6,909 research outputs found
Turbulent pitch-angle scattering and diffusive transport of hard-X-ray producing electrons in flaring coronal loops
Recent observations from {\em RHESSI} have revealed that the number of
non-thermal electrons in the coronal part of a flaring loop can exceed the
number of electrons required to explain the hard X-ray-emitting footpoints of
the same flaring loop. Such sources cannot, therefore, be interpreted on the
basis of the standard collisional transport model, in which electrons stream
along the loop while losing their energy through collisions with the ambient
plasma; additional physical processes, to either trap or scatter the energetic
electrons, are required. Motivated by this and other observations that suggest
that high energy electrons are confined to the coronal region of the source, we
consider turbulent pitch angle scattering of fast electrons off low frequency
magnetic fluctuations as a confinement mechanism, modeled as a spatial
diffusion parallel to the mean magnetic field. In general, turbulent scattering
leads to a reduction of the collisional stopping distance of non-thermal
electrons along the loop and hence to an enhancement of the coronal HXR source
relative to the footpoints. The variation of source size with electron
energy becomes weaker than the quadratic behavior pertinent to collisional
transport, with the slope of depending directly on the mean free path
again pitch angle scattering. Comparing the predictions of the model
with observations, we find that cm for
keV, less than the length of a typical flaring loop and smaller than, or
comparable to, the size of the electron acceleration region.Comment: 25 pages, 5 figures, accepted for publication in Astrophysical
Journa
Collisional relaxation of electrons in a warm plasma and accelerated nonthermal electron spectra in solar flares
Extending previous studies of nonthermal electron transport in solar flares
which include the effects of collisional energy diffusion and thermalization of
fast electrons, we present an analytic method to infer more accurate estimates
of the accelerated electron spectrum in solar flares from observations of the
hard X-ray spectrum. Unlike for the standard cold-target model, the spatial
characteristics of the flaring region, especially the necessity to consider a
finite volume of hot plasma in the source, need to be taken into account in
order to correctly obtain the injected electron spectrum from the
source-integrated electron flux spectrum (a quantity straightforwardly obtained
from hard X-ray observations). We show that the effect of electron
thermalization can be significant enough to nullify the need to introduce an
{\it ad hoc} low-energy cutoff to the injected electron spectrum in order to
keep the injected power in non-thermal electrons at a reasonable value. Rather
the suppression of the inferred low-energy end of the injected spectrum
compared to that deduced from a cold-target analysis allows the inference from
hard X-ray observations of a more realistic energy in injected non-thermal
electrons in solar flares.Comment: accepted for publication in Ap
Wave-particle interactions in non-uniform plasma and the interpretation of Hard X-ray spectra in solar flares
Context. High energy electrons accelerated during solar flare are abundant in
the solar corona and in the interplanetary space. Commonly, the number and the
energy of non-thermal electrons at the Sun is estimated using hard X-ray (HXR)
spectral observations (e.g. RHESSI) and a single-particle collisional
approximation. Aims. To investigate the role of the spectrally evolving
Langmuir turbulence on the population of energetic electrons in the solar
corona. Methods. We numerically simulate the relaxation of a power-law
non-thermal electron population in a collisional inhomogeneous plasma including
wave-particle, and wave-wave interactions. Results. The numerical simulations
show that the long-time evolution of electron population above 20 keV deviates
substantially from the collisional approximation when wave-particle
interactions in non-uniform plasma are taken into account. The evolution of
Langmuir wave spectrum towards smaller wavenumbers, due to large-scale density
fluctuations and wave-wave interactions, leads to an effective acceleration of
electrons. Furthermore, the time-integrated spectrum of non-thermal electrons,
which is normally observed with HXR above 20 keV, is noticeably increased due
to acceleration of non-thermal electrons by Langmuir waves. Conclusions. The
results show that the observed HXR spectrum, when interpreted in terms of
collisional relaxation, can lead to an overestimated number and energy of
energetic electrons accelerated in the corona.Comment: 8 pages, 6 figures, submitted to Astronomy and Astrophysics Journa
A Fokker-Planck Framework for Studying the Diffusion of Radio Burst Waves in the Solar Corona
Electromagnetic wave scattering off density inhomogeneities in the solar
corona is an important process which determines both the apparent source size
and the time profile of radio bursts observed at 1 AU. Here we model the
scattering process using a Fokker-Planck equation and apply this formalism to
several regimes of interest. In the first regime the density fluctuations are
