7 research outputs found
Double plasma resonance instability as a source of solar zebra emission
The double plasma resonance (DPR) instability plays a basic role in the
generation of solar radio zebras. In the plasma, consisting of the loss-cone
type distribution of hot electrons and much denser and colder background
plasma, this instability generates the upper-hybrid waves, which are then
transformed into the electromagnetic waves and observed as radio zebras. In the
present paper we numerically study the double plasma resonance instability from
the point of view of the zebra interpretation. We use a 3-dimensional
electromagnetic particle-in-cell (3-D PIC) relativistic model. First using the
multi-mode model, we study details of the double plasma resonance instability.
We show how the distribution function of hot electrons changes during this
instability. Then we show that there is a very good agreement between results
obtained by the multi-mode and specific-mode models, which is caused by a
dominance of the wave with the maximal growth rate. Therefore, for computations
in a broad range of model parameters, we use the specific-mode model. We
compute the maximal growth rates of the double plasma resonance instability.
The results are compared with the analytical ones. We find a very good
agreement between numerical and analytical growth rates. We also compute
saturation energies of the upper-hybrid waves in a very broad range of
parameters. We find that the saturation energies of the upper-hybrid waves show
maxima and minima at almost the same values of
as the growth rates. Furthermore, we
find that the saturation energy of the upper-hybrid waves is proportional to
the density of hot electrons. The maximum saturated energy can be up to one
percent of the kinetic energy of hot electrons. All these findings can be used
in the interpretation of solar radio zebras.Comment: 8 pages, 12 figure
Linear acceleration emission of pulsar relativistic streaming instability and interacting plasma bunches
Linear acceleration emission is one of the mechanisms that might explain
intense coherent radio emissions of radio pulsars. This mechanism is, however,
not well understood because the effects of collective plasma response and
nonlinear plasma evolution on the resulting emission power must be taken into
account. In addition, details of the radio emission properties of this
mechanism are unknown, which limits the observational verification of the
emission model.
By including collective and nonlinear plasma effects, we calculate radio
emission power properties by the linear acceleration emission mechanism that
occurs via the antenna principle for two instabilities in neutron star
magnetospheres: 1) a relativistic streaming instability and 2) interactions of
plasma bunches/clouds.
We utilize 1D electrostatic relativistic particle-in-cell simulations to
evolve the instabilities self-consistently. From the simulations, the power
properties of coherent emission are obtained by novel post-processing of
electric currents.
We found that the total radio power by plasma bunch interactions exceeds the
power of the streaming instability by eight orders of magnitude. The wave power
generated by a plasma bunch interaction can be as large as
~W. Therefore, simultaneously
interacting plasma bunches may account for the total observed radio power of
typical pulsars (-~W). The radio spectrum of the plasma bunch
is characterized by a flatter profile for lower frequencies and a power-law
index up to for higher frequencies. The plasma bunches
radiate in a wide range of frequencies simultaneously, fulfilling no specific
relation between emission frequency and height in the magnetosphere. The power
of the streaming instability is more narrowband than that of the interacting
bunches.Comment: 17 pages, 13 figures, 1 tabl
Refining pulsar radio emission due to streaming instabilities: Linear theory and PIC simulations in a wide parameter range
Several important mechanisms that explain the coherent pulsar radio emission
rely on streaming (or beam) instabilities of the relativistic pair plasma in a
pulsar magnetosphere. However, it is still not clear whether a streaming
instability by itself is sufficient to explain the observed coherent radio
emission. Due to the relativistic conditions that are present in the pulsar
magnetosphere, kinetic instabilities could be quenched. Moreover, uncertainties
regarding specific model-dependent parameters impede conclusions concerning
this question. We aim to constrain the possible parameter range for which a
streaming instability could lead to pulsar radio emission, focusing on the
transition between strong and weak beam models, beam drift speed, and
temperature dependence of the beam and background plasma components. We solve a
