arXiv subjects

Luca Franci

Publications and source records attributed to Luca Franci.

At least 19 recordsLinked to original sources

arXiv2026

Anomalous Electric Fields in Earth's Turbulent Magnetosheath: Insights From 3D Hybrid Simulations

In both collisional and collisionless plasmas the presence of a broad range of electromagnetic and plasma fluctuations provides anomalous electric fields that can be important for the dynamical evolution of the system as it is the case of magnetic reconnection, plasma turbulence and dynamo theory. In the context of plasma turbulence at scales larger than the ion's inertial length, the plasma satisfies the frozen-in condition, and the anomalous electric fields are produced by correlations between turbulent velocity and magnetic fields. Conversely, for collisionless plasmas and at kinetic scales, the total electric field has additional contributions that arise from kinetic phenomena, namely, charge separation, ambipolar electric fields and electron inertia, and each of these terms presents an anomalous counterpart. In this work we characterize the anomalous electric fields. We present a framework that can explain partial balance between the anomalous resistivity and anomalous transport, and we use it to study anomalous electric fields at kinetic scales. We establish how the different contributions to the anomalous electric field couple to the large-scale electric fields and show that the anomalous terms act back-reacting on magnetic-field organization and they may contribute to effective turbulent magnetic diffusion or shielding. Finally, we test two Sub-Grid-Scale models including anisotropic transport coefficients and show that although these models partially recover the spectral information, the phase coherence is not entirely recovered by these models suggesting a more complex non-linear contribution of the anomalous terms to the resolved scales.

physics.plasm-ph↗

arXiv2026

Shear interaction and acceleration of a corotating stream detected by Parker Solar Probe

Recent analysis of Parker Solar Probe (PSP) and Solar Orbiter data indicates that fluctuations energy is transferred to the bulk flow in fast streams of the solar wind at heliocentric distances between 15 and 120 Rsun. To gain insight into this process, we analyze data collected during the inbound orbit of PSP in Encounter 10, when it is in approximate corotation with the Sun and an accelerating stream is detected between 25 and 45 Rsun. The geometry of the flow indicates that a neighboring very fast streams is colliding with the corotating stream, causing large-scale magnetic field distortion, density enhancement, and acceleration. However, also deposition of fluctuations' energy can heat and do work on the solar wind plasma, resulting in a radial acceleration. We evaluate the energy lost by fluctuations via radial variation of conserved quantities in the corotating stream and find that it is almost equally partitioned into heating and work. While the heating is marginally consistent with the non-adiabatic decrease of proton temperature, the work exerted on the wind is insufficient to account for the measured solar wind acceleration. Although limited to this event, observations suggest that shear interaction is capable of accelerating slower streams within relatively short distances from the Sun, possibly leaving its imprint as large-scale density and magnetic fluctuations.

astro-ph.SR↗

arXiv2025

UK White Paper on Magnetic Reconnection

Magnetic reconnection powers explosive releases of magnetic energy, heating and particle acceleration throughout the plasma universe. Knowledge of this universal process is vital to understanding the Heliosphere, as it plays a key role in solar flares, coronal mass ejections, coronal heating, solar wind acceleration, geomagnetic storms, and interactions between the solar wind and planetary magnetospheres. As such, reconnection underpins multiple science objectives of multiple future space missions. The UK plays a leading role in this international field, through a combination of in situ measurements from Earth's magnetosphere and the solar wind, observations of the solar corona and chromosphere, and world-class numerical simulations and theory. This white paper identifies: Nine priority science objectives for reconnection research in the next decade; Recommendations to guide investment in theory, simulations and infrastructure; Mission priorities and required measurements to ensure the UK maintains and improves its world-class credentials in reconnection science.

