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Search Results: 1 - 10 of 4523 matches for " Henry; Kontar "
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Submarine groundwater discharge (SGD) patterns through a fractured rock: a case study in the Ubatuba coastal area, Brazil
Bokuniewicz,Henry; Kontar,Evgeny; Rodrigues,Marcelo; Klein,Daniel Andreas;
Revista de la Asociaci?3n Argentina de Sedimentolog?-a , 2004,
Abstract: the flow of groundwater out across the sea floor has the potential to influence sedimentary processes, sea floor morphology, pore water chemistry and benthic habitats. relatively few observations of the process of submarine groundwater discharge (sgd) have been made. measurements along the south american coast and over fractured rock aquifers are especially rare. the rate and distribution of sgd was measured using vented, benthic chambers on the floor of flamengo bay located at the southeast coast of brazil. discharge rates were found exceeding 200 cm3 s-1 of pore water per cm2 of sea floor per day (200 cm day-1). large variations in sgd rates were seen over distances of a few meters. we attribute the variation to the geomorphologic features of the fracture rock aquifer underlying a thin blanket of coastal sediments. clustering of fractures and the topography of the rock-sediment interface might be focusing or dispersing the discharge of groundwater.
Submarine groundwater discharge (SGD) patterns through a fractured rock: a case study in the Ubatuba coastal area, Brazil
Henry Bokuniewicz,Evgeny Kontar,Marcelo Rodrigues,Daniel Andreas Klein
Revista de la Asociación Argentina de Sedimentología , 2004,
Abstract: The flow of groundwater out across the sea floor has the potential to influence sedimentary processes, sea floor morphology, pore water chemistry and benthic habitats. Relatively few observations of the process of submarine groundwater discharge (SGD) have been made. Measurements along the South American coast and over fractured rock aquifers are especially rare. The rate and distribution of SGD was measured using vented, benthic chambers on the floor of Flamengo Bay located at the southeast coast of Brazil. Discharge rates were found exceeding 200 cm3 s-1 of pore water per cm2 of sea floor per day (200 cm day-1). Large variations in SGD rates were seen over distances of a few meters. We attribute the variation to the geomorphologic features of the fracture rock aquifer underlying a thin blanket of coastal sediments. Clustering of fractures and the topography of the rock-sediment interface might be focusing or dispersing the discharge of groundwater.
Implications of X-ray Observations for Electron Acceleration and Propagation in Solar Flares
Gordon D. Holman,Markus J. Aschwanden,Henry Aurass,Marina Battaglia,Paolo C. Grigis,Eduard P. Kontar,Wei Liu,Pascal Saint-Hilaire,Valentina V. Zharkova
Physics , 2011, DOI: 10.1007/s11214-010-9680-9
Abstract: High-energy X-rays and gamma-rays from solar flares were discovered just over fifty years ago. Since that time, the standard for the interpretation of spatially integrated flare X-ray spectra at energies above several tens of keV has been the collisional thick-target model. After the launch of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in early 2002, X-ray spectra and images have been of sufficient quality to allow a greater focus on the energetic electrons responsible for the X-ray emission, including their origin and their interactions with the flare plasma and magnetic field. The result has been new insights into the flaring process, as well as more quantitative models for both electron acceleration and propagation, and for the flare environment with which the electrons interact. In this article we review our current understanding of electron acceleration, energy loss, and propagation in flares. Implications of these new results for the collisional thick-target model, for general flare models, and for future flare studies are discussed.
