1,792 research outputs found
3D simulations of gyrosynchrotron emission from mildly anisotropic nonuniform electron distributions in symmetric magnetic loops
Microwave emission of solar flares is formed primarily by incoherent
gyrosynchrotron radiation generated by accelerated electrons in coronal
magnetic loops. The resulting emission depends on many factors, including
pitch-angle distribution of the emitting electrons and the source geometry. In
this work, we perform systematic simulations of solar microwave emission using
recently developed tools (GS Simulator and fast gyrosynchrotron codes) capable
of simulating maps of radio brightness and polarization as well as spatially
resolved emission spectra. A 3D model of a symmetric dipole magnetic loop is
used. We compare the emission from isotropic and anisotropic (of loss-cone
type) electron distributions. We also investigate effects caused by
inhomogeneous distribution of the emitting particles along the loop. It is
found that effect of the adopted moderate electron anisotropy is the most
pronounced near the footpoints and it also depends strongly on the loop
orientation. Concentration of the emitting particles at the loop top results in
a corresponding spatial shift of the radio brightness peak, thus reducing
effects of the anisotropy. The high-frequency (around 50 GHz) emission spectral
index is specified mainly by the energy spectrum of the emitting electrons;
however, at intermediate frequencies (around 10-20 GHz), the spectrum shape is
strongly dependent on the electron anisotropy, spatial distribution, and
magnetic field nonuniformity. The implications of the obtained results for the
diagnostics of the energetic electrons in solar flares are discussed.Comment: ApJ in press. 20 pp, 13 figs, on-line album and simulation source
code availabl
GRB spectral parameter modeling
Fireball model of the gamma-ray bursts (GRBs) predicts generation of numerous
internal shocks, which efficiently accelerate charged particles and generate
relatively small-scale stochastic magnetic and electric fields. 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 contribution we present key results of detailed modeling of the
GRB spectral parameters, which demonstrate 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.Comment: 3 pages; IAU symposium # 274 "Advances in Plasma Astrophysics
Thermal to Nonthermal Energy Partition at the Early Rise Phase of Solar Flares
In some flares the thermal component appears much earlier than the nonthermal
component in X-ray range. Using sensitive microwave observations we revisit
this finding made by Battaglia et al. (2009) based on RHESSI data analysis. We
have found that nonthermal microwave emission produced by accelerated electrons
with energy of at least several hundred keV, appears as early as the thermal
soft X-ray emission indicative that the electron acceleration takes place at
the very early flare phase. The non-detection of the hard X-rays at that early
stage of the flares is, thus, an artifact of a limited RHESSI sensitivity. In
all considered events, the microwave emission intensity increases at the early
flare phase. We found that either thermal or nonthermal gyrosynchrotron
emission can dominate the low-frequency part of the microwave spectrum below
the spectral peak occurring at 3-10 GHz. In contrast, the high-frequency
optically thin part of the spectrum is always formed by the nonthermal,
accelerated electron component, whose power-law energy spectrum can extend up
to a few MeV at this early flare stage. This means that even though the total
number of accelerated electrons is small at this stage, their nonthermal
spectrum is fully developed. This implies that an acceleration process of
available seed particles is fully operational. While, creation of this seed
population (the process commonly called `injection' of the particles from the
thermal pool into acceleration) has a rather low efficiency at this stage,
although, the plasma heating efficiency is high. This imbalance between the
heating and acceleration (in favor of the heating) is difficult to reconcile
within most of available flare energization models. Being reminiscent of the
tradeoff between the Joule heating and runaway electron acceleration, it puts
additional constraints on the electron injection into the acceleration process.Comment: 11 pages, 12 figures, accepted for Ap
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
The Causes of Business Cycles and the Cyclicality of Real Wages
This paper estimates the cyclicality of real wages using a VAR approach. Long-run restrictions on the behavior of aggregate hours and output identify labor supply, technology, oil price, and aggregate demand shocks. It is shown that real wages are procyclical in response to technology and oil price shocks but are countercyclical in response to aggregate demand shocks. The evidence is consistent with models where nominal wages are stickier than nominal prices. The results point out the importance of looking at the cyclicality of real wages and output or the unemployment rate.Center for Research on Economic and Social Theory, Department of Economics, University of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/100713/1/ECON182.pd
Optimized gyrosynchrotron algorithms and fast codes
Gyrosynchrotron (GS) emission of charged particles spiraling in magnetic
fields plays an exceptionally important role in astrophysics. In particular,
this mechanism makes a dominant contribution to the continuum solar and stellar
radio emissions. However, the available exact equations describing the emission
process are extremely slow computationally, thus limiting the diagnostic
capabilities of radio observations. In this work, we present approximate GS
codes capable of fast calculating the emission from anisotropic electron
distributions. The computation time is reduced by several orders of magnitude
compared with the exact formulae, while the computation error remains within a
few percent. The codes are implemented as the executable modules callable from
IDL; they are made available for users via web sites.Comment: Proceedings of the IAU Symposium 274 "Advances in Plasma
Astrophysics
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