858 research outputs found
Diffusive radiation in Langmuir turbulence produced by jet shocks
Anisotropic distributions of charged particles including two-stream
distributions give rise to generation of either stochastic electric fields (in
the form of Langmuir waves, Buneman instability) or random quasi-static
magnetic fields (Weibel and filamentation instabilities) or both. These
two-stream instabilities are known to play a key role in collisionless shock
formation, shock-shock interactions, and shock-induced electromagnetic
emission. This paper applies the general non-perturbative stochastic theory of
radiation to study electromagnetic emission produced by relativistic particles,
which random walk in the stochastic electric fields of the Langmuir waves. This
analysis takes into account the cumulative effect of uncorrelated Langmuir
waves on the radiating particle trajectory giving rise to angular diffusion of
the particle, which eventually modifies the corresponding radiation spectra. We
demonstrate that the radiative process considered is probably relevant for
emission produced in various kinds of astrophysical jets, in particular, prompt
gamma-ray burst spectra, including X-ray excesses and prompt optical flashes.Comment: 9 pages, 5 figures, MNRAS, accepte
GRB spectral parameters within the fireball model
Fireball model of the GRBs predicts generation of numerous internal shocks,
which then efficiently accelerate charged particles and generate magnetic and
electric fields. These fields are produced in the form of relatively
small-scale stochastic ensembles of waves, thus, the accelerated particles
diffuse in space due to interaction with the random waves and so emit so called
Diffusive Synchrotron Radiation (DSR) in contrast to standard synchrotron
radiation they would produce in a large-scale regular magnetic fields. In this
paper we present first results of comprehensive modeling of the GRB spectral
parameters within the fireball/internal shock concept. We have found that the
non-perturbative DSR emission mechanism in a strong random magnetic field is
consistent with observed distributions of the Band parameters and also with
cross-correlations between them; this analysis allowed to restrict GRB physical
parameters from the requirement of consistency between the model and observed
distributions.Comment: 14 pages, 17 figures, MNRAS in pres
Strongest coronal magnetic fields in solar cycles 23-24: probing, statistics, and implications
Strong coronal magnetic field, when present, manifests itself as bright
microwave sources at high frequencies produced by gyroresonant (GR) emission
mechanism in thermal coronal plasma. The highest frequency at which this
emission is observed is proportional to the absolute value of the strongest
coronal magnetic field on the line of sight. Although no coronal magnetic field
larger than roughly 2,000 G was expected, recently the field at least twice
larger has been reported. Here, we report a search for and statistical study of
such strong coronal magnetic fields using high-frequency GR emission. A
historic record of spatially resolved microwave observations at high
frequencies, 17 and 34 GHz, is available from Nobeyama RadioHeliograph for more
than 20 years (1995-2018). Here we employ this data set to identify sources of
bright GR emission at 34 GHz and perform a statistical analysis of the
identified GR cases to quantify the strongest coronal magnetic fields during
two solar cycles. We found that although active regions with the strong
magnetic field are relatively rare (less than 1% of all active regions), they
appear regularly on the Sun. These active regions are associated with prominent
manifestations of solar activity
Radio Spectral Evolution of an X-ray Poor Impulsive Solar Flare: Implications for Plasma Heating and Electron Acceleration
We present radio and X-ray observations of an impulsive solar flare that was
moderately intense in microwaves, yet showed very meager EUV and X-ray
emission. The flare occurred on 2001 Oct 24 and was well-observed at radio
wavelengths by the Nobeyama Radioheliograph (NoRH), the Nobeyama Radio
Polarimeters (NoRP), and by the Owens Valley Solar Array (OVSA). It was also
observed in EUV and X-ray wavelength bands by the TRACE, GOES, and Yohkoh
satellites. We find that the impulsive onset of the radio emission is
progressively delayed with increasing frequency relative to the onset of hard
X-ray emission. In contrast, the time of flux density maximum is progressively
delayed with decreasing frequency. The decay phase is independent of radio
frequency. The simple source morphology and the excellent spectral coverage at
radio wavelengths allowed us to employ a nonlinear chi-squared minimization
scheme to fit the time series of radio spectra to a source model that accounts
for the observed radio emission in terms of gyrosynchrotron radiation from
MeV-energy electrons in a relatively dense thermal plasma. We discuss plasma
heating and electron acceleration in view of the parametric trends implied by
the model fitting. We suggest that stochastic acceleration likely plays a role
in accelerating the radio-emitting electrons.Comment: 22 pages, 10 figure
Broadband microwave burst produced by electron beams