Stochastic acceleration by multi-island contraction during turbulent magnetic reconnection
Nicolas Bian,Eduard Kontar
Physics , 2013, DOI: 10.1103/PhysRevLett.110.151101
Abstract: The acceleration of charged particles in magnetized plasmas is considered during turbulent multi-island magnetic reconnection. The particle acceleration model is constructed for an ensemble of islands which produce adiabatic compression of the particles. The model takes into account the statistical fluctuations in the compression rate experienced by the particles during their transport in the acceleration region. The evolution of the particle distribution function is described as a simultaneous first and second-order Fermi acceleration process. While the efficiency of the first-order process is controlled by the average rate of compression, the second order process involves the variance in the compression rate. Moreover, the acceleration efficiency associated with the second-order process involves both the Eulerian properties of the compression field and the Lagrangian properties of the particles. The stochastic contribution to the acceleration is non-resonant and can dominate the systematic part in the case of a large variance in the compression rate. The model addresses the role of the second-order process, how the latter can be related to the large-scale turbulent transport of particles and explains some features of the numerical simulations of particle acceleration by multi-island contraction during magnetic reconnection.
Parallel electric field amplification by phase-mixing of Alfven waves
N. Bian,E. Kontar
Physics , 2010, DOI: 10.1051/0004-6361/201015385
Abstract: 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-$\beta$ 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.
Spatially-resolved Energetic Electron Properties for the 21 May 2004 Flare from Radio Observations and 3D Simulations
Alexey Kuznetsov,Eduard Kontar
Physics , 2014, DOI: 10.1007/s11207-014-0530-x
Abstract: We investigate in detail the 21 May 2004 flare using simultaneous observations of the Nobeyama Radioheliograph, Nobeyama Radiopolarimeters, Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Solar and Heliospheric Observatory (SOHO). The flare images in different spectral ranges reveal the presence of a well-defined single flaring loop in this event. We have simulated the gyrosynchrotron microwave emission using the recently developed interactive IDL tool GX Simulator. By comparing the simulation results with the observations, we have deduced the spatial and spectral properties of the non-thermal electron distribution. The microwave emission has been found to be produced by the high-energy electrons ($>100$ keV) with a relatively hard spectrum ($\delta\simeq 2$); the electrons were strongly concentrated near the loop top. At the same time, the number of high-energy electrons near the footpoints was too low to be detected in the RHESSI images and spatially unresolved data. The SOHO Extreme-ultraviolet Imaging Telescope images and the low-frequency microwave spectra suggest the presence of an extended "envelope" of the loop with lower magnetic field. Most likely, the energetic electron distribution in the considered flare reflects the localized (near the loop top) particle acceleration (injection) process accompanied by trapping and scattering.
Spatially resolved hard X-ray polarization in solar flares: effects of Compton scattering and bremsstrahlung
Natasha Jeffrey,Eduard Kontar
Physics , 2011, DOI: 10.1051/0004-6361/201117987
Abstract: This paper aims to study the polarization of hard X-ray (HXR) sources in the solar atmosphere, including Compton backscattering of photons in the photosphere (the albedo effect) and the spatial distribution of polarization across the source. HXR photon polarization and spectra produced via electron-ion bremsstrahlung are calculated from electron distributions typical for solar flares. Compton scattering and photoelectric absorption are then modelled using Monte Carlo simulations of photon transport in the photosphere. Polarization maps across HXR sources (primary and albedo components) for each of the modelled electron distributions are calculated at various source locations from the solar centre to the limb. We show that Compton scattering produces a distinct polarization variation across the albedo patch at peak albedo energies of 20-50 keV for all anisotropies modelled. The results show that there are distinct spatial polarization changes in both the radial and perpendicular to radial directions across the extent of the HXR source at a given disk location. In the radial direction, the polarization magnitude and direction at specific positions along the HXR source will either increase or decrease with increased photon distribution directivity towards the photosphere. We also show how high electron cutoff energies influence the direction of polarization at above ~100 keV. Spatially resolved HXR polarization measurements can provide important information about the directivity and energetics of the electron distribution. Our results indicate the preferred angular resolution of polarization measurements required to distinguish between the scattered and primary components. We also show how spatially resolved polarization measurements could be used to probe the emission pattern of an HXR source, using both the magnitude and the direction of polarization.
