160 research outputs found
Simulation study of the interaction between large-amplitude HF radio waves and the ionosphere
The time evolution of a large-amplitude electromagnetic (EM) wave injected
vertically into the overhead ionosphere is studied numerically. The EM wave has
a carrier frequency of 5 MHz and is modulated as a Gaussian pulse with a width
of approximately 0.1 milliseconds and a vacuum amplitude of 1.5 V/m at 50 km.
This is a fair representation of a modulated radio wave transmitted from a
typical high-power HF broadcast station on the ground. The pulse is propagated
through the neutral atmosphere to the critical points of the ionosphere, where
the L-O and R-X modes are reflected, and back to the neutral atmosphere. We
observe mode conversion of the L-O mode to electrostatic waves, as well as
harmonic generation at the turning points of both the R-X and L-O modes, where
their amplitudes rise to several times the original ones. The study has
relevance for ionospheric interaction experiments in combination with
ground-based and satellite or rocket observations.Comment: 6 pages, 7 figure
Conservation laws for the Maxwell-Dirac equations with a dual Ohm's law
Using a general theorem on conservation laws for arbitrary differential
equations proved by Ibragimov, we have derived conservation laws for Dirac's
symmetrized Maxwell-Lorentz equations under the assumption that both the
electric and magnetic charges obey linear conductivity laws (dual Ohm's law).
We find that this linear system allows for conservation laws which are
non-local in time
Neutrino orbital angular momentum in a plasma vortex
It is shown that an electron-neutrino beam, propagating in a background
plasma, can be decomposed into orbital momentum (OAM) states, similar to the
OAM photon states. Coupling between different OAM neutrino states, in the
presence of a plasma vortex, is considered. We show that plasma vorticity can
be transfered to the neutrino beam, which is relevant to the understanding of
the neutrino sources in astrophysics. Observation of neutrino OAM states could
eventually become possible.Comment: 4 pages. Submitted to Phys. Lett.
Zakharov simulation study of spectral features of on-demand Langmuir turbulence in an inhomogeneous plasma
We have performed a simulation study of Langmuir turbulence in the Earth's
ionosphere by means of a Zakharov model with parameters relevant for the F
layer. The model includes dissipative terms to model collisions and Landau
damping of the electrons and ions, and a linear density profile, which models
the ionospheric plasma inhomogeneity whose length scale is of the order 10--100
km. The injection of energy into the system is modeled by a constant source
term in the Zakharov equation. Langmuir turbulence is excited ``on-demand'' in
controlled ionospheric modification experiments where the energy is provided by
an HF radio beam injected into the overhead ionospheric plasma. The ensuing
turbulence can be studied with radars and in the form of secondary radiation
recorded by ground-based receivers. We have analyzed spectral signatures of the
turbulence for different sets of parameters and different altitudes relative to
the turning point of the linear Langmuir mode where the Langmuir frequency
equals the local plasma frequency. By a parametric analysis, we have derived a
simple scaling law, which links the spectral width of the turbulent frequency
spectrum to the physical parameters in the ionosphere. The scaling law provides
a quantitative relation between the physical parameters (temperatures, electron
number density, ionospheric length scale, etc.) and the observed frequency
spectrum. This law may be useful for interpreting experimental results.Comment: 7 pages, 8 figure
Linear and angular momentum of electromagnetic fields generated by an arbitrary distribution of charge and current densities at rest
Starting from Stratton-Panofsky-Phillips-Jefimenko equations for the electric
and magnetic fields generated by completely arbitrary charge and current
density distributions at rest, we derive far-zone approximations for the
fields, containing all components, dominant as well as sub-dominant. Using
these approximate formulas, we derive general formulas for the total
electromagnetic linear momentum and angular momentum, valid at large distances
from arbitrary, non-moving charge and current sources.Comment: Six pages, one figure. Errors correcte
Measurement of the spin of the M87 black hole from its observed twisted light
We present the first observational evidence that light propagating near a
rotating black hole is twisted in phase and carries orbital angular momentum
(OAM). This physical observable allows a direct measurement of the rotation of
the black hole. We extracted the OAM spectra from the radio intensity data
collected by the Event Horizon Telescope from around the black hole M87* by
using wavefront reconstruction and phase recovery techniques and from the
visibility amplitude and phase maps. This method is robust and complementary to
black-hole shadow circularity analyses. It shows that the M87* rotates
clockwise with an estimated rotation parameter with
confidence level (c.l.) and inclination , equivalent to
a magnetic arrested disk with inclination . From our
analysis we conclude, within a 6 c.l., that the M87* is rotating.Comment: Small addition on coherence. 5 pages, 2 figures Accepted for
publication in MNRAS Letter
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