arXiv subjects

Francesco Califano

Publications and source records attributed to Francesco Califano.

At least 19 recordsLinked to original sources

arXiv2026

Secondary Electron-Only Reconnection Driven by Large Scale Ion-Coupled Reconnection and Electron Kelvin-Helmholtz Instabilities in Hybrid Simulations of Solar Wind Turbulence

Electron-only reconnection (EREC) is a magnetic reconnection regime occurring within subion-scale current sheets (CSs), exhibiting only electron jets, without any ion outflows. EREC has been first observed in the Earth's magnetosheath, where its occurrence is linked to the small correlation length of magnetic fluctuations, limiting the growth of CSs to very large scales. On the other hand, the development of EREC in open systems with large magnetic correlation lengths, such as the solar wind (SW), remains an open question. To address this problem, we employ a large-scale 2D hybrid simulation with finite electron inertia, investigating the development of EREC driven by turbulence. By injecting energy at very large scales, we allow EREC to develop spontaneously due to the turbulent cascade, without any external small-scale forcing or imposed constraints on the turbulence correlation length. We find that EREC develops in our simulation via two distinct turbulence-driven mechanisms: (1) secondary EREC induced by the interaction of plasmoids in the outflows of large-scale ion-coupled reconnection; (2) EREC directly driven at subion scales by the electron Kelvin-Helmholtz instability in small-scale velocity shears. Furthermore, we perform a statistical analysis of CSs using the machine-learning clustering algorithm HDBSCAN, showing that subion-scale CSs capable of hosting EREC are dominant in our simulation. Our results suggest that EREC could occur even in large-scale space and astrophysical systems, like the SW, driven by secondary turbulent processes, potentially playing a key role in dissipating energy at kinetic scales.

physics.space-ph↗

arXiv2024

Formulation of a one-dimensional electrostatic plasma model for testing the validity of kinetic theory

We present a one-dimensional (1-D) model composed of aligned, electrostatically interacting charged disks, conceived in order to address in a computable model the validity of the Bogoliubov assumption on the decay of particle correlations in the Born-Bogoliubov-Green-Kirkwood-Yvon hierarchy. This assumption is a basic premise of plasma kinetic theory. The disk model exhibits spatially 1-D features at short distances, but retains 3-D features at large distances. Here the collective dynamics of this model plasma is investigated by solving the corresponding Vlasov equation. In addition, the implementation of the model for the numerical validation of the Bogoliubov assumption is formulated.

physics.plasm-ph↗

arXiv2024

Large Scale Linear Magnetic Holes with Magnetic Mirror Properties in Hybrid Simulations of Solar Wind Turbulence

Magnetic holes (MHs) are coherent magnetic field dips whose size ranges from fluid to kinetic scale, ubiquitously observed in the heliosphere and in planetary environments. Despite the longstanding effort in interpreting the abundance of observations, the origin and properties of MHs are still debated. In this letter, we investigate the interplay between plasma turbulence and MHs, using a 2D hybrid simulation initialized with solar wind parameters. We show that fully developed turbulence exhibits localized elongated magnetic depressions, whose properties are consistent with linear MHs frequently encountered in space. The observed MHs develop self-consistently from the initial magnetic field perturbations, by trapping hot ions with large pitch angles. Ion trapping produces an enhanced perpendicular temperature anysotropy that makes MHs stable for hundreds of ion gyroperiods, despite the surrounding turbulence. We introduce a new quantity, based on local magnetic field and ion temperature values, to measure the efficiency of ion trapping, with potential applications to the detection of MHs in satellite measurements. We complement this method by analyzing the ion velocity distribution functions inside MHs. Our diagnostics reveal the presence of trapped gyrotropic ion populations, whose velocity distribution is consistent with a loss cone, as expected for the motion of particles inside a magnetic mirror. Our results have potential implications for the theoretical and numerical modelling of MHs.

astro-ph.SR↗

arXiv2023

On the Cascade-Dissipation Balance in Astrophysical Plasmas

The differential heating of electrons and ions by turbulence in weakly collisional magnetized plasmas and the scales at which such energy dissipation is most effective are still debated. Using a large data sample measured in the Earth's magnetosheath by the Magnetospheric Multiscale mission and the coarse-grained energy equations derived from the Vlasov-Maxwell system we find evidence of a balance over two decades in scales between the energy cascade and dissipation rates. The decline of cascade rate at kinetic scales (in contrasts with a constant one in the inertial range), is balanced by an increasing ion and electron heating rates, estimated via the pressure-strain. Ion scales are found to contribute most effectively to ion heating, while electron heating originates equally from ion and electron scales. These results can potentially impact current understanding of particle heating in turbulent magnetized plasmas as well as their theoretical and numerical modeling.