Theoretical and experimental study of fast electron beams attracts a lot of
attention in the astrophysics and laboratory. In the case of solar flares the
problem of reliable beam detection and diagnostics is of exceptional
importance. This paper explores the fact that the electron beams moving oblique
to the magnetic field or along the field with some angular scatter around the
beam propagation direction can generate microwave continuum bursts via
gyrosynchrotron mechanism. The characteristics of the microwave bursts produced
by beams differ from those in case of isotropic or loss-cone distributions,
which suggests a new tool for quantitative diagnostics of the beams in the
solar corona. To demonstrate the potentiality of this tool, we analyze here a
radio burst occurred during an impulsive flare 1B/M6.7 on 10 March 2001 (AR
9368, N27W42). Based on detailed analysis of the spectral, temporal, and
spatial relationships, we obtained firm evidence that the microwave continuum
burst is produced by electron beams. For the first time we developed and
applied a new forward fitting algorithm based on exact gyrosynchrotron formulae
and employing both the total power and polarization measurements to solve the
inverse problem of the beam diagnostics. We found that the burst is generated
by a oblique beam in a region of reasonably strong magnetic field ( G) and the burst is observed at a quasi-transverse viewing angle. We
found that the life time of the emitting electrons in the radio source is
relatively short, s, consistent with a single reflection
of the electrons from a magnetic mirror at the foot point with the stronger
magnetic field. We discuss the implications of these findings for the electron
acceleration in flares and for beam diagnostics.Comment: Astrophysical Journal, accepted: 26 pages, 8 figure
Surface plasmon in 2D Anderson insulator with interactions
We study the effect of interactions on the zero-temperature a.c. conductivity
of 2D Anderson insulator at low frequencies. We show that the enhancement of
the real part of conductivity due to the Coulomb correlations in the occupation
numbers of localized states results in the change of the sign of imaginary part
within a certain frequency range. As a result, the propagation of a surface
plasmon in a localized system becomes possible. We analize the dispersion law
of the plasmon for the two cases: unscreened Coulomb interactions and the
interactions screened by a gate electrode spaced by some distance from the
electron plane.Comment: latex 22 pages + 2 uuencoded figure
Diffusive Radiation in One-dimensional Langmuir Turbulence
We calculate spectra of radiation produced by a relativistic particle in the
presence of one-dimensional Langmuir turbulence which might be generated by a
streaming instability in the plasma, in particular, in the shock front or at
the shock-shock interactions. The shape of the radiation spectra is shown to
depend sensitively on the angle between the particle velocity and electric
field direction. The radiation spectrum in the case of exactly transverse
particle motion is degenerate and similar to that of spatially uniform Langmuir
oscillations. In case of oblique propagation, the spectrum is more complex, it
consists of a number of power-law regions and may contain a distinct
high-frequency spectral peak. %at \omega=2\omega\pe \gamma^2. The emission
process considered is relevant to various laboratory plasma settings and for
astrophysical objects as gamma-ray bursts and collimated jets.Comment: 4 pages, 1 figure, accepted for Phys. Rev.
Spatiotemporal energy partitioning in a nonthermally dominated two-loop solar flare
Solar flares show remarkable variety in the energy partitioning between thermal and nonthermal components. Those with a prominent nonthermal component but only a modest thermal one are particularly well suited for study of the direct effect of the nonthermal electrons on plasma heating. Here, we analyze such a well-observed, impulsive single-spike nonthermal event, a solar flare SOL2013-11-05T035054, where the plasma heating can be entirely attributed to the energy losses of these impulsively accelerated electrons. Evolution of the energy budget of thermal and nonthermal components during the flare is analyzed using X-ray, microwave, and EUV observations and three-dimensional modeling. The results suggest that (i) the flare geometry is consistent with a two-loop morphology and the magnetic energy is likely released due to interaction between these two loops; (ii) the released magnetic energy is converted to the nonthermal energy of accelerated electrons only, which is subsequently converted to the thermal energy of the plasma; (iii) the energy is partitioned in these two flaring loops in comparable amounts; (iv) one of these flaring loops remained relatively tenuous but rather hot, while the other remained relatively cool but denser than the first. Therefore, this solar flare demonstrates an extreme efficiency of conversion of the free magnetic energy to the nonthermal energy of particle acceleration and the flow of energy into two loops from the nonthermal component to the thermal one with negligible direct heating
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