arXiv subjects

Silvia Perri

Publications and source records attributed to Silvia Perri.

18 recordsLinked to original sources

arXiv2026

A wavelet-based model of magnetic turbulence over a Parker spiral field

The propagation of Solar Energetic Particles (SEPs) through the heliosphere is strongly influenced by solar-wind magnetic turbulence, which governs particle scattering, field-line wandering, and cross-field transport. These processes shape SEP intensity profiles, anisotropies, and arrival times. Because heliospheric turbulence spans a vast range of scales, direct numerical simulations cannot capture its full spectrum. Synthetic turbulence models provide an efficient alternative by generating fields with prescribed statistical and spectral properties, enabling realistic studies of SEP transport. This work presents a numerical model able to generate synthetic magnetic turbulence in the heliosphere with the goal of studying field line and particle diffusion. The model is inspired by in situ solar wind observations, indicating that solar wind turbulence properties vary with the distance from the Sun. We extend the Cartesian wavelet-based Synthetic Turbulence Model (STM), presented in \cite{Malara2016PhRvE..94e3109M}, to the non-homogeneous expanding solar wind. This is achieved by finding a suitable change of coordinates that allows one to pass from a space in which the turbulence correlation length is homogeneous to one in which it is not. We present the analytical derivation, the numerical implementation, and testing. We obtain an improved and computationally efficient synthetic turbulence model characterized by a background Parker spiral, a broad inertial range and tunable radial scalings for both the correlation length and the magnetic-field fluctuation amplitude, as well as a controllable level of intermittency. The model capability of reproducing solar wind turbulence up to the particle Larmor scale and to take into account solar wind radial evolution makes it a powerful tool to study the propagation of SEPs throughout the heliosphere.

astro-ph.SR↗

arXiv2026

Numerical investigation of particle acceleration at interplanetary shocks: diffusive and superdiffusive scenarios

Energetic particles are ubiquitous in space and astrophysical plasmas, and interplanetary shocks are widely regarded as one of the main particle accelerators in the heliosphere. Indeed, in-situ measurements typically show that energetic particle fluxes peak at the shock, indicating a local acceleration process. Furthermore, the time profile of energetic particle fluxes is highly influenced by particle transport properties upstream and downstream of the shock. By advancing previous numerical test-particle models that simulate the transport of monoenergetic particles around an infinite planar shock, in this work we add the acceleration of such particles via energy gains at each shock crossing, in a first-order Fermi-type mechanism. Moreover, the acceleration of a 70 keV particle population, namely the seed population, is reproduced by integrating a Langevin-type equation upstream and downstream of an infinite planar shock. Particles can diffuse in the simulation box via random "kicks", which belong either to a Gaussian distribution (normal diffusion) or to a Levy distribution (superdiffusion). We perform several simulations by varying the parameters of the model. The particle energy spectra in both diffusive and superdiffusive simulations are in remarkable agreement with the theoretical predictions.The output energetic particle densities have been compared with those observed by the ACE spacecraft during an interplanetary shock crossing on December 14, 2006. We show not only that particle fluxes in different energy bins reproduce very well the observed ones upstream and downstream when superdiffusion is at work, but also that anomalous, superdiffusive transport speeds up the acceleration process and leads to values of particle energies consistent with observations.

astro-ph.SR↗

arXiv2025

Evolution of the Shock Properties of the 2023 March 13 Event from In-Situ and Remote-Sensing Data

Shocks driven by coronal mass ejections (CMEs) are the most powerful accelerators of gradual solar energetic particles (SEPs) in the inner heliosphere. On 2023 March 13, a halo CME, as seen from the Solar Heliospheric Observatory (SoHO) and the Sun TErrestrial Relations Observatory (STEREO), gave rise to a strong SEP event. In this work, we aim to analyze this CME-driven shock from multiple spacecraft, using both remote sensing observations from STEREO-A/COR2 and in-situ data from Parker Solar Probe (PSP), Solar Orbiter (SolO), and Wind. In order to determine its direction of propagation and kinematic properties, we model the shock geometry using STEREO-A/COR2 and SoHO/LASCO/C3 observations as an expanding ellipsoid. The density compression ratio of the shock is determined by fitting the brightness profile from the coronagraphic images with that obtained from raytracing simulations of a double-Gaussian shock density profile. We compare physical quantities such as compression ratio and Alfv\'enic Mach number derived from remote sensing observations with in-situ measurements by PSP, SolO, STEREO-A, and Wind. From STEREO-A/COR2, we determine the compression ratio around the entire shock front in the corona, finding significant non-homogeneities that can impact the values found during in-situ crossings. Following the evolution of the parameters characterizing the CME from the source to space, we find that closer to the Sun, both the gas compression ratio and the Alfv\'enic Mach number remain almost constant, while they increase at larger radial distances. This indicates a non-trivial evolution of the shock parameters during its journey through the interplanetary space.