astro-ph.IM↗

arXiv2025

The near-Sun Heliospheric Current Sheet, fluid and kinetic properties

The heliospheric current sheet (HCS) is an important large-scale structure of the heliosphere, and, for the first time, the Parker Solar Probe (PSP) mission enables us to study its properties statistically close to the Sun. We visually identify the 39 HCS crossings measured by PSP below 50~\Rs~during encounters 6 to 21, and investigate the occurrence and properties of magnetic reconnection, the behavior of the spectral properties of the turbulent energy cascade, and the occurrence of kinetic instabilities at the HCS. We find that 82\% of HCS crossings present signatures of reconnection jets, showing that the HCS is continuously reconnecting close to the Sun. The proportion of inward/outward jets depends on heliocentric distance, and the main HCS reconnection X-line has a higher probability of being located close to the Alfv\'en surface. We also observe a radial asymmetry in jet acceleration, where inward jets do not reach the local Alfv\'en speed, contrary to outward jets. We find that turbulence levels are enhanced in the ion kinetic range, consistent with the triggering of an inverse cascade by magnetic reconnection. Finally, we highlight the ubiquity of magnetic hole trains in the high $\beta$ environment of the HCS, showing that the mirror mode instability plays a key role in regulating the ion temperature anisotropy in HCS reconnection. Our findings shed new light on the properties of magnetic reconnection in the high $\beta$ plasma environment of the HCS, its interplay with the turbulent cascade and the role of the mirror mode instability.

astro-ph.SR↗

arXiv2025

Evolution of ion distribution functions in ionospheric plasmas perturbed by Alfv\'en waves

This study investigates ion kinetic effects during the parametric decay instability (PDI) of parallel-propagating Alfv\'en waves under plasma conditions characteristic of the Earth's ionosphere. By using a series of hybrid particle-in-cell simulations, we examine the evolution of ion velocity distribution functions (VDFs) in ultra-low-beta plasmas. Our numerical campaign systematically explores the dependence on key parameters (plasma beta, pump-wave amplitude and polarization, and ion composition). To emphasize the role of kinetic effects, we choose to trigger the PDI with a dispersive mother wave with wavelength comparable to the ion characteristic inertial length. Our results reveal pronounced nonthermal VDF modifications, including parallel heating and the formation of secondary ion beams, linked to the nonlinear evolution of parametric decay instability. By varying the plasma beta and the pump-wave amplitude, we identify a critical regime where rapid and complete broadening of the velocity distribution function is observed, triggering bidirectional ion acceleration. Notably, simulations modeling realistic ionospheric conditions demonstrate that even low-amplitude Alfv\'enic perturbations can induce significant VDF spreading and ion beam generation, with hydrogen ions exhibiting stronger effects than oxygen. These nonthermal microscopic processes offer a plausible mechanism for particle precipitation in space weather events. This work represents the first comprehensive study with hybrid simulations of PDI-driven ion kinetics in ultra-low-beta plasmas, providing quantitative estimates for the time delay between electromagnetic wave impact and ion VDF modification and new insights into wave-particle interactions that may contribute to ion acceleration, precipitation processes and space plasma dynamics.

physics.plasm-ph↗

arXiv2024

Temperature anisotropy instabilities driven by intermittent velocity shears in the solar wind

Where and under what conditions the transfer of energy between electromagnetic fields and particles takes place in the solar wind remains an open question. We investigate the conditions that promote the growth of kinetic instabilities predicted by linear theory, to infer how turbulence and temperature-anisotropy-driven instabilities are interrelated. Using a large dataset from Solar Orbiter, we introduce the radial rate of strain, a novel measure computed from single-spacecraft data, that we interpret as a proxy for the double-adiabatic strain rate. The solar wind exhibits high absolute values of the radial rate of strain at locations with large temperature anisotropy. We measure the kurtosis and skewness of the radial rate of strain from the statistical moments to show that it is non-Gaussian for unstable intervals and increasingly intermittent at smaller scales with a power-law scaling. We conclude that the velocity field fluctuations in the solar wind contribute to the presence of temperature anisotropy sufficient to create potentially unstable conditions.

physics.space-ph↗

arXiv2024

Turbulent Energy Conversion Associated with Kinetic Microinstabilities in Earth's Magnetosheath

