The process of magnetic reconnection when studied in nature or when modeled in 3D simulations differs in one key way from the standard 2D paradigmatic cartoon: it is accompanied by many fluctuations ...in the electromagnetic fields and plasma properties. We developed a diagnostics to study the spectrum of fluctuations in the various regions around a reconnection site. We define the regions in terms of the local value of the flux function that determines the distance from the reconnection site, with positive values in the outflow and negative values in the inflow. We find that fluctuations belong to two very different regimes depending on the local plasma beta (defined as the ratio of plasma and magnetic pressures). The first regime develops in the reconnection outflows where beta is high and it is characterized by a strong link between plasma and electromagnetic fluctuations, leading to momentum and energy exchanges via anomalous viscosity and resistivity. But there is a second, low-beta regime: it develops in the inflow and in the region around the separatrix surfaces, including the reconnection electron diffusion region itself. It is remarkable that this low-beta plasma, where the magnetic pressure dominates, remains laminar even though the electromagnetic fields are turbulent.
Space plasmas are dominated by the presence of large-amplitude waves, large-scale inhomogeneities, kinetic effects and turbulence. Beside the homogeneous turbulence, the generation of small scale ...fluctuations can take place also in other realistic configurations, namely, when perturbations superpose to an inhomogeneous background magnetic field. When an Alfvén wave propagates in a medium where the Alfvén speed varies in a direction transverse to the mean field, it undergoes phase-mixing, which progressively bends wavefronts, generating small scales in the transverse direction. As soon as transverse scales become of the order of the proton inertial length dp, kinetic Alfvén waves (KAWs) are naturally generated. KAWs belong to the branch of Alfvén waves and propagate almost perpendicularly to the ambient magnetic field, at scales close to dp. Many numerical, observational and theoretical works have suggested that these fluctuations may play a determinant role in the development of the solar-wind turbulent cascade. In the present paper, the generation of large amplitude KAW fluctuations in inhomogeneous background, as well as their effect on the protons, have been investigated by means of hybrid Vlasov-Maxwell direct numerical simulations. Imposing a pressure balanced magnetic shear, the kinetic dynamics of protons has been investigated by varying both the magnetic configuration and the amplitude of the initial perturbations. Of particular interest here is the transition from quasi-linear to turbulent regimes, focusing in particular on the development of important non-Maxwellian features in the proton distribution function driven by KAW fluctuations. Several indicators to quantify the deviations of the protons from thermodynamic equilibrium are presented. These numerical results might help to explain the complex dynamics of inhomogeneous and turbulent astrophysical plasmas, such as the heliospheric current sheet, the magnetospheric boundary layer, and the solar corona.
ABSTRACT We study the linear and nonlinear evolution of the tearing instability on thin current sheets by means of two-dimensional numerical simulations, within the framework of compressible, ...resistive MHD. In particular we analyze the behavior of current sheets whose inverse aspect ratio scales with the Lundquist number S as . This scaling has been recently recognized to yield the threshold separating fast, ideal reconnection, with an evolution and growth that are independent of S provided this is high enough, as it should be natural having the ideal case as a limit for . Our simulations confirm that the tearing instability growth rate can be as fast as , where is the ideal Alfvénic time set by the macroscopic scales, for our least diffusive case with . The expected instability dispersion relation and eigenmodes are also retrieved in the linear regime, for the values of S explored here. Moreover, in the nonlinear stage of the simulations we observe secondary events obeying the same critical scaling with S, here calculated on the local, much smaller lengths, leading to increasingly faster reconnection. These findings strongly support the idea that in a fully dynamic regime, as soon as current sheets develop, thin, and reach this critical threshold in their aspect ratio, the tearing mode is able to trigger plasmoid formation and reconnection on the local (ideal) Alfvénic timescales, as required to explain the explosive flaring activity often observed in solar and astrophysical plasmas.