physics.plasm-ph↗

arXiv2023

Comparative simulations of Kelvin-Helmholtz induced magnetic reconnection at the Earth's magnetospheric flanks

This study presents three-dimensional (3D) resistive Hall-magnetohydrodynamic simulations of the Kelvin-Helmholtz instability (KHI) dynamics at Earth's magnetospheric flanks during northward interplanetary magnetic field periods. By comparing two simulations with and without initial magnetic shear, we analyze the impact of distinct magnetic field orientations on plasma dynamics and magnetic reconnection events taking into account 3D mechanisms, such as KHI high latitude stabilization. The identical nature of the simulations, except for the presence/absence of an initial magnetic shear, enables, for the first time, a complete and coherent comparative analysis of the latitudinal distribution of KH vortices, current sheets, reconnection events, and the evolution of the mixing layer. In one configuration, a uniform magnetic field leads to double mid-latitude reconnection (MLR), while in the other, magnetic shear induces both type I vortex-induced reconnection (VIR) and MLR. Notably, the type I VIR observed in this second scenario results from the combined action of line advection and vortex-induced current sheet pinching (the classic mechanism driving two-dimensional type I VIR). Of particular importance is our quantification of newly closed field lines that experienced double reconnection, ultimately becoming embedded in solar wind plasma at low latitudes while remaining connected to magnetospheric plasma at high latitudes. The varying abundance of such lines in the two simulations holds implications for plasma transport at the magnetopause.

physics.space-ph↗

arXiv2023

Generation of sub-ion scale magnetic holes from electron shear flow instabilities in plasma turbulence

Magnetic holes (MHs) are coherent structures associated with strong magnetic field depressions in magnetized plasmas. They are observed in many astrophysical environments at a wide range of scales but their origin is still under debate. In this work we investigate the formation of sub-ion scale MHs using a fully kinetic 2D simulation of plasma turbulence initialized with parameters typical of the Earth's magnetosheath. Our analysis shows that the turbulence is capable of generating sub-ion scale MHs from large scale fluctuations via the following mechanism: first, the nonlinear large scale dynamics spontaneously leads to the development of thin and elongated electron velocity shears; these structures then become unstable to the electron Kelvin-Helmholtz instability and break up into small scale electron vortices; the electric current carried by these vortices locally reduces the magnetic field, inducing the formation of sub-ion scale MHs. The MHs thus produced exhibit features consistent with satellite observations and with previous numerical studies. We finally discuss the kinetic properties of the observed sub-ion scale MHs, showing that they are characterized by complex non-Maxwellian electron velocity distributions exhibiting anisotropic and agyrotropic features.

physics.plasm-ph↗

arXiv2023

Maps of solar wind plasma precipitation onto Mercury's surface: a geographical perspective

Mercury is the closest planet to the Sun, possesses a weak intrinsic magnetic field and has only a very tenuous atmosphere (exosphere). These three conditions result in a direct coupling between the plasma emitted from the Sun (namely the solar wind) and Mercury's surface. The planet's magnetic field leads to a non-trivial pattern of plasma precipitation onto the surface, that is expected to contribute to the alteration of the regolith over geological time scales. The goal of this work is to study the solar wind plasma precipitation onto the surface of Mercury from a geographical perspective, as opposed to the local-time-of-day approach of previous precipitation modeling studies. We employ solar wind precipitation maps for protons and electrons from two fully-kinetic numerical simulations of Mercury's plasma environment. These maps are then integrated over two full Mercury orbits (176 Earth days). We found that the plasma precipitation pattern at the surface is most strongly affected by the upstream solar wind conditions, particularly by the interplanetary magnetic field direction, and less by Mercury's 3:2 spin-orbit resonance. We also found that Mercury's magnetic field is able to shield the surface from roughly 90% of the incoming solar wind flux. At the surface, protons have a broad energy distribution from below 500 eV to more than 1.5 keV; while electrons are mostly found in the range 0.1-4 keV. These results will help to better constrain space weathering and exosphere source processes at Mercury, as well as to interpret observations by the ongoing ESA/JAXA BepiColombo mission.

