436 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
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 ionâneutral 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
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
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
The Role of Current Sheet Scattering in the Proton Isotropic Boundary Formation During Geomagnetic Storms
There is considerable evidence that current sheet scattering (CSS) plays an important role in isotropic boundary (IB) formation during quiet time. However, IB formation can also result from scattering by electromagnetic ion cyclotron waves, which are much more prevalent during storm time. The effectiveness of CSS can be estimated by the parameter K=Rcrg, the ratio of the field line radius of curvature to the particle gyroradius. Using magnetohydrodynamic and empirical models, we estimated the parameter K associated with storm time IB observations on the nightside. We used magnetic field observations from spacecraft in the magnetotail to estimate and correct for errors in the K values computed by the models. We find that the magnetohydrodynamic and empirical models produce fairly similar results without correction and that correction increases this similarity. Accounting for uncertainty in both the latitude of the IB and the threshold value of K required for CSS, we found that 29â54% of the IB observations satisfied the criteria for CSS. We found no correlation between the corrected K and magnetic local time, which further supports the hypothesis that CSS played a significant role in forming the observed IBs.Key PointsWe corrected estimates of the adiabaticity parameter K using in situ data, obtaining consistent results between empirical and MHD modelsA significant fraction of the K estimates was below the threshold for current sheet scatteringCorrected K estimates are independent of SymâH* and MLT, even for high values of KPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149533/1/jgra54932.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149533/2/jgra54932_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/149533/3/jgra54932-sup-0001-supplementary.pd
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
On Molecular Hydrogen Formation and the Magnetohydrostatic Equilibrium of Sunspots
We have investigated the problem of sunspot magnetohydrostatic equilibrium
with comprehensive IR sunspot magnetic field survey observations of the highly
sensitive Fe I lines at 15650 \AA\ and nearby OH lines. We have found that some
sunspots show isothermal increases in umbral magnetic field strength which
cannot be explained by the simplified sunspot model with a single-component
ideal gas atmosphere assumed in previous investigations. Large sunspots
universally display non-linear increases in magnetic pressure over temperature,
while small sunspots and pores display linear behavior. The formation of
molecules provides a mechanism for isothermal concentration of the umbral
magnetic field, and we propose that this may explain the observed rapid
increase in umbral magnetic field strength relative to temperature. Existing
multi-component sunspot atmospheric models predict that a significant amount of
molecular hydrogen (H2) exists in the sunspot umbra. The formation of H2 can
significantly alter the thermodynamic properties of the sunspot atmosphere and
may play a significant role in sunspot evolution. In addition to the survey
observations, we have performed detailed chemical equilibrium calculations with
full consideration of radiative transfer effects to establish OH as a proxy for
H2, and demonstrate that a significant population of H2 exists in the coolest
regions of large sunspots.Comment: 17 pages, 19 figures, accepted for publication in Ap
Gradient expansion, curvature perturbations and magnetized plasmas
The properties of magnetized plasmas are always investigated under the
hypothesis that the relativistic inhomogeneities stemming from the fluid
sources and from the geometry itself are sufficiently small to allow for a
perturbative description prior to photon decoupling. The latter assumption is
hereby relaxed and pre-decoupling plasmas are described within a suitable
expansion where the inhomogeneities are treated to a given order in the spatial
gradients. It is argued that the (general relativistic) gradient expansion
shares the same features of the drift approximation, customarily employed in
the description of cold plasmas, so that the two schemes are physically
complementary in the large-scale limit and for the low-frequency branch of the
spectrum of plasma modes. The two-fluid description, as well as the
magnetohydrodynamical reduction, are derived and studied in the presence of the
spatial gradients of the geometry. Various solutions of the coupled system of
evolution equations in the anti-Newtonian regime and in the quasi-isotropic
approximation are presented. The relation of this analysis to the so-called
separate Universe paradigm is outlined. The evolution of the magnetized
curvature perturbations in the nonlinear regime is addressed for the magnetized
adiabatic mode in the plasma frame.Comment: 40 pages, no figure
ExB mean flows in finite ion temperature plasmas
The impact of ion pressure dynamics on E x B mean flows is investigated.
Using a simplified, two-dimensional, drift ordered fluid model in the
thin-layer approximation, three stresses in addition to the Reynolds stress are
shown to modify the E x B mean flow. These additional terms in the stress
tensor all require ion pressure fluctuations. Quasi-linear analysis show that
these additional stresses are as important as the Reynolds stress, and hence
must be taken into account in analysis of transport barriers in which sheared E
x B mean flows are key ingredients
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