11,053 research outputs found
Effects of Volcanic Emissions on Clouds During Kilauea Degassing Events
Aerosols influence Earths radiative balance directly by scattering and absorbing solar radiation, and indirectly by modifying cloud properties. Current scientific consensus indicates that these effects may offset as much as 50% of the warming due to greenhouse gas emissions. Over the last two decades dramatic volcanic events in Hawaii have produced localized aerosol emissions in otherwise clean environments. These are natural experiments" where the aerosol effects on clouds and climate can be partitioned from other effects like meteorology and industrial emissions. Therefore, these events provide a unique opportunity to learn about possible effects of aerosol pollution on climate through cloud modification. In this work we use the version 5 of the NASA Goddard Earth Observing System (GEOS-5) and satellite retrievals to analyze and evaluate the strength of the aerosol indirect effect on liquid and ice clouds during the 2008 and 2018 Kilauea degassing events using different emissions scenarios (0, 1, and 5 actual emissions). Our results suggested that the 2018 event was stronger and more regionally significant with respect to cloud formation process for both liquid and ice clouds, while the 2008 affected local liquid clouds only. GEOS-5 predictions reproduced spatial patterns for all parameters, however better precision could be gained by using more accurate plume parameters for height and ash concentration
The role of diffusion in the transport of energetic electrons during solar flares
The transport of the energy contained in suprathermal electrons in solar
flares plays a key role in our understanding of many aspects of flare physics,
from the spatial distributions of hard X-ray emission and energy deposition in
the ambient atmosphere to global energetics. Historically the transport of
these particles has been largely treated through a deterministic approach, in
which first-order secular energy loss to electrons in the ambient target is
treated as the dominant effect, with second-order diffusive terms (in both
energy and angle) being generally either treated as a small correction or even
neglected. We here critically analyze this approach, and we show that spatial
diffusion through pitch-angle scattering necessarily plays a very significant
role in the transport of electrons. We further show that a satisfactory
treatment of the diffusion process requires consideration of non-local effects,
so that the electron flux depends not just on the local gradient of the
electron distribution function but on the value of this gradient within an
extended region encompassing a significant fraction of a mean free path. Our
analysis applies generally to pitch-angle scattering by a variety of
mechanisms, from Coulomb collisions to turbulent scattering. We further show
that the spatial transport of electrons along the magnetic field of a flaring
loop can be modeled rather effectively as a Continuous Time Random Walk with
velocity-dependent probability distribution functions of jump sizes and
occurrences, both of which can be expressed in terms of the scattering mean
free path.Comment: 11 pages, to be published in Astrophysical Journa
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Twin roll casting and melt conditioned twin-roll casting of magnesium alloys
Recently, BCAST at Brunel University has developed a MCAST (melt conditioning by advanced shear technology) process for conditioning liquid metal at temperature either above or bellow the alloy liquidus using a high shear twin-screw mechanism. The MCAST process has now been combined with the twin roll casting (TRC) process to form an innovative technology, namely, the melt conditioned twin roll casting (MC-TRC) process for casting Al-alloy and Mg-alloy strips. During the MC-TRC process, liquid alloy with a specified temperature is continuously fed into the MCAST machine. By intensive shearing under the high shear rate and high intensity of turbulence, the liquid is transformed into conditioned melt with uniform temperature and composition throughout the whole volume. The conditioned melt is then fed continuously into the twin-roll caster for strip production. The experimental results show that the AZ91D MC-TRC strips with different thicknesses have fine and uniform microstructure. The strip consists of equiaxed grains with a mean size of 60-70μm. The strip displays extremely uniform grain size and composition throughout the whole cross-section. Investigation also shows that both TRC and MC-TRC processes with reduced deformation are effective to reduce the formation of defects, particularly the formation of the central line segregations
Suppression of parallel transport in turbulent magnetized plasmas and its impact on the non-thermal and thermal aspects of solar flares
The transport of the energy contained in electrons, both thermal and suprathermal, in solar flares plays a key role in our understanding of many aspects of the flare phenomenon, from the spatial distribution of hard X-ray emission to global energetics. Motivated by recent RHESSI observations that point to the existence of a mechanism that confines electrons to the coronal parts of flare loops more effectively than Coulomb collisions, we here consider the impact of pitch-angle scattering off turbulent magnetic fluctuations on the parallel transport of electrons in flaring coronal loops. It is shown that the presence of such a scattering mechanism in addition to Coulomb collisional scattering can significantly reduce the parallel thermal and electrical conductivities relative to their collisional values. We provide illustrative expressions for the resulting thermoelectric coefficients that relate the thermal flux and electrical current density to the temperature gradient and the applied electric field. We then evaluate the effect of these modified transport coefficients on the flare coronal temperature that can be attained, on the post-impulsive-phase cooling of heated coronal plasma, and on the importance of the beam-neutralizing return current on both ambient heating and the energy loss rate of accelerated electrons. We also discuss the possible ways in which anomalous transport processes have an impact on the required overall energy associated with accelerated electrons in solar flares
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
On the variation of solar flare coronal x-ray source sizes with energy
Observations with {\em RHESSI} have enabled the detailed study of the
structure of dense hard X-ray coronal sources in solar flares. The variation of
source extent with electron energy has been discussed in the context of
streaming of non-thermal particles in a one-dimensional cold-target model, and
the results used to constrain both the physical extent of, and density within,
the electron acceleration region. Here we extend this investigation to a more
physically realistic model of electron transport that takes into account the
finite temperature of the ambient plasma, the initial pitch-angle distribution
of the accelerated electrons, and the effects of collisional pitch-angle
scattering. The finite temperature results in the thermal diffusion of
electrons, that leads to the observationally-inferred value of the acceleration
region volume being an overestimate of its true value. The different directions
of the electron trajectories, a consequence of both the non-zero injection
pitch-angle and scattering within the target, cause the projected propagation
distance parallel to the guiding magnetic field to be reduced, so that a
one-dimensional interpretation can overestimate the actual density by a factor
of up to . The implications of these results for the determination of
acceleration region properties (specific acceleration rate, filling factor,
etc.) are discussed.Comment: 45 pages, 9 figures, accepted for publication in Ap
Adversarial Sparse-View CBCT Artifact Reduction
We present an effective post-processing method to reduce the artifacts from
sparsely reconstructed cone-beam CT (CBCT) images. The proposed method is based
on the state-of-the-art, image-to-image generative models with a perceptual
loss as regulation. Unlike the traditional CT artifact-reduction approaches,
our method is trained in an adversarial fashion that yields more perceptually
realistic outputs while preserving the anatomical structures. To address the
streak artifacts that are inherently local and appear across various scales, we
further propose a novel discriminator architecture based on feature pyramid
networks and a differentially modulated focus map to induce the adversarial
training. Our experimental results show that the proposed method can greatly
correct the cone-beam artifacts from clinical CBCT images reconstructed using
1/3 projections, and outperforms strong baseline methods both quantitatively
and qualitatively
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
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