20%, the polytropic index is about 5/3, whereas for pickup ion densities greater than 20%, the polytropic index tends toward 2.2, suggesting a fundamental change in the character of the shock, as seen in the simulations, when the pickup ion density is large. The model and the simulations both indicate for the upstream parameters chosen for Voyager 2 conditions that the pickup ion density is about 25% and the pickup ions gain the larger share ( approximately 90%) of the downstream thermal pressure, consistent with Voyager 2 observations near the shock
An exact solution of the moving boundary problem for the expansion of a plasma cylinder in a magnetic field
An exact analytic solution has been obtained for a uniformly expanding,
neutral, infinitely conducting plasma cylinder in an external uniform and
constant magnetic field. The electrodynamical aspects related to the emission
and transformation of energy have been considered as well. The results obtained
can be used in analysing the recent experimental and simulation data.Comment: 5 pages, 1 figur
Pitch‐angle scattering of cometary ions: Computer simulations
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95535/1/grl5552.pd
On the state of low luminous accreting neutron stars
Observational appearance of a neutron star in the subsonic propeller state
which is a companion of a wind-fed mass-exchange close binary system is
discussed. During the subsonic propeller state the neutron star magnetosphere
is surrounded by a spherical quasi-static plasma envelope, which is extended
from the magnetospheric boundary up to the star accretion radius. The energy
input to the envelope due to the propeller action by the neutron star exceeds
the radiative losses and the plasma temperature in the envelope is of the order
of the free-fall temperature. Under this condition the magnetospheric boundary
is interchange stable. Nevertheless, I find that the rate of plasma penetration
from the envelope into the magnetic field of the neutron star due to diffusion
and magnetic field line reconnection processes is large enough for the
accretion power to dominate the spindown power. I show that the accretion
luminosity of the neutron star in the subsonic propeller state is 5*10**{30} -
10**{33} (dM/dt)_{15} erg/s, where dM/dt is the strength of the normal
companion stellar wind which is parametrized in terms of the maximum possible
mass accretion rate onto the neutron star magnetosphere. On this basis I
suggest that neutron stars in the subsonic propeller state are expected to be
observed as low luminous accretion-powered pulsars. The magnetospheric radius
of the neutron star in this state is determined by the strength of the stellar
wind, (dM/dt)_c, while the accretion luminosity is determined by the rate of
plasma penetration into the star magnetosphere, (dM/dt)_a, which is (dM/dt)_a
<< (dM/dt)_c. That is why the classification of the neutron star state in these
objects using the steady accretion model (i.e. setting (dM/dt)_a = (dM/dt)_c)
can lead to a mistaken conclusion.Comment: 6 pages, accepted for publication in A&
Pitch angle scattering of cometary ions into monospherical and bispherical distributions
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95043/1/grl5478.pd
Particle Energization in an Expanding Magnetized Relativistic Plasma
Using a 2-1/2-dimensional particle-in-cell (PIC) code to simulate the
relativistic expansion of a magnetized collisionless plasma into a vacuum, we
report a new mechanism in which the magnetic energy is efficiently converted
into the directed kinetic energy of a small fraction of surface particles. We
study this mechanism for both electron-positron and electron-ion (mi/me=100, me
is the electron rest mass) plasmas. For the electron-positron case the pairs
can be accelerated to ultra-relativistic energies. For electron-ion plasmas
most of the energy gain goes to the ions.Comment: 7 pages text plus 5 figures, accepted for publication by Physical
Review Letter
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