astro-ph.SR↗

arXiv2025

Revisiting X-ray polarization of the shell of Cassiopeia A using spectropolarimetric analysis

X-ray synchrotron radiation is expected to be highly polarized. Thanks to the Imaging X-ray Polarimetry Explorer (IXPE), it is now possible to evaluate the degree of X-ray polarization in sources such as supernova remnants (SNRs). Jointly using IXPE data and high-resolution Chandra observations, we perform a spatially resolved spectropolarimetric analysis of SNR Cassiopeia A (Cas A). We focus in the 3-6 keV energy band on regions near the shell dominated by nonthermal synchrotron emission. By combining IXPE's polarization sensitivity with Chandra's higher spatial and spectral resolution, we constrain the local polarization degree (PD) and polarization angle (PA) across the remnant. Our analysis reveals PD values ranging locally from 10% to 26%, showing significant regional variations that underscore the complex magnetic field morphology of Cas A. The polarization vectors indicate a predominantly radial magnetic field, consistent with previous studies. Thanks to the improved modeling of thermal contamination using Chandra data, we retrieve higher PD values compared to earlier IXPE analysis and more significant detections with respect to the standard IXPEOBSSIM analysis. Finally, we also estimate the degree of magnetic turbulence {\eta} from the measured photon index and PD, under the assumption of an isotropic fluctuating field across the shell of Cas A.

astro-ph.HE↗

arXiv2025

Flat-top electron velocity distributions driven by wave-particle resonant interactions

The role of kinetic electrons in the excitation and sustainment of ion-bulk electrostatic waves in collisionless plasmas is investigated, with a focus on the physical mechanisms responsible for the generation of small-scale structures in space plasmas. Building on the work of F. Valentini et al., PRL, 106, 165002 (2011), we numerically solve the Vlasov-Poisson system in one spatial and one velocity dimension for both ions and electrons. Our findings reveal that a significant fraction of the energy supplied by an external driving electric field, used to trigger ion-bulk waves excitation, is transferred to electrons, which become trapped within the wave potential well. As a result, multiple phase-space vortices, generated during the early time evolution, undergo a merging process in the long-time limit, ultimately resulting in a single, coherent, and persistent phase-space hole in the distributions of both species. Furthermore, the resonant interaction between electrons and ion-bulk fluctuations induces a velocity-space diffusion process, leading to the development of a "flat-top" profile in the electron velocity distribution, routinely observed in near Earth space. To establish observational relevance, virtual spacecraft measurements were performed to evaluate the detectability of the velocity distribution features observed in the simulations using modern spaceborne instruments. The results presented here are consistent with observations of electrostatic phenomena in space plasmas, and underscore the widespread occurrence of such structures across various plasma environments.

physics.plasm-ph↗

arXiv2025

The Interaction of a Supernova Remnant with background interstellar turbulence

Supernova explosions (SNe) are among the most energetic events in the Universe. After the explosion, the material ejected by the Supernova expands throughout the interstellar medium (ISM) forming what is called Supernova Remnant (SNR). Shocks associated with the expanding SNR are sources of galactic cosmic rays, that can reach energy of the PeV order. In these processes, a key role is played by the magnetic field. It is known that the ISM is turbulent with an observed magnetic field of about a few $\mu$G, made by the superposition of a uniform and a fluctuating component. During the SNR expansion, the shock interacts with a turbulent environment, leading to a distortion of the shock front and a compression of the medium. In this work, we use the MagnetoHydroDynamics (MHD) PLUTO code to mimic the evolution of the blast wave associated with the SNR. We make a parametric study varying the level of density and magnetic field fluctuations in the interstellar medium, with the aim of understanding the best parameter values able to reproduce real observations. We introduce a novel analysis technique based on two-dimensional autocorrelation function $C({\ell})$ and the second order structure function $S_2(\ell)$, quantifying the level of anisotropy and the turbulence correlation lengths. By interpolating the autocorrelation function on a polar grid, we extract the power spectra of turbulence at the SNR. Finally, a preliminary comparison with Chandra observations of SN 1006 is also presented.