Electron Distribution Functions in Solar Flares from combined X-ray and EUV Observations
Marina Battaglia,Eduard P. Kontar
Physics , 2013, DOI: 10.1088/0004-637X/779/2/107
Abstract: Simultaneous solar flare observations with SDO and RHESSI provide spatially resolved information about hot plasma and energetic particles in flares. RHESSI allows the properties of both hot (> 8 MK) thermal plasma and nonthermal electron distributions to be inferred, while SDO/AIA is more sensitive to lower temperatures. We present and implement a new method to reconstruct electron distribution functions from SDO/AIA data. The combined analysis of RHESSI and AIA data allows the electron distribution function to be inferred over the broad energy range from ~0.1 keV up to a few tens of keV. The analysis of two well observed flares suggests that the distributions in general agree to within a factor of three when the RHESSI values are extrapolated into the intermediate range 1-3 keV, with AIA systematically predicting lower electron distributions. Possible instrumental and numerical effects, as well as potential physical origins for this discrepancy are discussed. The inferred electron distribution functions in general show one or two nearly Maxwellian components at energies below ~ 15 keV and a non-thermal tail above.
Height structure of X-ray, EUV and white-light emission in a solar flare
Marina Battaglia,Eduard P. Kontar
Physics , 2011, DOI: 10.1051/0004-6361/201117605
Abstract: The bulk of solar flare emission originates from very compact sources located in the lower solar atmosphere and seen in various wavelength ranges: near optical, UV, EUV, soft and hard X-rays, and gamma-ray emission, yet very few spatially resolved imaging observations to determine the structure of these regions exist. We investigate the above-the-photosphere heights of hard X-ray (HXR), EUV and white-light continuum sources in the low atmosphere and the corresponding densities at these heights. Simultaneous EUV/continuum images from SDO and HXR RHESSI images are compared to study a well observed gamma-ray limb flare. Using RHESSI X-ray visibilities we determine the height of the HXR sources as a function of energy above the photosphere. Co-aligning AIA/SDO and HMI/SDO images with RHESSI we infer, for the first time, the heights and characteristic densities of HXR, EUV and continuum sources in a flaring footpoint. 35-100 keV HXR sources are found at heights between 1.7 and 0.8 Mm above the photosphere, below the white-light continuum emission which appears at heights 1.5-3 Mm, and the peak of EUV emission originating near 3 Mm. The EUV emission locations are consistent with energy deposition from low energy electrons of ~12 keV occurring in the top layers of the fully ionized chromosphere/low corona and not by >20 keV electrons that produce HXR footpoints in the lower neutral chromosphere. The maximum of white-light emission appears between the HXR and EUV emission, presumably in the transition between ionized and neutral atmospheres suggesting free-bound and free-free continuum emission. We note that the energy deposited by low energy electrons is sufficient to explain the energetics of optical and UV emissions.
Plasma radio emission from inhomogeneous collisional plasma of a flaring loop
Heather Ratcliffe,Eduard. P. Kontar
Physics , 2013, DOI: 10.1051/0004-6361/201322263
Abstract: The evolution of a solar flare accelerated non-thermal electron population and associated plasma emission is considered in collisional inhomogeneous plasma. Non-thermal electrons collisionally evolve to become unstable and generate Langmuir waves, which may lead to intense radio emission. We self-consistently simulated the collisional relaxation of electrons, wave-particle interactions, and non-linear Langmuir wave evolution in plasma with density fluctuations. Additionally, we simulated the scattering, decay, and coalescence of the Langmuir waves which produce radio emission at the fundamental or the harmonic of the plasma frequency, using an angle-averaged emission model. Long-wavelength density fluctuations, such as are observed in the corona, are seen to strongly suppress the levels of radio emission, meaning that a high level of Langmuir waves can be present without visible radio emission. Additionally, in homogeneous plasma, the emission shows time and frequency variations that could be smoothed out by density inhomogeneities.
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