87 research outputs found
Fire Hose instability driven by alpha particle temperature anisotropy
We investigate properties of a solar wind-like plasma including a secondary
alpha particle population exhibiting a parallel temperature anisotropy with
respect to the background magnetic field, using linear and quasi-linear
predictions and by means of one-dimensional hybrid simulations. We show that
anisotropic alpha particles can drive a parallel fire hose instability
analogous to that generated by protons, but that, remarkably, the instability
can be triggered also when the parallel plasma beta of alpha particles is below
unity. The wave activity generated by the alpha anisotropy affects the
evolution of the more abundant protons, leading to their anisotropic heating.
When both ion species have sufficient parallel anisotropies both of them can
drive the instability, and we observe generation of two distinct peaks in the
spectra of the fluctuations, with longer wavelengths associated to alphas and
shorter ones to protons. If a non-zero relative drift is present, the unstable
modes propagate preferentially in the direction of the drift associated with
the unstable species. The generated waves scatter particles and reduce their
temperature anisotropy to marginally stable state, and, moreover, they
significantly reduce the relative drift between the two ion populations. The
coexistence of modes excited by both species leads to saturation of the plasma
in distinct regions of the beta/anisotropy parameter space for protons and
alpha particles, in good agreement with in situ solar wind observations. Our
results confirm that fire hose instabilities are likely at work in the solar
wind and limit the anisotropy of different ion species in the plasma.Comment: 10 pages, 9 figures, Accepted for publication in The Astrophysical
Journa
Solar wind turbulence from MHD to sub-ion scales: high-resolution hybrid simulations
We present results from a high-resolution and large-scale hybrid (fluid
electrons and particle-in-cell protons) two-dimensional numerical simulation of
decaying turbulence. Two distinct spectral regions (separated by a smooth break
at proton scales) develop with clear power-law scaling, each one occupying
about a decade in wave numbers. The simulation results exhibit simultaneously
several properties of the observed solar wind fluctuations: spectral indices of
the magnetic, kinetic, and residual energy spectra in the magneto-hydrodynamic
(MHD) inertial range along with a flattening of the electric field spectrum, an
increase in magnetic compressibility, and a strong coupling of the cascade with
the density and the parallel component of the magnetic fluctuations at
sub-proton scales. Our findings support the interpretation that in the solar
wind large-scale MHD fluctuations naturally evolve beyond proton scales into a
turbulent regime that is governed by the generalized Ohm's law.Comment: 5 pages, 5 figures; introduction and conclusions changed, references
updated, accepted for publication in ApJ
Two-dimensional Hybrid Simulations of Kinetic Plasma Turbulence: Current and Vorticity vs Proton Temperature
Proton temperature anisotropies between the directions parallel and
perpendicular to the mean magnetic field are usually observed in the solar wind
plasma. Here, we employ a high-resolution hybrid particle-in-cell simulation in
order to investigate the relation between spatial properties of the proton
temperature and the peaks in the current density and in the flow vorticity. Our
results indicate that, although regions where the proton temperature is
enhanced and temperature anisotropies are larger correspond approximately to
regions where many thin current sheets form, no firm quantitative evidence
supports the idea of a direct causality between the two phenomena. On the other
hand, quite a clear correlation between the behavior of the proton temperature
and the out-of-plane vorticity is obtained.Comment: 4 pages, 2 figures, Proceedings of the Fourteenth International Solar
Wind Conferenc
High-resolution hybrid simulations of kinetic plasma turbulence at proton scales
We investigate properties of plasma turbulence from magneto-hydrodynamic
(MHD) to sub-ion scales by means of two-dimensional, high-resolution hybrid
particle-in-cell simulations. We impose an initial ambient magnetic field,
perpendicular to the simulation box, and we add a spectrum of large-scale
magnetic and kinetic fluctuations, with energy equipartition and vanishing
correlation. Once the turbulence is fully developed, we observe a MHD inertial
range, where the spectra of the perpendicular magnetic field and the
perpendicular proton bulk velocity fluctuations exhibit power-law scaling with
spectral indices of -5/3 and -3/2, respectively. This behavior is extended over
a full decade in wavevectors and is very stable in time. A transition is
observed around proton scales. At sub-ion scales, both spectra steepen, with
the former still following a power law with a spectral index of ~-3. A -2.8
slope is observed in the density and parallel magnetic fluctuations,
highlighting the presence of compressive effects at kinetic scales. The
spectrum of the perpendicular electric fluctuations follows that of the proton
bulk velocity at MHD scales, and flattens at small scales. All these features,
which we carefully tested against variations of many parameters, are in good
agreement with solar wind observations. The turbulent cascade leads to on
overall proton energization with similar heating rates in the parallel and
perpendicular directions. While the parallel proton heating is found to be
independent on the resistivity, the number of particles per cell and the
resolution employed, the perpendicular proton temperature strongly depends on
these parameters.Comment: 15 pages, 13 figures, submitted to Ap
Studio della trasformazione di 1,2-propandiolo ad acido metacrilico mediante processo multi-stadio
In questo elaborato di tesi è stata studiata la reazione di trasformazione diretta di 1,2-propandiolo ad acido propionico in fase gas, la quale necessita di una catalisi combinata acido-redox. Per questo motivo sono stati testati catalizzatori bifunzionali: eteropoliacidi, ossidi misti W-V-O con struttura HTB e alluminofosfati drogati con V e Co. Tuttavia, la buona riuscita del processo sembra essere impedita da reazioni di scissione ossidativa, che potrebbero avvenire sul reagente 1,2-propandiolo, sul prodotto intermedio, la propionaldeide, e probabilmente anche sul prodotto di ossidazione selettiva, l’acido propionico. Per comprendere le relazioni cinetiche esistenti tra prodotti di reazione e reagente sono state svolte delle prove in funzione del tempo di contatto. Inoltre è stata studiata separatamente la reazione di ossidazione dell’intermedio del processo: la propionaldeide. In seguito è stato iniziato lo studio che riguarda la reazione di condensazione aldolica tra l’acido propionico e la formaldeide generata in situ a partire da metanolo, al fine di ottenere acido metacrilico. La reazione è stata testata utilizzando un catalizzatore a base di AlPO, studiando il comportamento catalitico in funzione del rapporto di alimentazione dei reagenti, della temperatura e del tempo di contatto
On the competition between radial expansion and Coulomb collisions in shaping the electron velocity distribution function: Kinetic simulations
International audienc
Analytic thin wall false vacuum decay rate
We derive a closed-form false vacuum decay rate at one loop in the thin wall
limit, where the true and false vacua are nearly degenerate. We obtain the
bounce configuration in dimensions, together with the Euclidean action with
a higher order correction, counter-terms and renormalization group running. We
extract the functional determinant via the Gel'fand-Yaglom theorem for low and
generic orbital multipoles. The negative and zero eigenvalues appear for low
multipoles and the translational zeroes are removed. We compute the
fluctuations for generic multipoles, multiply and regulate the orbital modes.
We find an explicit finite renormalized decay rate in and give a
closed-form expression for the finite functional determinant in any dimension.Comment: 22 pages plus 5 appendices, published in JHEP. In v4 we updated the
final result in D=4, after the addition of a term missed in the previous
versions. The result in D=3 is unchange
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