considered quasi-static and diffusion in wavevector space is dominated by
angular diffusion on the surface of a constant energy sphere. In the
small-angle ("pencil beam") approximation, this diffusion further occurs over a
small solid angle in wavevector space. The second regime corresponds to a much
later time, by which scattering has rendered the photon distribution
near-isotropic resulting in a spatial diffusion of the radiation. The third
regime involves time-dependent fluctuations and, therefore, Fermi acceleration
of photons. Combined, these results provide a comprehensive theoretical
framework within which to understand several important features of propagation
of radio burst waves in the solar corona: emitted photons are accelerated in a
relatively small inner region and then diffuse outwards to larger distances. En
route, angular diffusion results both in source sizes which are substantially
larger than the intrinsic source, and in observed intensity-versus-time
profiles that are asymmetric, with a sharp rise and an exponential decay. Both
of these features are consistent with observations of solar radio bursts.Comment: 28 pages , 1 figure, submitted to Ap
A Fokker-Planck Framework for Studying the Diffusion of Radio Burst Waves in the Solar Corona
Electromagnetic wave scattering off density inhomogeneities in the solar
corona is an important process which determines both the apparent source size
and the time profile of radio bursts observed at 1 AU. Here we model the
scattering process using a Fokker-Planck equation and apply this formalism to
several regimes of interest. In the first regime the density fluctuations are
considered quasi-static and diffusion in wavevector space is dominated by
angular diffusion on the surface of a constant energy sphere. In the
small-angle ("pencil beam") approximation, this diffusion further occurs over a
small solid angle in wavevector space. The second regime corresponds to a much
later time, by which scattering has rendered the photon distribution
near-isotropic resulting in a spatial diffusion of the radiation. The third
regime involves time-dependent fluctuations and, therefore, Fermi acceleration
of photons. Combined, these results provide a comprehensive theoretical
framework within which to understand several important features of propagation
of radio burst waves in the solar corona: emitted photons are accelerated in a
relatively small inner region and then diffuse outwards to larger distances. En
route, angular diffusion results both in source sizes which are substantially
larger than the intrinsic source, and in observed intensity-versus-time
profiles that are asymmetric, with a sharp rise and an exponential decay. Both
of these features are consistent with observations of solar radio bursts.Comment: 28 pages , 1 figure, submitted to Ap
GAP: From sound design to practical implementation in clinical trials for traditional chinese medicine
The past few years have witnessed encouraging progress in improving the methodological quality of clinical research of traditional Chinese medicine (TCM). This improvement has contributed to wider academic acceptance of the findings of TCM clinical studies, which were previously deemed dubious. As a proof of this statement, one clinical study testing the effects of a Chinese patent drug Qili Qiangxin Capsules on chronic heart failure has just published a research article on the Journal of the American College of Cardiology, a medical journal of international prestige. However, a sound and scientific design does not always see to its practicality in the conduct of the study, and in fact we observed a widening gap between the two elements. In this special issue, we called for papers discussing efforts to bridge the gap between scientific design and practical implementation of clinical research with TCM
Performance Analysis and Optimization of Sizable 6-Axis Force Sensor Based on Stewart Platform
Parallel electric field generation by Alfven wave turbulence
{This work aims to investigate the spectral structure of the parallel
electric field generated by strong anisotropic and balanced Alfvenic turbulence
in relation with the problem of electron acceleration from the thermal
population in solar flare plasma conditions.} {We consider anisotropic Alfvenic
fluctuations in the presence of a strong background magnetic field. Exploiting
this anisotropy, a set of reduced equations governing non-linear, two-fluid
plasma dynamics is derived. The low- limit of this model is used to
follow the turbulent cascade of the energy resulting from the non-linear
interaction between kinetic Alfven waves, from the large magnetohydrodynamics
(MHD) scales with down to the small "kinetic" scales
with , being the ion sound gyroradius.}
{Scaling relations are obtained for the magnitude of the turbulent
electromagnetic fluctuations, as a function of and ,
showing that the electric field develops a component parallel to the magnetic
field at large MHD scales.} {The spectrum we derive for the parallel electric
field fluctuations can be effectively used to model stochastic resonant
acceleration and heating of electrons by Alfven waves in solar flare plasma
conditions
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