physics.space-ph↗

arXiv2023

Subion Scale Turbulence Driven by Magnetic Reconnection

The interplay between plasma turbulence and magnetic reconnection remains an unsettled question in astrophysical and laboratory plasmas. Here we report the first observational evidence that magnetic reconnection drives subion scale turbulence in magnetospheric plasmas by transferring energy to small scales. We employ a spatial coarse-grained model of Hall magnetohydrodynamics, enabling us to measure the nonlinear energy transfer rate across scale $\ell$ at position $x$. Its application to Magnetospheric Multiscale mission data shows that magnetic reconnection drives intense energy transfer to subion scales. This observational evidence is remarkably supported by the results from Hybrid Vlasov-Maxwell simulations of turbulence to which the coarse-grained model is also applied. These results can potentially answer some open questions on plasma turbulence in planetary environments.

physics.plasm-ph↗

arXiv2023

Role of FLR effects in magnetopause equilibrium

The Earth magnetopause, when sufficiently plane and stationary at a local scale, can be considered as a "quasi-tangential" discontinuity, since the normal component of the magnetic field Bn is typically very small but not zero. Contrary to observations, the "Classic Theory of Discontinuities" (CTD) predicts that rotational and compressional jumps should be mutually exclusive in the general case Bn n.e.t. 0, but allows only one exception: the tangential discontinuity provided that Bn is strictly zero. Here we show that Finite Larmor Radius (FLR) effects play an important role in the quasi-tangential case, whenever the ion Larmor radius is not fully negligible with respect to the magnetopause thickness. By including FLR effects, the results suggest that a rotational discontinuity undergoes a change comparable to the change of a Shear Alfven into a Kinetic Alfven wave when considering linear modes. For this new kind of discontinuity, the co-existence of rotational and compressional variations at the magnetopause does no more imply that this boundary is a strict tangential discontinuity, even in 1D-like regions far from X-lines if any. This result may lead to important consequences concerning the oldest and most basic questions of magnetospheric physics: how can the magnetopause be open, where and when? The role of FLR being established theoretically, the paper then shows that it can be proved experimentally. We make use of MMS data and process them with the most recent available 4 spacecraft tools. First, we present the different processing techniques that we use to estimate spatial derivatives, such as grad(B) and div(P), and the magnetopause normal direction. We point out why this normal direction must be determined with extremely high accuracy to make the conclusions unambiguous. The results obtained by these techniques are presented in a detailed case study and on a statistical basis.

physics.plasm-ph↗

arXiv2022

Spectral properties and energy transfer at kinetic scales in collisionless plasma turbulence

By means of a fully kinetic simulation of freely decaying plasma turbulence, we study the spectral properties and the energy exchanges characterizing the turbulent cascade in the kinetic range. We find that the magnetic field spectrum follows the ${k^{-α}\,exp(-λ\, k)}$ law at kinetic scales with $α\!\simeq\!2.73$ and $λ\!\simeq\!ρ_e$ (where ${ρ_e}$ is the electron gyroradius). The same law with $α\!\simeq\!0.94$ and an exponential decay characterized by $λ\!\simeq\!0.87ρ_e$ is observed in the electron velocity spectrum but not in the ion velocity spectrum that drops like a steep power law $\sim k^{-3.25}$ before reaching electron scales. By analyzing the filtered energy conversion channels, we find that the electrons play a major role with respect to the ions in driving the magnetic field dynamics at kinetic scales. Our analysis reveals the presence of an indirect electron-driven mechanism that channels the e.m. energy from large to sub-ion scale more efficiently than the direct nonlinear scale-to-scale transfer of e.m. energy. This mechanism consists of three steps: in the first step the e.m. energy is converted into electron fluid flow energy at large scales; in the second step the electron fluid flow energy is nonlinearly transferred towards sub-ion scales; in the final step the electron fluid flow energy is converted back into e.m. energy at sub-ion scales. This electron-driven transfer drives the magnetic field cascade up to fully developed turbulence, after which dissipation becomes dominant and the electrons start to subtract energy from the magnetic field and dissipate it via the pressure-strain interaction at sub-ion scales.

physics.plasm-ph↗

arXiv2022

Solar-wind electron precipitation on weakly magnetized bodies: the planet Mercury