Kinetic Alfvén waves are believed to primary form fluctuations in a hydromagnetic turbulence at scales of the order of the ion inertial length. We study a model where an initial Alfvén wave ...propagates inside an equilibrium structure which is inhomogeneous in the direction perpendicular to the equilibrium magnetic field. In a previous paper this situation has been considered in a particular configuration where the initial wave vector is parallel to the magnetic field and the wave is polarized perpendicular to the inhomogeneity direction. Here we consider other configurations, with a different polarization and possible initial oblique propagation. We employ numerical simulations, using both a Hall‐magnetohydrodynamics and a Hybrid Vlasov‐Maxwell model. Results show that in all the considered cases the time evolution leads to the formation of kinetic Alfvén waves within the inhomogeneity regions, which are identified by a comparison with analytical linear theory results. Then, in this context the formation of kinetic Alfvén waves seems to be a general phenomenon which could be also extended to more complex situations, like turbulence. Kinetic simulations show that kinetic Alfvén waves modify the ion distribution function, generating temperature anisotropy of both parallel and perpendicular to the local magnetic field as well as particle beams aligned along the local magnetic field. These results could be relevant both in the solar corona and in large‐scale structures of the solar wind, where Alfvénic fluctuations are present along with large‐scale inhomogeneities.
Key Points
Mechanisms to produce kinetic Alfvén waves in pressure‐balance structures
Linear and nonlinear simulations of propagating Alfvén waves
Kinetic effects due to the propagation of kinetic Alfvén waves
In this work we report the realization of the first low-threshold cryogenic detector that uses diamond as absorber for astroparticle physics applications. We tested two 0.175 g CVD diamond samples, ...each instrumented with a W-TES. The sensors showed transitions at about 25 mK. We present the performance of the diamond detectors and we highlight the best performing one, where we obtained an energy threshold as low as 16.8 eV. This promising result lays the foundation for the use of diamond for different fields of applications where low threshold and excellent energy resolution are required, as i.e. light dark matter searches and BSM physics with coherent elastic neutrino nucleus scattering.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The transport of energetic particles in the presence of magnetic turbulence is an important but unsolved problem of space physics and astrophysics. Here, we aim at advancing the understanding of ...energetic particle transport by means of a new numerical model of synthetic magnetic turbulence. The model builds up a turbulent magnetic field as a superposition of space-localized fluctuations at different spatial scales. The resulting spectrum is isotropic with an adjustable spectral index. The model allows us to reproduce a spectrum broader than four decades, and to regulate the level of intermittency through a technique based on the p-model. Adjusting the simulation parameters close to solar wind conditions at 1 au, we inject ∼1 MeV protons in the turbulence realization and compute the parallel and perpendicular diffusion coefficients as a function of spectral extension, turbulence level, and intermittency. While a number of previous results are recovered in the appropriate limits, including anomalous transport regimes for low turbulence levels, we find that long spectral extensions tend to reduce the diffusion coefficients. Furthermore, we find for the first time that intermittency has an influence on parallel transport but not on perpendicular transport, with the parallel diffusion coefficient increasing with the level of intermittency. We also obtain the distribution of particle inversion times for parallel velocity, a power law for more than one decade, and compare it with the pitch angle scattering times observed in the solar wind. This parametric study can be useful to interpret particle propagation properties in astrophysical systems.
Models of coronal heating, turbulence and fast reconnection Velli, M.; Pucci, F.; Rappazzo, F. ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
05/2015, Letnik:
373, Številka:
2042
Journal Article
Recenzirano
Odprti dostop
Coronal heating is at the origin of the EUV and X-ray emission and mass loss from the sun and many other stars. While different scenarios have been proposed to explain the heating of magnetically ...confined and open regions of the corona, they must all rely on the transfer, storage and dissipation of the abundant energy present in photospheric motions, which, coupled to magnetic fields, give rise to the complex phenomenology seen at the chromosphere and transition region (i.e. spicules, jets, 'tornadoes'). Here we discuss models and numerical simulations which rely on magnetic fields and electric currents both for energy transfer and for storage in the corona. We will revisit the sources and frequency spectrum of kinetic and electromagnetic energies, the role of boundary conditions, and the routes to small scales required for effective dissipation. Because reconnection in current sheets has been, and still is, one of the most important processes for coronal heating, we will also discuss recent aspects concerning the triggering of reconnection instabilities and the transition to fast reconnection.
