8,010 research outputs found
Parallel electric field amplification by phase-mixing of Alfven waves
Previous numerical studies have identified "phase mixing" of low-frequency
Alfven waves as a mean of parallel electric field amplification and
acceleration of electrons in a collisionless plasma. Theoretical explanations
are given of how this produces an amplification of the parallel electric field,
and as a consequence, also leads to enhanced collisionless damping of the wave
by energy transfer to the electrons. Our results are based on the properties of
the Alfven waves in a warm plasma which are obtained from drift-kinetic theory,
in particular, the rate of their electron Landau damping. Phase mixing in a
collisionless low- plasma proceeds in a manner very similar to the
visco-resistive case, except for the fact that electron Landau damping is the
primary energy dissipation channel. The time and length scales involved are
evaluated. We also focus on the evolution of the parallel electric field and
calculate its maximum value in the course of its amplification
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
How consumers’ need for variety and social consumption influences festival patronage and spending
This paper investigates the influence of motivational goals such as variety seeking and social consumption on consumers’ patronage and spending at craft beer festivals. In doing so, we develop and test a number of hypotheses by examining information collected via means of a survey questionnaire proposed in 2017 to visitors of a large beer festival in the UK. Findings of our analysis unveil how cognitive engagement affects individuals’ behavior with regard to responding to and financially engage with beer festivals. Results also identify cognitive engagement as an important mediator of the effects related to variety-seeking and social consumption. From a managerial perspective, findings reveal important attributes affecting consumers’ drivers towards craft beers, contributing to understand which dimension of consumer engagement influence their behaviors. Overall, the study provides fresh empirical evidence in terms of identifying and recognizing consumers’ behaviors with regard to defining future trends in the craft beer secto
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Investigation of LED light effects on plant growth in improved protected horticulture system
In controlled environment agriculture, energy is the predominant factor in production costs. Lighting is the one major consumers of energy. Commercial crop production in greenhouses can be enhanced by supplemental lighting which provides low moderate intensity light levels to increase photosynthesis and plant growth. Traditionally, horticultural lights were high-intensity discharge lamps such as high-pressure sodium (HPS), metal-halide (MH), and mercury (HPMV). The disadvantages of these lamps are high-energy costs, heat generation and suboptimal spectrum for photosynthesis
Suppressed star formation in circumnuclear regions in Seyfert galaxies
Feedback from black hole activity is widely believed to play a key role in
regulating star formation and black hole growth. A long-standing issue is the
relation between the star formation and fueling the supermassive black holes in
active galactic nuclei (AGNs). We compile a sample of 57 Seyfert galaxies to
tackle this issue. We estimate the surface densities of gas and star formation
rates in circumnuclear regions (CNRs). Comparing with the well-known
Kennicutt-Schmidt (K-S) law, we find that the star formation rates in CNRs of
most Seyfert galaxies are suppressed in this sample. Feedback is suggested to
explain the suppressed star formation rates.Comment: 1 color figure and 1 table. ApJ Letters in pres
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
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