arXiv subjects

Federico Fraternale

Publications and source records attributed to Federico Fraternale.

13 recordsLinked to original sources

arXiv2026

Shocks, compressible perturbations, and intermittency in the very local interstellar medium: Voyager 1 and 2 observations and numerical modeling

Voyager spacecraft (V1 and V2) provide unique in situ measurements of perturbations propagating beyond the heliopause through the very local interstellar medium (VLISM), including the shocks and pressure fronts whose origin is debated. In particular, a jump in magnetic field strength, observed by V1 in 2020.4 at 149.3 au from the Sun, was followed by a distinct "hump" and persistently strong magnetic field, both requiring theoretical explanation. This paper offers an interpretation of those observations using a self-consistent, MHD model of the solar wind - LISM interaction driven by the OMNI and interplanetary scintillation data combined with a turbulence analysis of Voyager data. Our simulations convincingly demonstrate that global, solar-cycle-driven compressions, on hitting the heliopause, can reproduce those puzzling V1 observations. They appear to be associated with solar cycle 24, whereas similar interstellar magnetic field structures can occur once per cycle. The turbulence analysis reveals time-dependent magnetic compressibility that persists up to 165 au at scales below 10 days. Turbulence intermittency at scales below 1 hour is mostly confined to specific intervals, possibly associated with a broad foreshock region. The apparent disappearance of intermittency since 2022 reflects the turbulence weakening rather than a fundamental change in VLISM properties. We predict that V1 will record relatively strong magnetic field strengths until $\sim$2030, followed by weaker, infrequent perturbations. At V2, we expect multiple solar-driven compressions before 2026, followed by a major event induced by solar cycle 25 around 2030. New Horizons is expected to cross the termination shock at 80$\pm$2 au in 2031.

physics.space-ph↗

arXiv2025

Magnetohydrodynamic/Kinetic Modeling of the Interaction between the Solar Wind and the Local Interstellar Medium with He$^+$, He$^{2+}$, and Pickup H$^+$ ions and H, He Atoms

The interaction of the solar wind with the local interstellar medium (LISM) spans a wide range of interacting particle populations, energies, and scales. Sophisticated models are required to capture the global picture, interpret near-Earth observations, and ultimately understand the properties of the LISM at distances of thousands of AUs, where the medium is presumed to be unperturbed by this interaction. We present a new extension of our MHD-plasma/kinetic-neutral heliospheric model, implemented within the Multi-Scale Fluid-Kinetic Simulation Suite (MS-FLUKSS). The new model treats singly and doubly charged helium ions, pickup protons, and electrons as separate, self-consistently coupled populations, interacting through six charge exchange processes and photoionization with kinetically treated neutral hydrogen and helium atoms. In this paper, we provide detailed information on the implementation, including new fits for the charge-exchange cross sections, and demonstrate the functionality and performance of the new code.

physics.space-ph↗

arXiv2022

Turbulence in the outer heliosphere

The solar wind (SW) and local interstellar medium (LISM) are turbulent media. Their interaction is governed by complex physical processes and creates heliospheric regions with significantly different properties in terms of particle populations, bulk flow and turbulence. Our knowledge of the solar wind turbulence \nature and dynamics mostly relies on near-Earth and near-Sun observations, and has been increasingly improving in recent years due to the availability of a wealth of space missions, including multi-spacecraft missions. In contrast, the properties of turbulence in the outer heliosphere are still not completely understood. In situ observations by Voyager and New Horizons, and remote neutral atom measurements by IBEX strongly suggest that turbulence is one of the critical processes acting at the heliospheric interface. It is intimately connected to charge exchange processes responsible for the production of suprathermal ions and energetic neutral atoms. This paper reviews the observational evidence of turbulence in the distant SW and in the LISM, advances in modeling efforts, and open challenges.

physics.space-ph↗

arXiv2021

Wave focusing and related multiple dispersion transitions in plane Poiseuille flows

Motivated by the recent discovery of a dispersive-to-nondispersive transition for linear waves in shear flows, we accurately explored the wavenumber-Reynolds number parameter map of the plane Poiseuille flow, in the limit of least-damped waves. We have discovered the existence of regions of the map where the dispersion and propagation features vary significantly from their surroundings. These regions are nested in the dispersive, low-wavenumber part of the map. This complex dispersion scenario demonstrates the existence of linear dispersive focusing in wave envelopes evolving out of an initial, spatially localized, three-dimensional perturbation. An asymptotic wave packet's representation, based on the saddle-point method, allows to enlighten the nature of the packet's morphology, in particular the arrow-shaped structure and spatial spreading rates. A correlation is also highlighted between the regions of largest dispersive focusing and the regions which are most subject to strong nonlinear coupling in observations.

physics.flu-dyn↗

arXiv2021

Intermittency acceleration of water droplet population dynamics inside the interfacial layer between cloudy and clear air environments