Plasma in the terrestrial magnetosheath is characterised by very weak particle-particle collisions, so kinetic microinstabilities are thought to be responsible for regulating the thermodynamics of the plasma. By exciting electromagnetic waves, these instabilities redistribute free energy in velocity space, moulding the velocity distribution function (VDF) into a lower energy state. In the high-beta magnetosheath, relatively small perturbations to the VDF can easily excite instabilities compared to in the low-beta inner heliosphere. Since magnetic fields cannot do work on the particles, electric fields mediate energy exchange between the electromagnetic field and the bulk fluid properties of the plasma. We investigate signatures of non-ideal energy conversion associated with turbulent fluctuations in the context of electron and ion temperature anisotropy-beta instabilities, utilising over 24 hours of data spread over 163 distinct intervals of in situ magnetosheath observations from Magnetospheric Multiscale (MMS). We find that average energy conversion into fluid flow is enhanced along instability boundaries, suggesting that turbulence is playing a role in how free energy is redistributed in the plasma. The work enables a quantification of the energetics which are associated with the role of kinetic microinstabilities in regulating collisionless plasma thermodynamics. This work provides insight into the open question of how specific plasma processes couple into the turbulent dynamics and ultimately lead to energy dissipation and particle energisation in collisionless plasmas.

physics.space-ph↗

arXiv2022

The essential role of multi-point measurements in investigations of turbulence, three-dimensional structure, and dynamics: the solar wind beyond single scale and the Taylor Hypothesis

Space plasmas are three-dimensional dynamic entities. Except under very special circumstances, their structure in space and their behavior in time are not related in any simple way. Therefore, single spacecraft in situ measurements cannot unambiguously unravel the full space-time structure of the heliospheric plasmas of interest in the inner heliosphere, in the Geospace environment, or the outer heliosphere. This shortcoming leaves numerous central questions incompletely answered. Deficiencies remain in at least two important subjects, Space Weather and fundamental plasma turbulence theory, due to a lack of a more complete understanding of the space-time structure of dynamic plasmas. Only with multispacecraft measurements over suitable spans of spatial separation and temporal duration can these ambiguities be resolved. We note that these characterizations apply to turbulence across a wide range of scales, and also equally well to shocks, flux ropes, magnetic clouds, current sheets, stream interactions, etc. In the following, we will describe the basic requirements for resolving space-time structure in general, using turbulence' as both an example and a principal target or study. Several types of missions are suggested to resolve space-time structure throughout the Heliosphere.

physics.space-ph↗

arXiv2022

Anisotropic electron heating in turbulence-driven magnetic reconnection in the near-Sun solar wind

We perform a high-resolution two-dimensional fully-kinetic numerical simulation of a turbulent plasma system with observation-driven conditions, in order to investigate the interplay between turbulence, magnetic reconnection, and particle heating from ion to sub-electron scales in the near-Sun solar wind. We find that the power spectra of the turbulent plasma and electromagnetic fluctuations show multiple power-law intervals down to scales smaller than the electron gyroradius. Magnetic reconnection is observed to occur in correspondence of current sheets with a thickness of the order of the electron inertial length, which form and shrink due to interacting ion-scale vortexes. In some cases, both ion and electron outflows are observed (the classic reconnection scenario), while in others -- typically for the shortest current sheets -- only electron jets are presents ("electron-only reconnection"). At the onset of reconnection, the electron temperature starts to increase and a strong parallel temperature anisotropy develops. This suggests that in strong turbulence electron-scale coherent structures may play a significant role for electron heating, as impulsive and localized phenomena such as magnetic reconnection may transfer energy from the electromagnetic fields to particles more efficiently than damping mechanisms related to interactions with wave-like fluctuations.

physics.space-ph↗

arXiv2022

Ion-scale transition of plasma turbulence: Pressure-strain effect

We investigate properties of solar wind-like plasma turbulence using direct numerical simulations. We analyze the transition from large, magnetohydrodynamic (MHD) scales to the ion characteristic ones using two-dimensional hybrid (fluid electrons, kinetic ions) simulations. To capture and quantify turbulence properties, we apply the Karman-Howarth-Monin (KHM) equation for compressible Hall MHD (extended by considering the plasma pressure as a tensor quantity) to the numerical results. The KHM analysis indicates that the transition from MHD to ion scales (the so called ion break in the power spectrum) results from a combination of an onset of Hall physics and of an effective dissipation owing to the pressure-strain energy-exchange channel and resistivity. We discuss the simulation results in the context of the solar wind.

astro-ph.SR↗

arXiv2021

Spacetime Hall-MHD turbulence at sub-ion scales: structures or waves?