linear relativistic kinetic dispersion relation appropriate for pulsar
conditions in a more general way than in previous studies, considering a wider
parameter range. The analytical results are validated by comparison with
relativistic kinetic particle-in-cell (PIC) numerical simulations. We obtain
growth rates as a function of background and beam densities, temperatures, and
streaming velocities while finding a remarkable agreement of the linear
dispersion predictions and numerical simulation results in a wide parameter
range. Monotonous growth is found when increasing the beam-to-background
density ratio. With growing beam velocity, the growth rates firstly increase,
reach a maximum and decrease again for higher beam velocities. A monotonous
dependence on the plasma temperatures is found, manifesting in an asymptotic
behaviour when reaching colder temperatures. We show that the generated waves
are phase-coherent by calculating the fractional bandwidth. We provide an
explicit parameter range of plasma conditions for efficient pulsar radio
emission.Comment: 15 pages, 7 figures. An abridged version of the abstract is shown
here. Accepted in A&
Radio Emission by Soliton Formation in Relativistically Hot Streaming Pulsar Pair Plasmas
A number of possible pulsar radio emission mechanisms are based on streaming
instabilities in relativistically hot electron-positron pair plasmas. At
saturation the unstable waves can form, in principle, stable solitary waves
which could emit the observed intense radio signals. We searched for the proper
plasma parameters which would lead to the formation of solitons, investigated
their properties and dynamics as well as the resulting oscillations of
electrons and positrons possibly leading to radio wave emission. We utilized a
one-dimensional version of the relativistic Particle-in-Cell code ACRONYM
initialized with an appropriately parameterized one-dimensional
Maxwell-J\"uttner velocity space particle distribution to study the evolution
of the resulting streaming instability in a pulsar pair plasma. We found that
strong electrostatic superluminal L-mode solitons are formed for plasmas with
normalized inverse temperatures or relative beam drift speeds
with Lorentz factors . The parameters of the solitons fulfill the
wave emission conditions. For appropriate pulsar parameters the resulting
energy densities of superluminal solitons can reach up to
ergcm, while those of subluminal solitons reach only up to ergcm. Estimated energy densities of up to ergcm suffice to explain pulsar nanoshots.Comment: 20 pages, 15 figures, 1 tabl
Bunch Expansion as a cause for Pulsar Radio Emissions
Electromagnetic waves due to electron-positron clouds (bunches), created by
cascading processes in pulsar magnetospheres, have been proposed to explain the
pulsar radio emission. In order to verify this hypothesis, we utilized for the
first time Particle-in-Cell (PIC-) code simulations to study the nonlinear
evolution of electron-positron bunches in dependence on the relative drift
speeds of electrons and positrons, on the initial plasma temperature, and on
the initial distance between the bunches. For this sake, we utilized the
PIC-code ACRONYM with a high-order field solver and particle weighting factor,
appropriate to describe relativistic pair plasmas. We found that the bunch
expansion is mainly determined by the relative electron-positron drift speed.
Finite drift speeds were found to cause the generation of strong electric
fields that reach up to V/cm () and strong plasma heating. As a result, up
to 15~\% of the initial kinetic energy is transformed into the electric field
energy. Assuming the same electron- and positron-distributions we found that
the fastest (in the bunch reference frame) particles of consecutively emitted
bunches eventually overlap in the momentum (velocity) space. This overlap
causes two-stream instabilities that generate (electrostatic) subluminal L-mode
waves with electric field amplitudes reaching up to
V/cm (). We found that
the interaction of electron-position bunches leads to plasma heating, to the
generation of strong electric fields and of intense superluminal L-mode waves
which, in principle, can be behind the observed electromagnetic emissions of
pulsars in the radio wave range.Comment: 18 pages, 13 figure
Determining Factors of the Czech Foreign Trade Balance:Structural Issues in Trade Creation
Using panel data for 29 industries, the study tests alternative specifications of Czech export and import functions. The balance of trade is primarily influenced by the real exchange rate, aggregate demand and tariff changes. Reduced growth of the Czech economy after 1996 was an important factor that has kept the balance of trade at a sustainable level in the medium-term, contributing even to the appreciation of the real exchange rate