Mercury is the archetype of a weakly magnetized, airless, telluric body immersed in the solar wind. Due to the lack of any substantial atmosphere, the solar wind directly precipitates on Mercury's surface. Using a 3D fully-kinetic self-consistent plasma model, we show for the first time that solar-wind electron precipitation drives (i) efficient ionization of multiple species (H, He, O and Mn) in Mercury's neutral exosphere and (ii) emission of X-rays from the planet's surface. This is the first, independent evidence of X-ray auroras on Mercury using a numerical approach.

physics.space-ph↗

arXiv2022

Electron dynamics in small magnetospheres: insights from global fully-kinetic plasma simulations of planet Mercury

The planet Mercury possesses a small but highly dynamic magnetosphere in which the role and dynamics of electrons are still largely unknown. We aim at modeling the global dynamics of solar wind electrons impinging on Mercury's magnetosphere. Particular relevance is given to local acceleration processes and the global circulation patterns. The goals of this work are pursued by means of three-dimensional, fully kinetic particle-in-cell simulations modeling the interaction of the solar wind with the Hermean magnetosphere. This method allows a self-consistent representation of the plasma dynamics from the large planetary scale down to the electron kinetic scale. Numerical simulations are carried out using two different solar wind conditions: purely northward or purely southward interplanetary magnetic field direction. We find a high plasma current (of the order of few $μ$A/m2) flowing at the magnetospheric boundaries (bow shock and magnetopause) dominated by electrons. This current is driven by the small-scale electron physics resolved in our model. Furthermore, we observe strong electron acceleration up to tens of keV as a consequence of magnetic reconnection when the interplanetary magnetic field is directed southward. Such energetic electrons are partially trapped in the dipolar magnetic field of the planet mainly at nightside. Finally, by studying the distribution of electrons in our simulations along Mariner10 and BepiColombo first-Mercury-flyby trajectories, we propose that both spacecraft observed this energetic quasi-trapped electron population around closest approach.

physics.space-ph↗

arXiv2022

Local cascade and dissipation in incompressible Hall magnetohydrodynamic turbulence: the Coarse-Graining approach

We derive the coarse-graining (CG) equations of incompressible Hall Magnetohydrodynamics (HMHD) turbulence to investigate the {\it local (in space)} energy cascade rate as a function of the filtering scale $\ell$. First, the CG equations are space averaged to obtain the analytical expression of the mean cascade rate. Its application to 3 dimensional (3D) simulations of (weakly compressible) HMHD shows a cascade rate consistent with the value of the mean dissipation rate in the simulations and with the classical estimates based on the "third-order" law. Furthermore, we developed an anisotropic version of CG that allows us to study the magnitude of the cascade rate along different directions with respect to the mean magnetic field. Its implementation on the numerical data with moderate background magnetic field shows a weaker cascade along the magnetic field than in the perpendicular plane, while an isotropic cascade is recovered in the absence of a background field. The strength of the CG approach is further revealed when considering the {\it local-in-space} energy transfer, which is shown theoretically and numerically to match at a given position $\bm{x}$, when locally averaged over a neighboring region, the (quasi-)local dissipation. Prospects of exploiting this new model to investigate local dissipation in spacecraft data are discussed.

physics.plasm-ph↗

arXiv2021

Characterizing current structures in 3D hybrid-kinetic simulations of plasma turbulence

In space and astrophysical plasmas turbulence leads to the development of coherent structures characterized by a strong current density and important magnetic shears. Using hybrid-kinetic simulations of turbulence (3D with different energy injection scales) we investigate the development of these coherent structures and characterize their shape. First, we present different methods to estimate the overall shape of the 3D structure using local measurements, foreseeing application on satellite data. Then we study the local magnetic configuration inside and outside current peak regions, comparing the statistics in the two cases. Last, we compare the statistical properties of the local configuration obtained in simulations with the ones obtained analyzing an MMS dataset having similar plasma parameters. Thanks to our analysis, 1) we validate the possibility to study the overall shape of 3D structures using local methods, 2) we provide an overview of local magnetic configuration emerging in different turbulent regimes, 3) we show that our 3D-3V simulations can reproduce the structures that emerge in MMS data for the periods studied by Phan et al. (2018), Stawarz et al. (2019).

physics.plasm-ph↗

arXiv2021

Bridging hybrid- and full-kinetic models with Landau-fluid electrons: I. 2D magnetic reconnection