Spatiotemporal properties of two-dimensional (2D) Hall-magnetohydrodynamic turbulence at intermediate plasma $\beta=2$ are studied by means of Fast Iterative Filtering, a new technique for the decomposition of nonstationary nonlinear signals. Results show that the magnetic energy at sub-ion scales is concentrated in perturbations with frequencies smaller than the ion-cyclotron (IC) frequency and with polarization properties that are incompatible with both kinetic Alfv\'en waves (KAWs) and IC waves. At higher frequencies, we clearly identify signatures of both whistler waves and KAWs, however their energetic contribution to the magnetic power spectrum is negligible. We conclude that the dynamics of 2D Hall-MHD turbulence at sub-ion scales is mainly driven by localized intermittent structures, with no significant contribution of wavelike fluctuations.

physics.plasm-ph↗

arXiv2021

Spectral transfer and K\'arm\'an-Howarth-Monin equations for compressible Hall magnetohydrodynamics

We derive two new forms of the K\'arm\'an-Howarth-Monin equation for decaying compressible Hall magnetohydrodynamic (MHD) turbulence. We test them on results of a weakly-compressible, two-dimensional, moderate-Reynolds-number Hall MHD simulation and compare them with an isotropic spectral transfer (ST) equation. The KHM and ST equations are automatically satisfied during the whole simulation owing to the periodic boundary conditions and have complementary cumulative behavior. They are used here to analyze the onset of turbulence and its properties when it is fully developed. These approaches give equivalent results characterizing: the decay of the kinetic + magnetic energy at large scales, the MHD and Hall cross-scale energy transfer/cascade, the pressure dilatation, and the dissipation. The Hall cascade appears when the MHD one brings the energy close to the ion inertial range and is related to the formation of reconnecting current sheets. At later times, the pressure-dilation energy-exchange rate oscillates around zero with no net effect on the cross-scale energy transfer when averaged over a period of its oscillations. A reduced one-dimensional analysis suggests that all three methods may be useful to estimate the energy cascade rate from in situ observations.

physics.plasm-ph↗

arXiv2020

Evidence of a "current-mediated" turbulent regime in space and astrophysical plasmas

How the turbulent energy cascade develops below the magnetohydrodynamic scales in space and astrophysical plasmas is a major open question. Here, we measure the power spectrum of magnetic fluctuations in Parker Solar Probe's observations close to the Sun and in state-of-the-art numerical simulations of plasma turbulence. Both reveal a power-law behavior with a slope compatible with $-11/3$ at scales smaller than the ion characteristic scales, steeper than what is typically observed in the solar wind and in the Earth's magnetosheath. We explain such behavior by developing a simple two-fluid model which does not require any kinetic processes nor electron-inertia effects. This is characterized by a significant contribution of the ion kinetic energy to the total turbulent energy cascade at sub-ion scales, although the dynamics is driven by the magnetic field through the current density. We expect that this regime may be relevant for a broad class of low-beta plasmas, e.g. the solar corona, non-relativistic magnetized jets and disks, and laboratory plasmas.

physics.plasm-ph↗

arXiv2020

In-situ observation of Hall Magnetohydrodynamic Cascade in Space Plasma

We present estimates of the turbulent energy cascade rate, derived from a Hall-MHD third-order law. We compute the contribution from the Hall term and the MHD term to the energy flux. We use MMS data accumulated in the magnetosheath and the solar wind, and compare the results with previously established simulation results. We find that in observation, the MHD contribution is dominant at inertial scales, as in the simulations, but the Hall term becomes significant in observations at larger scales than in the simulations. Possible reasons are offered for this unanticipated result.

physics.space-ph↗

arXiv2020

Multidimensional Iterative Filtering: a new approach for investigating plasma turbulence in numerical simulations

