arXiv subjects

Francesco Califano

Publications and source records attributed to Francesco Califano.

31 records · Page 2Linked to original sources

arXiv2019

ViDA: a Vlasov-DArwin solver for plasma physics at electron scales

We present a Vlasov-DArwin numerical code (ViDA) specifically designed to address plasma physics problems, where small-scale high accuracy is requested even during the non linear regime to guarantee a clean description of the plasma dynamics at fine spatial scales. The algorithm provides a low-noise description of proton and electron kinetic dynamics, by splitting in time the multi-advection Vlasov equation in phase space. Maxwell equations for the electric and magnetic fields are reorganized according to Darwin approximation to remove light waves. Several numerical tests show that ViDA successfully reproduces the propagation of linear and nonlinear waves and captures the physics of magnetic reconnection. We also discuss preliminary tests of the parallelization algorithm efficiency, performed at CINECA on the Marconi-KNL cluster. ViDA will allow to run Eulerian simulations of a non-relativistic fully-kinetic collisionless plasma and it is expected to provide relevant insights on important problems of plasma astrophysics such as, for instance, the development of the turbulent cascade at electron scales and the structure and dynamics of electron-scale magnetic reconnection, such as the electron diffusion region.

physics.plasm-ph↗

arXiv2019

Interplay between Kelvin-Helmholtz and Lower-Hybrid Drift instabilities

Boundary layers in space and astrophysical plasmas are the location of complex dynamics where different mechanisms coexist and compete eventually leading to plasma mixing. In this work, we present fully kinetic Particle-In-Cell simulations of different boundary layers characterized by the following main ingredients: a velocity shear, a density gradient and a magnetic gradient localized at the same position. In particular, the presence of a density gradient drives the development of the lower hybrid drift instability (LHDI), which competes with the Kelvin-Helmholtz instability (KHI) in the development of the boundary layer. Depending on the density gradient, the LHDI can even dominate the dynamics of the layer. Because these two instabilities grow on different spatial and temporal scales, when the LHDI develops faster than the KHI an inverse cascade is generated, at least in 2D. This inverse cascade, starting at the LHDI kinetic scales, generates structures at scale lengths at which the KHI would typically develop. When that is the case, those structures can suppress the KHI itself because they significantly affect the underlying velocity shear gradient. We conclude that depending on the density gradient, the velocity jump and the width of the boundary layer, the LHDI in its nonlinear phase can become the primary instability for plasma mixing. These numerical simulations show that the LHDI is likely to be a dominant process at the magnetopause of Mercury. These results are expected to be of direct impact to the interpretation of the forthcoming BepiColombo observations.

physics.plasm-ph↗

arXiv2019

In situ spacecraft observations of a structured electron diffusion region during magnetopause reconnection

The Electron Diffusion Region (EDR) is the region where magnetic reconnection is initiated and electrons are energized. Because of experimental difficulties, the structure of the EDR is still poorly understood. A key question is whether the EDR has a homogeneous or patchy structure. Here we report Magnetospheric MultiScale (MMS) novel spacecraft observations providing evidence of inhomogeneous current densities and energy conversion over a few electron inertial lengths within an EDR at the terrestrial magnetopause, suggesting that the EDR can be rather structured. These inhomogenenities are revealed through multi-point measurements because the spacecraft separation is comparable to a few electron inertial lengths, allowing the entire MMS tetrahedron to be within the EDR most of the time. These observations are consistent with recent high-resolution and low-noise kinetic simulations.

physics.plasm-ph↗

arXiv2018

Coherent structures and spectral energy transfer in turbulent plasma: a space-filter approach

Plasma turbulence at scales of the order of the ion inertial length is mediated by several mechanisms, including linear wave damping, magnetic reconnection, formation and dissipation of thin current sheets, stochastic heating. It is now understood that the presence of localized coherent structures enhances the dissipation channels and the kinetic features of the plasma. However, no formal way of quantifying the relationship between scale-to-scale energy transfer and the presence of spatial structures has so far been presented. In this letter we quantify such relationship analyzing the results of a two-dimensional high-resolution Hall-MHD simulation. In particular, we employ the technique of space-filtering to derive a spectral energy flux term which defines, in any point of the computational domain, the signed flux of spectral energy across a given wavenumber. The characterization of coherent structures is performed by means of a traditional two-dimensional wavelet transformation. By studying the correlation between the spectral energy flux and the wavelet amplitude, we demonstrate the strong relationship between scale-to-scale transfer and coherent structures. Furthermore, by conditioning one quantity with respect to the other, we are able for the first time to quantify the inhomogeneity of the turbulence cascade induced by topological structures in the magnetic field. Taking into account the low filling-factor of coherent structures (i.e. they cover a small portion of space), it emerges that 80% of the spectral energy transfer (both in the direct and inverse cascade directions) is localized in about 50% of space, and 50% of the energy transfer is localized in only 25% of space.

physics.plasm-ph↗

arXiv2015

Pressure anisotropy and small spatial scales induced by velocity shear

Non-Maxwellian metaequilibria can exist in low-collisionality plasmas as evidenced by satellite and laboratory measurements. By including the full pressure tensor dynamics in a fluid plasma model, we show that a sheared velocity field can provide an effective mechanism that makes an initial isotropic state anisotropic and agyrotropic. We discuss how the propagation of magneto-elastic waves can affect the pressure tensor anisotropization and its spatial filamentation which are due to the action of both the magnetic field and flow strain tensor. We support this analysis by a numerical integration of the nonlinear equations describing the pressure tensor evolution.

physics.plasm-ph↗

arXiv2014

Kinetic plasma turbulence during the nonlinear stage of the Kelvin-Helmholtz instability