Magnetic reconnection (MR) plays a fundamental role in plasma dynamics under many different conditions, from space and astrophysical environments to laboratory devices. High-resolution in-situ measurements from space missions allow to study naturally occurring MR processes in great detail. Alongside direct measurements, numerical simulations play a key role in investigating the fundamental physics underlying MR. The choice of an adequate plasma model to be employed in numerical simulations, while also compromising with their computational cost, is crucial to efficiently address the problem. We consider a new plasma model that includes a refined electron response within the hybrid-kinetic framework (kinetic ions, fluid electrons). The extent to which this new model can reproduce a full-kinetic description of 2D MR, with particular focus on its robustness during the non-linear stage, is evaluated. We perform 2D simulations of MR with moderate guide field by means of three different plasma models: a hybrid-Vlasov-Maxwell model with isotropic, isothermal electrons, a hybrid-Vlasov-Landau-fluid (HVLF) model where an anisotropic electron fluid is equipped with a Landau-fluid closure, and a full-kinetic one. When compared to the full-kinetic case, the HVLF model effectively reproduces the main features of MR, as well as several aspects of the associated electron micro-physics and its feedback onto proton dynamics. This includes the global evolution of MR and the local physics occurring within the so-called electron-diffusion region, as well as the evolution of species pressure anisotropy. In particular, anisotropy driven instabilities (such as firehose, mirror, and cyclotron instabilities) play a relevant role in regulating electrons anisotropy during the non-linear stage of MR. As expected, the HVLF model captures all these features, except for the electron-cyclotron instability.

physics.plasm-ph↗

arXiv2020

Detecting Reconnection Events in Kinetic Vlasov Hybrid Simulations Using Clustering Techniques

Kinetic turbulence in magnetized space plasmas has been extensively studied via in situ observations, numerical simulations and theoretical models. In this context, a key point concerns the formation of coherent current structures and their disruption through magnetic reconnection. We present automatic techniques aimed at detecting reconnection events in large data set of numerical simulations. We make use of clustering techniques known as K-means and DBscan (usually referred in literature as unsupervised machine learning approaches), and other methods based on thresholds of standard reconnection proxies. All our techniques use also a threshold on the aspect ratio of the regions selected. We test the performance of our algorithms. We propose an optimal aspect ratio to be used in the automated machine learning algorithm: AR=18. The performance of the unsupervised approach results to be strongly competitive with respect to those of other methods based on thresholds of standard reconnection proxies.

physics.plasm-ph↗

arXiv2020

Statistical Properties of Turbulent Fluctuations Associated with Electron-only Magnetic Reconnection

Context: Recent satellite measurements in the turbulent magnetosheath of Earth have given evidence of an unusual reconnection mechanism that is driven exclusively by electrons. This newly observed process was called electron-only reconnection, and its inter-play with plasma turbulence is a matter of great debate. Aims: By using 2D-3V hybrid Vlasov-Maxwell simulations of freely decaying plasma turbulence, we study the role of electron-only reconnection in the development of plasma turbulence. In particular, we search for possible differences with respect to the turbulence associated with standard ion-coupled reconnection. Methods: We analyzed the structure functions of the turbulent magnetic field and ion fluid velocity fluctuations to characterize the structure and the intermittency properties of the turbulent energy cascade. Results: We find that the statistical properties of turbulent fluctuations associated with electron-only reconnection are consistent with those of turbulent fluctuations associated with standard ion-coupled reconnection, and no peculiar signature related to electron-only reconnection is found in the turbulence statistics. This result suggests that the turbulent energy cascade in a collisionless magnetized plasma does not depend on the specific mechanism associated with magnetic reconnection. The properties of the dissipation range are discussed as well, and we claim that only electrons contribute to the dissipation of magnetic field energy at sub-ion scales.

physics.plasm-ph↗

arXiv2020

Simulation of plasmaspheric plume impact on dayside magnetic reconnection

During periods of strong magnetic activity, cold dense plasma from the plasmasphere typically forms a plume extending towards the dayside magnetopause, eventually reaching it. In this work, we present a large-scale two-dimensional fully kinetic Particle-In-Cell simulation of a reconnecting magnetopause hit by a propagating plasmaspheric plume. The simulation is designed so that it undergoes four distinct phases: initial unsteady state, steady state prior to plume arrival at the magnetospause, plume interaction and steady state once the plume is well engulfed in the reconnection site. We show the evolution of the magnetopause's dynamics subjected to the modification of the inflowing plasma. Our main result is that the change in the plasma temperature (cold protons in the plume) have no effects on the magnetic reconnection rate, which on average depends only on the inflowing magnetic field and total ion density, before, during and after the impact.

physics.plasm-ph↗