We use direct numerical simulation to study the temporal evolution of a perturbation localized on the turbulent layer that typically separates a cloud from the surrounding clear air. Across this shearless layer, a turbulent kinetic energy gradient naturally forms. Here, a finite perturbation in the form of a local initial temperature fluctuation is applied to simulate a hydrodynamic instability inside the background turbulent air flow. A numerical initial value problem for two diametrically opposite types of drop population distributions is then solved. Specifically, we consider a mono-disperse population of droplets of 15 $μ$m of radius and a poly-disperse distribution with radii in the range 0.6 - 30 $μ$m. For both distributions, it is observed that the evaporation and condensation have a dramatically different weight inside the homogeneous cloudy region and the interfacial anisotropic mixing region. It is observed that the dynamics of drop collisions is highly effected by the turbulence structure of the host region. The two populations show a common aspect during their energy decay transient. That is the increased probability of collisions in the interfacial layer hat houses intense anisotropic velocity fluctuations. This layer, in fact, induces an enhanced differentiation on droplets kinetic energy and sizes. Both polydisperse and monodisperse initial particle distributions contain $10^7$ droplets, matching an initial liquid water content of $0.8 g/m^3$. An estimate of the turbulent collision kernel for geometric collisions used in the population balance equations is given. A preliminary discussion is presented on the structure of the two unsteady non ergodic collision kernels obtained inside the cloud interface region.

physics.flu-dyn↗

arXiv2019

Sign singularity of the local energy transfer in space plasma turbulence

In weakly collisional space plasmas, the turbulent cascade provides most of the energy that is dissipated at small scales by various kinetic processes. Understanding the characteristics of such dissipative mechanisms requires the accurate knowledge of the fluctuations that make energy available for conversion at small scales, as different dissipation processes are triggered by fluctuations of a different nature. The scaling properties of different energy channels are estimated here using a proxy of the local energy transfer, based on the third-order moment scaling law for magnetohydrodynamic turbulence. In particular, the sign-singularity analysis was used to explore the scaling properties of the alternating positive-negative energy fluxes, thus providing information on the structure and topology of such fluxes for each of the different type of fluctuations. The results show the highly complex geometrical nature of the flux, and that the local contributions associated with energy and cross-helicity nonlinear transfer have similar scaling properties. Consequently, the fractal properties of current and vorticity structures are similar to those of the Alfvénic fluctuations.

physics.space-ph↗

arXiv2019

The structure of magnetic turbulence in the heliosheath region observed by Voyager 2 at 106 AU

It is currently believed that the turbulent fluctuations pervade the outermost heliosphere. Turbulence, magnetic reconnection, and their link may be responsible for magnetic energy conversion in these regions. The governing mechanisms of such anisotropic and compressible magnetic turbulence in the inner heliosheath (IHS) and in the local interstellar medium (LISM) still lack a thorough description. The present literature mainly concerns large scales which are not representative of the inertial-cascade dynamics of turbulence. Moreover, lack of broadband spectral analysis makes the IHS dynamics critically understudied. Our recent study shows that 48 s magnetic-field data from the Voyager mission are appropriate for a spectral analysis over a frequency range of six decades, from 5 x 10-8 Hz to 10-2 Hz. Here, focusing on the Voyager 2 observation interval from 2013.824 to 2016.0, we describe the structure of turbulence in a sector zone of the IHS. A spectral break around 7 x 10-7 Hz (magnetic structures with size l~1.3 Astronomical Units) separates the energy-injection regime from the inertial-cascade regime of turbulence. A second scale is observed around 6 x 10-5 Hz (l~ 0.017 AU) and corresponds to a peak of compressibility and intermittency of fluctuations.

physics.space-ph↗

arXiv2019

Magnetic turbulence spectra and intermittency in the heliosheath and in the local interstellar medium

The understanding of inertial-scale dynamics in the heliosheath is not yet thorough. Magnetic field fluctuations across the inner heliosheath (IHS) and the local interstellar medium (LISM) are here considered to provide accurate and highly resolved statistics over different plasma conditions between 88 and 136 au. By using the unique in situ 48 s measurements from the Voyager Interstellar Mission, we investigate different fluctuation regimes at the magnetohydrodynamic (MHD) scales, down to the MHD-to-kinetic transition. We focus on a range of scales exceeding five frequency decades (5 x 10-8 < f < 10-2 Hz), which is unprecedented in literature analysis. A set of magnetic field data for eight intervals in the IHS, in both unipolar and sector regions, and four intervals in the LISM is used for the analysis. Results are set forth in terms of the power spectral density, spectral compressibility, structure functions, and intermittency of magnetic field increments. In the heliosheath, we identify the energy-injection regime displaying a ~1/f energy decay, and the inertial-cascade regime. Here, the power spectrum is anisotropic and dominated by compressive modes, with intermittency that can reach kurtosis values of up to 10. In the interstellar medium the structure of turbulence is anisotropic as well, with transverse fluctuations clearly prevailing after 2015 May. Here, we show that intermittent features occur only at scales smaller than 10-6 Hz.

