5,013 research outputs found
Gyrofluid simulations of collisionless reconnection in the presence of diamagnetic effects
The effects of the ion Larmor radius on magnetic reconnection are
investigated by means of numerical simulations, with a Hamiltonian gyrofluid
model. In the linear regime, it is found that ion diamagnetic effects decrease
the growth rate of the dominant mode. Increasing ion temperature tends to make
the magnetic islands propagate in the ion diamagnetic drift direction. In the
nonlinear regime, diamagnetic effects reduce the final width of the island.
Unlike the electron density, the guiding center density does not tend to
distribute along separatrices and at high ion temperature, the electrostatic
potential exhibits the superposition of a small scale structure, related to the
electron density, and a large scale structure, related to the ion
guiding-center density
Gyrofluid simulations of collisionless reconnection in the presence of diamagnetic effects
The effects of the ion Larmor radius on magnetic reconnection are
investigated by means of numerical simulations, with a Hamiltonian gyrofluid
model. In the linear regime, it is found that ion diamagnetic effects decrease
the growth rate of the dominant mode. Increasing ion temperature tends to make
the magnetic islands propagate in the ion diamagnetic drift direction. In the
nonlinear regime, diamagnetic effects reduce the final width of the island.
Unlike the electron density, the guiding center density does not tend to
distribute along separatrices and at high ion temperature, the electrostatic
potential exhibits the superposition of a small scale structure, related to the
electron density, and a large scale structure, related to the ion
guiding-center density
Gyrofluid simulations of collisionless reconnection in the presence of diamagnetic effects
The effects of the ion Larmor radius on magnetic reconnection are
investigated by means of numerical simulations, with a Hamiltonian gyrofluid
model. In the linear regime, it is found that ion diamagnetic effects decrease
the growth rate of the dominant mode. Increasing ion temperature tends to make
the magnetic islands propagate in the ion diamagnetic drift direction. In the
nonlinear regime, diamagnetic effects reduce the final width of the island.
Unlike the electron density, the guiding center density does not tend to
distribute along separatrices and at high ion temperature, the electrostatic
potential exhibits the superposition of a small scale structure, related to the
electron density, and a large scale structure, related to the ion
guiding-center density
Geochemical monitoring of volcanic lakes. A generalized box model for active crater lakes
In the past, variations in the chemical contents (SO4
2−, Cl−, cations) of
crater lake water have not systematically demonstrated any relationships
with eruptive activity. Intensive parameters (i.e., concentrations,
temperature, pH, salinity) should be converted into extensive parameters
(i.e., fluxes, changes with time of mass and solutes), taking into account
all the internal and external chemical–physical factors that affect the
crater lake system. This study presents a generalized box model approach
that can be useful for geochemical monitoring of active crater lakes, as
highly dynamic natural systems. The mass budget of a lake is based on
observations of physical variations over a certain period of time: lake
volume (level, surface area), lake water temperature, meteorological
precipitation, air humidity, wind velocity, input of spring water, and
overflow of the lake. This first approach leads to quantification of the
input and output fluxes that contribute to the actual crater lake volume.
Estimating the input flux of the "volcanic" fluid (Qf - kg/s) –– an
unmeasurable subsurface parameter –– and tracing its variations with
time is the major focus during crater lake monitoring. Through expanding
the mass budget into an isotope and chemical budget of the lake, the box
model helps to qualitatively characterize the fluids involved. The
(calculated) Cl− content and dD ratio of the rising "volcanic" fluid defines
its origin. With reference to continuous monitoring of crater lakes, the
present study provides tips that allow better calculation of Qf in the future.
At present, this study offers the most comprehensive and up-to-date
literature review on active crater lakes
Mode signature and stability for a Hamiltonian model of electron temperature gradient turbulence
Stability properties and mode signature for equilibria of a model of electron
temperature gradient (ETG) driven turbulence are investigated by Hamiltonian
techniques. After deriving the infinite families of Casimir invariants,
associated with the noncanonical Poisson bracket of the model, a sufficient
condition for stability is obtained by means of the Energy-Casimir method. Mode
signature is then investigated for linear motions about homogeneous equilibria.
Depending on the sign of the equilibrium "translated" pressure gradient, stable
equilibria can either be energy stable, i.e.\ possess definite linearized
perturbation energy (Hamiltonian), or spectrally stable with the existence of
negative energy modes (NEMs). The ETG instability is then shown to arise
through a Kre\u{\i}n-type bifurcation, due to the merging of a positive and a
negative energy mode, corresponding to two modified drift waves admitted by the
system. The Hamiltonian of the linearized system is then explicitly transformed
into normal form, which unambiguously defines mode signature. In particular,
the fast mode turns out to always be a positive energy mode (PEM), whereas the
energy of the slow mode can have either positive or negative sign
Coupling between reconnection and Kelvin-Helmholtz instabilities in collisionless plasmas
In a collisionless plasma, when reconnection instability takes place, strong shear flows may develop. Under appropriate conditions these shear flows become unstable to the Kelvin-Helmholtz instability. Here, we investigate the coupling between these instabilities in the framework of a four-field model. Firstly, we recover the known results in the low β limit, β being the ratio between the plasma and the magnetic pressure. We concentrate our attention on the dynamical evolution of the current density and vorticity sheets which evolve coupled together according to a laminar or a turbulent regime. A three-dimensional extension in this limit is also discussed. Secondly, we consider finite values of the β parameter, allowing for compression of the magnetic and velocity fields along the ignorable direction. We find that the current density and vorticity sheets now evolve separately. The Kelvin-Helmholtz instability involves only the vorticity field, which ends up in a turbulent regime, while the current density maintains a laminar structure
Coupling between reconnection and Kelvin-Helmholtz instabilities in collisionless plasmas
Abstract. In a collisionless plasma, when reconnection instability takes place, strong shear flows may develop. Under appropriate conditions these shear flows become unstable to the Kelvin-Helmholtz instability. Here, we investigate the coupling between these instabilities in the framework of a four-field model. Firstly, we recover the known results in the low β limit, β being the ratio between the plasma and the magnetic pressure. We concentrate our attention on the dynamical evolution of the current density and vorticity sheets which evolve coupled together according to a laminar or a turbulent regime. A three-dimensional extension in this limit is also discussed. Secondly, we consider finite values of the β parameter, allowing for compression of the magnetic and velocity fields along the ignorable direction. We find that the current density and vorticity sheets now evolve separately. The Kelvin-Helmholtz instability involves only the vorticity field, which ends up in a turbulent regime, while the current density maintains a laminar structure
Mediastinitis and sternal prosthesis infection successfully treated by minimally invasive omental flap transposition
Purulent mediastinitis is a possible serious complication after mediastinal surgery. We report the case of a localized
sternal plasmocytoma treated by sternectomy and prosthetic repair, who needed a second surgery for a fistulizing
mediastinitis. Five months earlier, in another Hospital, the patient underwent sternal resection and reconstruction
with a “sandwich” prosthesis (Methyl-methacrylate and Marlex mesh). Suppurative mediastinitis occurred and septic
shock resolution was observed after the spontaneous opening of a mediastinal cutaneous fistula. After referring to
our Unit the patient underwent extensive local and systemic preparation and nutritional support; the infected
prosthesis was then removed and the gap filled by a laparoscopically-prepared omental flap. Adequate
preoperative management, removal of any infected material and minimally invasive omental flap transposition
allowed the successful treatment of this life-threatening condition
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