Using a full kinetic, implicit particle-in-cell code, iPiC3D, we studied the properties of plasma kinetic turbulence, such as would be found at the interface between the solar wind and the Earth magnetosphere at low latitude during northwards periods. In this case, in the presence of a magnetic field B oriented mostly perpendicular to the velocity shear, turbulence is fed by the disruption of a Kelvin-Helmholtz vortex chain via secondary instabilities, vortex pairing and non-linear interactions. We found that the magnetic energy spectral cascade between ion and electron inertial scales, $d_i$ and $d_e$, is in agreement with satellite observations and other previous numerical simulations; however, in our case the spectrum ends with a peak beyond $d_e$ due to the occurrence of the lower hybrid drift instability. The electric energy spectrum is influenced by effects of secondary instabilities: anomalous resistivity, fed by the development of the lower hybrid drift instability, steepens the spectral decay and, depending on the alignment or anti-alignment of B and the shear vorticity, peaks due to ion-Bernstein waves may dominate the spectrum around $d_i$. These waves are generated by counter-streaming flow structures, through flux freezing also responsible for reconnection of the in-plane component of the magnetic field, which then generates electron pressure anisotropy and flattening of the field-aligned component of the electron distribution function.

physics.plasm-ph↗

arXiv2013

Vlasov-Poisson simulations of electrostatic parametric instability for localized Langmuir wave packets in the solar wind

Recent observation of large amplitude Langmuir waveforms during a Type III event in the solar wind have been interpreted as the signature of the electrostatic decay of beam-driven Langmuir waves. This mechanism is thought to be a first step to explain the generation of solar Type III radio emission. The threshold for this parametric instability in typical solar wind condition is investigated here by means of 1D-1V Vlasov-Poisson simulations. We show that the amplitude of the observed Langmuir beat-like waveforms is of the order of the effective threshold computed from the full kinetic simulations. The expected level of associated ion acoustic density fluctuations have also been computed for comparison with observations.

astro-ph.SR↗

arXiv2012

On the breaking of a plasma wave in a thermal plasma: I. The structure of the density singularity

The structure of the singularity that is formed in a relativistically large amplitude plasma wave close to the wavebreaking limit is found by using a simple waterbag electron distribution function. The electron density distribution in the breaking wave has a typical "peakon" form. The maximum value of the electric field in a thermal breaking plasma is obtained and compared to the cold plasma limit. The results of computer simulations for different initial electron distribution functions are in agreement with the theoretical conclusions.

physics.plasm-ph↗

arXiv2012

On the breaking of a plasma wave in a thermal plasma: II. Electromagnetic wave interaction with the breaking plasma wave

The structure of the density singularity formed in a relativistically large amplitude plasma wave close to the wavebreaking limit leads to a refraction coefficient which has a coordinate dependence with discontinuous derivatives. This results in a non-exponentially small above-barrier reflection of an electromagnetic wave interacting with the nonlinear plasma wave.

physics.plasm-ph↗

arXiv2012

Coupling between whistler waves and slow-mode solitary waves

The interplay between electron-scale and ion-scale phenomena is of general interest for both laboratory and space plasma physics. In this paper we investigate the linear coupling between whistler waves and slow magnetosonic solitons through two-fluid numerical simulations. Whistler waves can be trapped in the presence of inhomogeneous external fields such as a density hump or hole where they can propagate for times much longer than their characteristic time scale, as shown by laboratory experiments and space measurements. Space measurements have detected whistler waves also in correspondence to magnetic holes, i.e., to density humps with magnetic field minima extending on ion-scales. This raises the interesting question of how ion-scale structures can couple to whistler waves. Slow magnetosonic solitons share some of the main features of a magnetic hole. Using the ducting properties of an inhomogeneous plasma as a guide, we present a numerical study of whistler waves that are trapped and transported inside propagating slow magnetosonic solitons.

physics.plasm-ph↗

arXiv2007

Exploring the thermodynamic limit of Hamiltonian models: convergence to the Vlasov equation

We here discuss the emergence of Quasi Stationary States (QSS), a universal feature of systems with long-range interactions. With reference to the Hamiltonian Mean Field (HMF) model, numerical simulations are performed based on both the original $N$-body setting and the continuum Vlasov model which is supposed to hold in the thermodynamic limit. A detailed comparison unambiguously demonstrates that the Vlasov-wave system provides the correct framework to address the study of QSS. Further, analytical calculations based on Lynden-Bell's theory of violent relaxation are shown to result in accurate predictions. Finally, in specific regions of parameters space, Vlasov numerical solutions are shown to be affected by small scale fluctuations, a finding that points to the need for novel schemes able to account for particles correlations.

cond-mat.stat-mech↗

arXiv2004

Propagation of finite amplitude electrostatic disturbances in an inhomogeneous magnetized plasma

A 1D2V open boundary Vlasov-Ampere code has been implemented with the aim of making a detailed investigation of the propagation of finite amplitude electromagnetic disturbances in an inhomogeneous magnetized plasma. The code is being applied to study the propagation of an externally driven electromagnetic signal, localized at one boundary of the integration interval, through a given equilibrium plasma configuration with inhomogeneous plasma density and magnetic field.

physics.plasm-ph↗

arXiv2004

Impact of dissipative effects on the macroscopic evolution of a Vlasov system

Numerical diffusion is introduced by any numerical scheme as soon as small scales fluctuations, generated during the dynamical evolution of a collisionless plasma, become comparable to the grid size. Here we investigate the role of numerical dissipation in collisionless plasma simulations by studying the non linear regime of the two stream instability. We show that the long time evolution of the Vlasov - Poisson system can be affected by the used algorithm.

physics.plasm-ph↗