658 research outputs found
Non-linear effects in the cyclotron resonance of a massless quasi-particle in graphene
We consider the classical motion of a massless quasi-particle in a magnetic
field and under a weak electromagnetic radiation with the frequency .
Due to the non-parabolic, linear energy dispersion, the particle responds not
only at the frequency but generates a broad frequency spectrum around
it. The linewidth of the cyclotron resonance turns out to be very broad even in
a perfectly pure material which allows one to explain recent experimental data
in graphene. It is concluded that the linear response theory does not work in
graphene in finite magnetic fields.Comment: 5 pages, 4 figure
Plus Charge Prevalence in Cosmic Rays: Room for Dark Matter in the Positron Spectrum
The unexpected energy spectrum of the positron/electron ratio is interpreted
astrophysically, with a possible exception of the 100-300 GeV range. The data
indicate that this ratio, after a decline between GeV, rises steadily
with a trend towards saturation at 200-400GeV. These observations (except for
the trend) appear to be in conflict with the diffusive shock acceleration (DSA)
mechanism, operating in a \emph{single} supernova remnant (SNR) shock. We argue
that ratio can still be explained by the DSA if positrons are
accelerated in a \emph{subset} of SNR shocks which: (i) propagate in clumpy gas
media, and (ii) are modified by accelerated CR \emph{protons}. The protons
penetrate into the dense gas clumps upstream to produce positrons and,
\emph{charge the clumps positively}. The induced electric field expels
positrons into the upstream plasma where they are shock-accelerated. Since the
shock is modified, these positrons develop a harder spectrum than that of the
CR electrons accelerated in other SNRs. Mixing these populations explains the
increase in the ratio at GeV. It decreases at GeV
because of a subshock weakening which also results from the shock modification.
Contrary to the expelled positrons, most of the antiprotons, electrons, and
heavier nuclei, are left unaccelerated inside the clumps. Scenarios for the
100-300 GeV AMS-02 fraction exceeding the model prediction, including, but not
limited to, possible dark matter contribution, are also discussed.Comment: 36 pages, 6 figure
On fast radial propagation of parametrically excited geodesic acoustic mode
The spatial and temporal evolution of parametrically excited geodesic
acoustic mode (GAM) initial pulse is investigated both analytically and
numerically. Our results show that the nonlinearly excited GAM propagates at a
group velocity which is, typically, much larger than that due to finite ion
Larmor radius as predicted by the linear theory. The nonlinear dispersion
relation of GAM driven by a finite amplitude drift wave pump is also derived,
showing a nonlinear frequency increment of GAM. Further implications of these
findings for interpreting experimental observations are also discussed
Global limits on kinetic Alfv\'{e}non speed in quasineutral plasmas
Large amplitude kinetic Alfv\'{e}non (exact Alfv\'{e}n soliton) matching
condition is investigated in quasineutral electron-ion and
electron-positron-ion plasmas immersed in a uniform magnetic field. Using the
standard pseudopotential method, the magnetohydrodynamics (MHD) equations are
exactly solved and a global allowed matching condition for propagation of
kinetic solitary waves is derived. It is remarked that, depending on the plasma
parameters, the kinetic solitons can be sub- or super-Alfv\'{e}nic, in general.
It is further revealed that, either upper or lower soliton speed-limit is
independent of fractional plasma parameters. Furthermore, the soliton
propagation angle with respect to that of the uniform magnetic field is found
to play a fundamental role in controlling the soliton matching speed-range.Comment: To be published in Physics of Plasma
The effects of strong temperature anisotropy on the kinetic structure of collisionless slow shocks and reconnection exhausts. Part II: Theory
Simulations of collisionless oblique propagating slow shocks have revealed
the existence of a transition associated with a critical temperature anisotropy
epsilon=1-mu_0(P_parallel-P_perpendicular)/ B^2 = 0.25 (Liu, Drake and Swisdak
(2011)). An explanation for this phenomenon is proposed here based on
anisotropic fluid theory, in particular the Anisotropic Derivative
Nonlinear-Schrodinger-Burgers equation, with an intuitive model of the energy
closure for the downstream counter-streaming ions. The anisotropy value of 0.25
is significant because it is closely related to the degeneracy point of the
slow and intermediate modes, and corresponds to the lower bound of the coplanar
to non-coplanar transition that occurs inside a compound slow shock
(SS)/rotational discontinuity (RD) wave. This work implies that it is a pair of
compound SS/RD waves that bound the outflows in magnetic reconnection, instead
of a pair of switch-off slow shocks as in Petschek's model. This fact might
explain the rareness of in-situ observations of
Petschek-reconnection-associated switch-off slow shocks.Comment: 18 pages, 10 figure
Ion-acoustic solitons in warm magnetoplasmas with super-thermal electrons
In this work, the phenomenon of formation of localised electrostatic waves
(ESW) or soliton is considered in a warm magnetoplasma with the possibility of
non-thermal electron distribution. The parameter regime considered here is
relevant in case of magnetospheric plasmas. We show that deviation from a usual
relaxed Maxwellian distribution of the electron population has a significant
bearing in the allowed parameter regime, where these ESWs can be found. We
further consider the presence of more than one electron temperature, which is
inspired by recent space-based observations[key-2].Comment: 10 pages, 5 figure
Chaos from turbulence: stochastic-chaotic equilibrium in turbulent convection at high Rayleigh numbers
It is shown that correlation function of the mean wind velocity generated by
a turbulent thermal convection (Rayleigh number ) exhibits
exponential decay with a very long correlation time, while corresponding
largest Lyapunov exponent is certainly positive. These results together with
the reconstructed phase portrait indicate presence of chaotic component in the
examined mean wind. Telegraph approximation is also used to study relative
contribution of the chaotic and stochastic components to the mean wind
fluctuations and an equilibrium between these components has been studied in
detail
Equilibrium statistical mechanics for single waves and wave spectra in Langmuir wave-particle interaction
Under the conditions of weak Langmuir turbulence, a self-consistent
wave-particle Hamiltonian models the effective nonlinear interaction of a
spectrum of M waves with N resonant out-of-equilibrium tail electrons. In order
to address its intrinsically nonlinear time-asymptotic behavior, a Monte Carlo
code was built to estimate its equilibrium statistical mechanics in both the
canonical and microcanonical ensembles. First the single wave model is
considered in the cold beam/plasma instability and in the O'Neil setting for
nonlinear Landau damping. O'Neil's threshold, that separates nonzero
time-asymptotic wave amplitude states from zero ones, is associated to a second
order phase transition. These two studies provide both a testbed for the Monte
Carlo canonical and microcanonical codes, with the comparison with exact
canonical results, and an opportunity to propose quantitative results to
longstanding issues in basic nonlinear plasma physics. Then the properly
speaking weak turbulence framework is considered through the case of a large
spectrum of waves. Focusing on the small coupling limit, as a benchmark for the
statistical mechanics of weak Langmuir turbulence, it is shown that Monte Carlo
microcanonical results fully agree with an exact microcanonical derivation. The
wave spectrum is predicted to collapse towards small wavelengths together with
the escape of initially resonant particles towards low bulk plasma thermal
speeds. This study reveals the fundamental discrepancy between the long-time
dynamics of single waves, that can support finite amplitude steady states, and
of wave spectra, that cannot.Comment: 15 pages, 7 figures, to appear in Physics of Plasma
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