1,041 research outputs found
Last scattering, relic gravitons and the circular polarization of the CMB
The tensor contribution to the -mode polarization induced by a magnetized
plasma at last scattering vanishes exactly. Conversely a polarized background
of relic gravitons cannot generate a -mode polarization. The reported
results suggest that, in the magnetized CDM paradigm, the dominant
source of circular dichroism stems from the large-scale fluctuations of the
spatial curvature.Comment: 8 pages, no figure
Breakdown of adiabatic invariance in spherical tokamaks
Thermal ions in spherical tokamaks have two adiabatic invariants: the
magnetic moment and the longitudinal invariant. For hot ions, variations in
magnetic-field strength over a gyro period can become sufficiently large to
cause breakdown of the adiabatic invariance. The magnetic moment is more
sensitive to perturbations than the longitudinal invariant and there exists an
intermediate regime, super-adiabaticity, where the longitudinal invariant
remains adiabatic, but the magnetic moment does not. The motion of
super-adiabatic ions remains integrable and confinement is thus preserved.
However, above a threshold energy, the longitudinal invariant becomes
non-adiabatic too, and confinement is lost as the motion becomes chaotic. We
predict beam ions in present-day spherical tokamaks to be super-adiabatic but
fusion alphas in proposed burning-plasma spherical tokamaks to be
non-adiabatic.Comment: 6 pages, 8 figure
The evolution of electron overdensities in magnetic fields
When a neutral gas impinges on a stationary magnetized plasma an enhancement in the ionization rate occurs when the neutrals exceed a threshold velocity. This is commonly known as the critical ionization velocity effect. This process has two distinct timescales: an ion–neutral collision time and electron acceleration time. We investigate the energization of an ensemble of electrons by their self-electric field in an applied magnetic field. The evolution of the electrons is simulated under different magnetic field and density conditions. It is found that electrons can be accelerated to speeds capable of electron impact ionization for certain conditions. In the magnetically dominated case the energy distribution of the excited electrons shows that typically 1% of the electron population can exceed the initial electrostatic potential associated with the unbalanced ensemble of electrons
Double-layer shocks in a magnetized quantum plasma
The formation of small but finite amplitude electrostatic shocks in the
propagation of quantum ion-acoustic waves (QIAWs) obliquely to an external
magnetic field is reported in a quantum electron-positron-ion (e-p-i) plasma.
Such shocks are seen to have double-layer (DL) structures composed of the
compressive and accompanying rarefactive slow-wave fronts. Existence of such DL
shocks depends critically on the quantum coupling parameter associated with
the Bohm potential and the positron to electron density ratio . The
profiles may, however, steepen initially and reach a steady state with a number
of solitary waves in front of the shocks. Such novel DL shocks could be a good
candidate for particle acceleration in intense laser-solid density plasma
interaction experiments as well as in compact astrophysical objects, e.g.,
magnetized white dwarfs.Comment: 4 pages, 1 figure (to appear in Physical Review E
Birefringence, CMB polarization and magnetized B-mode
Even in the absence of a sizable tensor contribution, a B-mode polarization
can be generated because of the competition between a pseudo-scalar background
and pre-decoupling magnetic fields. By investigating the dispersion relations
of a magnetoactive plasma supplemented by a pseudo-scalar interaction, the
total B-mode polarization is shown to depend not only upon the plasma and
Larmor frequencies but also on the pseudo-scalar rotation rate. If the
(angular) frequency channels of a given experiment are larger than the
pseudo-scalar rotation rate, the only possible source of (frequency dependent)
B-mode autocorrelations must be attributed to Faraday rotation. In the opposite
case the pseudo-scalar contribution dominates and the total rate becomes, in
practice, frequency-independent. The B-mode cross-correlations can be used,
under certain conditions, to break the degeneracy by disentangling the two
birefringent contributions.Comment: 8 pages, 2 figure
What happens when the geomagnetic field reverses?
During geomagnetic field reversals the radiation belt high-energy proton
populations become depleted. Their energy spectra become softer, with the
trapped particles of highest energies being lost first, and eventually
recovering after a field reversal. The radiation belts rebuild in a dynamical
way with the energy spectra flattening on the average during the course of many
millennia, but without ever reaching complete steady state equilibrium between
successive geomagnetic storm events determined by southward turnings of the IMF
orientation. Considering that the entry of galactic cosmic rays and the solar
energetic particles with energies above a given threshold are strongly
controlled by the intensity of the northward component of the interplanetary
magnetic field, we speculate that at earlier epochs when the geomagnetic dipole
was reversed, the entry of these energetic particles into the geomagnetic field
was facilitated when the interplanetary magnetic field was directed northward.
Unlike in other complementary work where intensive numerical simulations have
been used, our demonstration is based on a simple analytical extension of
Stormer's theory. The access of GCR and SEP beyond geomagnetic cut-off
latitudes is enhanced during epochs when the Earth's magnetic dipole is
reduced, as already demonstrated earlier.Comment: 12 pages, 3 figures (at the end of file), paper accepted for AGU
Geophysical Monograph on "Dynamics of the Earth's Radiation Belts and Inner
Magnetosphere". Editors : D.Summers, I.R.Mann, D.N.Baker, and M.Schul
An electrically powered binary star?
We propose a model for stellar binary systems consisting of a magnetic and a
non-magnetic white-dwarf pair which is powered principally by electrical
energy. In our model the luminosity is caused by resistive heating of the
stellar atmospheres due to induced currents driven within the binary. This
process is reminiscent of the Jupiter-Io system, but greatly increased in power
because of the larger companion and stronger magnetic field of the primary.
Electrical power is an alternative stellar luminosity source, following on from
nuclear fusion and accretion. We find that this source of heating is sufficient
to account for the observed X-ray luminosity of the 9.5-min binary RX J1914+24,
and provides an explanation for its puzzling characteristics.Comment: accepted by MNRA
Dark mammoth trunks in the merging galaxy NGC 1316 and a mechanism of cosmic double helices
NGC 1316 is a giant, elliptical galaxy containing a complex network of dark,
dust features. The morphology of these features has been examined in some
detail using a Hubble Space Telescope, Advanced Camera for Surveys image. It is
found that most of the features are constituted of long filaments. There also
exist a great number of dark structures protruding inwards from the filaments.
Many of these structures are strikingly similar to elephant trunks in H II
regions in the Milky Way Galaxy, although much larger. The structures, termed
mammoth trunks, generally are filamentary and often have shapes resembling the
letters V or Y. In some of the mammoth trunks the stem of the Y can be resolved
into two or more filaments, many of which showing signs of being intertwined. A
model of the mammoth trunks, related to a recent theory of elephant trunks, is
proposed. Based on magnetized filaments, the model is capable of giving an
account of the various shapes of the mammoth trunks observed, including the
twined structures.Comment: Accepted for publication in Astrophysics & Space Scienc
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