physics.space-ph↗

arXiv2019

Turbulence-driven ion beams in the magnetospheric Kelvin-Helmholtz instability

The description of the local turbulent energy transfer, and the high-resolution ion distributions measured by the Magnetospheric Multiscale mission, together provide a formidable tool to explore the cross-scale connection between the fluid-scale energy cascade and plasma processes at sub-ion scales. When the small-scale energy transfer is dominated by Alfvénic, correlated velocity and magnetic field fluctuations, beams of accelerated particles are more likely observed. Here, for the first time we report observations suggesting the nonlinear wave-particle interaction as one possible mechanism for the energy dissipation in space plasmas.

physics.space-ph↗

arXiv2018

Linear waves in sheared flows. Lower bound of the vorticity growth and propagation discontinuities in the parameters space

This study provides sufficient conditions for the temporal monotonic decay of enstrophy for two-dimensional perturbations traveling in the incompressible, viscous, plane Poiseuille and Couette flows. Extension of J. L. Synge's procedure (1938) to the initial-value problem allowed us to find the region of the wavenumber-Reynolds number map where the enstrophy of any initial disturbance cannot grow. This region is wider than the kinetic energy's one. We also show that the parameters space is split in two regions with clearly distinct propagation and dispersion properties.

physics.flu-dyn↗

arXiv2016

Dispersive to non-dispersive transition and phase velocity transient for linear waves in plane wake and channel flows

In this study we analyze the phase and group velocity of three-dimensional linear traveling waves in two sheared flows, the plane channel and the wake flows. This was carried out by varying the wave number over a large interval of values at a given Reynolds number inside the ranges 20-100, 1000-8000, for the wake and channel flow, respectively. Evidence is given about the possible presence of both dispersive and non-dispersive effects which are associated with the long and short ranges of wavelength. We solved the Orr-Sommerfeld and Squire eigenvalue problem and observed the least stable mode. It is evident that, at low wave numbers, the least stable eigenmodes in the left branch of the spectrum beave in a dispersive manner. By contrast, if the wavenumber is above a specific threshold, a sharp dispersive to non-dispersive transition can be observed. Beyond this transition, the dominant mode belongs to the right branch of the spectrum. The transient behavior of the phase velocity of small three-dimensional traveling waves was also considered. Having chosen the initial conditions, we then show that the shape of the transient highly depends on the transition wavelength threshold value. We show that the phase velocty can oscillate with a frequency which is equal to the frequency width of the eigenvalue spectrum. Furthermore, evidence of intermediate self-similarity is given for the perturbation field.

physics.flu-dyn↗

arXiv2015

Voyager 2 solar plasma and magnetic field spectral analysis for intermediate data sparsity

The Voyager probes are the furthest, still active, spacecraft ever launched from Earth. During their 38-year trip, they have collected data regarding solar wind properties (such as the plasma velocity and magnetic field intensity). Unfortunately, a complete time evolution of the measured physical quantities is not available. The time series contains many gaps which increase in frequency and duration at larger distances. The aim of this work is to perform a spectral and statistical analysis of the solar wind plasma velocity and magnetic field using Voyager 2 data measured in 1979, when the gaps/signal ratio is of order of unity. This analysis is achieved using four different data reconstruction techniques: averages on linearly interpolated subsets, correlation of linearly interpolated data, compressed sensing spectral estimation, and maximum likelihood data reconstruction. With five frequency decades, the spectra we obtained have the largest frequency range ever computed at 5 astronomical units from the Sun; spectral exponents have been determined for all the components of the velocity and magnetic field fluctuations. Void analysis is also useful in recovering other spectral properties such as integral scales (see for instance Table 4) and, if the confidence level of the measurements is sufficiently high, the decay variation in the small scale range due, for instance, to dissipative effects.

physics.space-ph↗

arXiv2013

Frequency Transient of Three-Dimensional Perturbations in Shear Flows. Similarity Properties and Wave Packets Linear Formation

The present thesis deals with the non-modal linear analysis of 3D perturbations in wall flows. In the first part,a solution to the Orr-Sommerfeld and Squire IVP, in the form of orthogonal functions expansion, is researched. The Galerkin method is successfully implemented to numerically compute approximate solutions for bounded flows. The Chandrasekhar functions revealed to ensure a fifth order of accuracy. The focus of the subsequent analysis is on the transient behavior of the perturbation frequency and phase velocity. The results confirm recent observations about a jump in the temporal evolution of the frequency of the wall-normal velocity signal, considered as the end of an Early Transient. After this jump, the wave frequency for Plane Couette flow experiences a periodic modulation about the asymptotic value, which is motivated and investigated in detail. A new result is the presence of a second frequency jump for the wall-normal vorticity. This fact, together with the possibility for different values of the signals asymptotic frequency, shows the existence of an Intermediate Transient. Moreover, a connection between the frequency jumps and the establishing of a self-similarity condition in time for both the velocity and vorticity profiles is found and investigated for both Plane Couette flow and Plane Poiseuille flow. Eventually, through superposition of waves with limited wavenumber range, a wave packet is reconstructed for Plane Couette flow and Blasius boundary-layer flow . The linear spot evolution revealed to have many common features with the early stages of a turbulent spot, particularly the streaky structure and the spot shape.

physics.flu-dyn↗