The properties of the turbulence that develops in the outflows of magnetic reconnection have been investigated using self-consistent plasma simulations, in three dimensions. As commonly observed in ...space plasmas, magnetic reconnection is characterized by the presence of turbulence. Here we provide a direct comparison of our simulations with reported observations of reconnection events in the magnetotail, investigating the properties of the electromagnetic field and the energy conversion mechanisms. In particular, simulations show the development of a turbulent cascade consistent with spacecraft observations, statistics of the dissipation mechanisms in the turbulent outflows similar to the ones observed in reconnection jets in the magnetotail, and that the properties of turbulence vary as a function of the distance from the reconnecting X-line.
We present direct and scaled comparisons between laboratory and in situ space observations of magnetic reconnection with a guide field, comparing results from the Magnetospheric Multiscale Mission ...and the Magnetic Reconnection eXperiment (MRX). While Magnetospheric Multiscale Mission observations obtain high‐resolution and fully kinetic data, MRX observations fully cover the 2‐D reconnection plane near the current sheet, removing uncertainties in situating the measurements compared to the reconnection region. Through scaling transformations, we show a quantitative agreement in magnetic field and current density profiles, which agree within a factor of 2 from each other. The introduction of the guide field causes the energy conversion J·E in the current sheet to be dominated by J||E|| in both cases. However, parallel electric fields reported by recent spacecraft crossings are significantly (5–10 times) larger than values obtained on MRX, highlighting an important issue for understanding energy conversion by reconnection.
Plain Language Summary
Magnetic reconnection is an important process in the Earth's magnetosphere which rapidly converts magnetic field energy to kinetic energy in the plasma. It is studied both in situ by spacecraft such as the Magnetospheric Multiscale Mission and laboratory experiments. Directly comparing the two is important because the different experiments provide complementary insights about the same process. We find scaling transformations which allow direct comparisons of the results of the experiments, showing agreement on fundamental quantities, such as the width of the current sheet, and also interesting areas of disagreement, especially noting the extremely large electric fields and associated enhanced energy dissipation observed by Magnetospheric Multiscale Mission. These results prompt new questions about nature of energy conversion in space current sheets.
Key Points
Direct scaled comparisons between space and laboratory plasmas show common structure of reconnection current sheets with a guide field
During guide field reconnection the energy conversion is dominated by J||E|| in both experiments
This study evaluated the potential of using poly(lactic acid)/poly(s-caprolactone) (PLA/PCL) blends for fused filament fabrication (FFF) and assembly with pure PLA for biomedical applications. ...PLA/PCL binary blends were meltblended in a twin-screw extruder at different ratios (20/80 to 80/20) and then formed into filaments with a calibrated diameter for FFF. The microstructure, surface properties, and rheological and mechanical behaviors of the blends were assessed. The blends were immiscible but showed signs of adhesion between the phases. It was determined that the fibrillar morphology of inclusions for PLA/PCL ratios higher than 30/70 proved to be driven by the manufacturing process. The tensile mechanical behaviors of printed and injected samples were similar, and their Young's modulus was simulated using Halpin-Tsai and Mori Tanaka models based on the sample microstructure. The ductility of the blends was strongly driven by the behavior of its majority phase. Finally, specific samples were designed to characterize the tensile strength between PLA and its blends by entangling layers of both materials. The strength of the assembly was found to be dependent on the phase that was continuous and was governed by the strength and the viscosity of the blend.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, ODKLJ, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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