Turbulent space and astrophysical plasmas exhibit a complex dynamics, which involves nonlinear coupling across different temporal and spatial scales. There is growing evidence that impulsive events, such as magnetic reconnection instabilities, lead to a spatially localized enhancement of energy dissipation, thus speeding up the energy transfer at small scales. Capturing such a diverse dynamics is challenging. Here, we employ the Multidimensional Iterative Filtering (MIF) method, a novel technique for the analysis of nonstationary multidimensional signals. Unlike other traditional methods (e.g., based on Fourier or wavelet decomposition), MIF does not require any previous assumption on the functional form of the signal to be identified. Using MIF, we carry out a multiscale analysis of Hall-magnetohydrodynamic (HMHD) and hybrid particle-in-cell (HPIC) numerical simulations of decaying plasma turbulence. The results assess the ability of MIF to spatially identify and separate the different scales (the MHD inertial range, the sub-ion kinetic, and the dissipation scales) of the plasma dynamics. Furthermore, MIF decomposition allows to detect localized current structures and to characterize their contribution to the statistical and spectral properties of turbulence. Overall, MIF arises as a very promising technique for the study of turbulent plasma environments.

physics.plasm-ph↗

arXiv2020

Fast acceleration of transrelativistic electrons in astrophysical turbulence

Highly energetic, relativistic electrons are commonly present in many astrophysical systems, from solar flares to the intra-cluster medium, as indicated by observed electromagnetic radiation. However, open questions remain about the mechanisms responsible for their acceleration, and possible re-acceleration. Ubiquitous plasma turbulence is one of the possible universal mechanisms. We study the energization of transrelativistic electrons in turbulence using hybrid particle-in-cell, which provide a realistic model of Alfvénic turbulence from MHD to sub-ion scales, and test particle simulations for electrons. We find that, depending on the electron initial energy and turbulence strength, electrons may undergo a fast and efficient phase of energization due to the magnetic curvature drift during the time they are trapped in dynamic magnetic structures. In addition, electrons are accelerated stochastically which is a slower process that yields lower maximum energies. The combined effect of these two processes determines the overall electron acceleration. With appropriate turbulence parameters, we find that superthermal electrons can be accelerated up to relativistic energies. For example, with heliospheric parameters and a relatively high turbulence level, rapid energization to MeV energies is possible.

physics.plasm-ph↗

arXiv2020

On cascade of kinetic energy in compressible hydrodynamic turbulence

Properties of the turbulent cascade of kinetic energy are studied using direct numerical simulations of three-dimensional hydrodynamic decaying turbulence with a moderate Reynolds number and the initial Mach number $M=1$. Compressible and incompressible versions of the Karman-Howarth-Monin (KHM) and low-pass filtering/coarse-graining approaches are compared. In the simulation the total energy is well conserved; the scale dependent KHM and coarse-grained energy equations are also well conserved; the two approaches show similar results, the system does not have an inertial range for the cascade of kinetic energy, the region where this cascade dominates also have a non-negligible contribution of the kinetic-energy decay, dissipation, and pressure-dilatation effects. While the two approaches give semi-quantitatively similar results for the kinetic energy cascade, dissipation and pressure-dilatation rates, they differ in the increment separation and filtering scales; these scales are not simply related. The two approaches may be used to find the inertial range and to determine the cascade/dissipation rate of the kinetic energy.

physics.flu-dyn↗

arXiv2019

Modeling Kelvin-Helmholtz instability-driven turbulence with hybrid simulations of Alfv\'enic turbulence

Magnetospheric Multiscale (MMS) observations of plasma turbulence generated by a Kelvin-Helmholtz (KH) event at the Earth's magnetopause are compared with a high-resolution two-dimensional (2D) hybrid direct numerical simulation (DNS) of decaying plasma turbulence driven by large-scale balanced Alfv\'enic fluctuations. The simulation, set up with four observation-driven physical parameters (ion and electron betas, turbulence strength, and injection scale) exhibits a quantitative agreement on the spectral, intermittency, and cascade-rate properties with in situ observations, despite the different driving mechanisms. Such agreement demonstrates a certain universality of the turbulent cascade from magnetohydrodynamic (MHD) to sub-ion scales, whose properties are mainly determined by the selected parameters, also indicating that the KH instability-driven turbulence has a quasi-2D nature. The validity of the Taylor hypothesis in the sub-ion spatial range suggests that the fluctuations at sub-ion scales have predominantly low frequencies, consistent with a kinetic Alfv\'en wave-like nature or with quasi-static structures. Finally, the third-order structure function analysis indicates that the cascade rate of the turbulence generated by a KH event in the magnetopause is an order of magnitude larger than in the ambient magnetosheath.

astro-ph.EP↗