astro-ph.HE↗

arXiv2023

Interpretation of flat energy spectra upstream of fast interplanetary shocks

Interplanetary shocks are large-scale heliospheric structures often caused by eruptive phenomena at the Sun, and represent one of the main sources of energetic particles. Several interplanetary shock crossings by spacecraft at $1$ AU have revealed enhanced energetic-ion fluxes that extend far upstream of the shock. Surprisingly, in some shock events, ion fluxes with energies between $100$ keV and about $2$ MeV acquire similar values (which we refer to as ``overlapped'' fluxes), corresponding to flat energy spectra in that range. In contrast, closer to the shock, the fluxes are observed to depend on energy. In this work, we analyze three interplanetary shock-related energetic particle events observed by the Advanced Composition Explorer spacecraft where flat ion energy spectra were observed upstream of the shock. We interpret these observations via a velocity filter mechanism for particles in a given energy range. This reveals that low energy particles tend to be confined to the shock front and cannot easily propagate upstream, while high energy particles can. The velocity filter mechanism has been corroborated from observations of particle flux anisotropy by the Solid-State Telescope of Wind/3DP.

astro-ph.SR↗

arXiv2022

Probing the Density Fine Structuring of the Solar Corona with Comet Lovejoy

The passage of sungrazing comets in the solar corona can be a powerful tool to probe the local plasma properties. Here, we carry out a study of the striae pattern appearing in the tail of sungrazing Comet Lovejoy, as observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) during the inbound and outbound phases of the comet orbit. We consider the images in EUV in the 171 Å bandpass, where emission from oxygen ions O$^{4+}$ and O$^{5+}$ is found. The striae are described as due to a beam of ions injected along the local magnetic field, with the initial beam velocity decaying because of collisions. Also, ion collisional diffusion contributes to ion propagation. Both the collision time for velocity decay and the diffusion coefficient for spatial spreading depend on the ambient plasma density. A probabilistic description of the ion beam density along the magnetic field is developed, where the beam position is given by the velocity decay and the spreading of diffusing ions is described by a Gaussian probability distribution. Profiles of emission intensity along the magnetic field are computed and compared with the profiles along the striae observed by AIA, showing a good agreement for most considered striae. The inferred coronal densities are then compared with a hydrostatic model of the solar corona. The results confirm that the coronal density is strongly spatially structured.

astro-ph.SR↗

arXiv2021

Nature of electrostatic fluctuations in the terrestrial magnetosheath

The high cadence plasma, electric, and magnetic field measurements by the Magnetospheric MultiScale spacecraft allow us to explore the near-Earth space plasma with an unprecedented time and spatial resolution, resolving electron-scale structures that naturally emerge from plasma complex dynamics. The formation of small-scale turbulent features is often associated to structured, non-Maxwellian particle velocity distribution functions that are not at thermodynamic equilibrium. Using measurements in the terrestrial magnetosheath, this study focuses on regions presenting bumps in the power spectral density of the parallel electric field at sub-proton scales. Correspondingly, it is found that the ion velocity distribution functions exhibit beam-like features at nearly the local ion thermal speed. Ion cyclotron waves in the ion-scale range are frequently observed at the same locations. These observations, supported by numerical simulations, are consistent with the generation of ion-bulk waves that propagate at the ion thermal speed. This represents a new branch of efficient energy transfer at small scales, which may be relevant to weakly collisional astrophysical plasmas.

physics.plasm-ph↗

arXiv2020

In situ evidence of ion acceleration between consecutive reconnection jet fronts

Processes driven by unsteady reconnection can efficiently accelerate particles in many astrophysical plasmas. An example are the reconnection jet fronts in an outflow region. We present evidence of suprathermal ion acceleration between two consecutive reconnection jet fronts observed by the Magnetospheric Multiscale mission in the terrestrial magnetotail. An earthward propagating jet is approached by a second faster jet. Between the jets, the thermal ions are mostly perpendicular to magnetic field, are trapped and are gradually accelerated in the parallel direction up to 150 keV. Observations suggest that ions are predominantly accelerated by a Fermi-like mechanism in the contracting magnetic bottle formed between the two jet fronts. The ion acceleration mechanism is presumably efficient in other environments where jet fronts produced by variable rates of reconnection are common and where the interaction of multiple jet fronts can also develop a turbulent environment, e.g. in stellar and solar eruptions.

physics.plasm-ph↗

arXiv2020

Coherent events at ion scales in the inner Heliosphere: \textit{Parker Solar Probe} observations during the first Encounter

\textit{Parker Solar Probe} has shown the ubiquitous presence of strong magnetic field deflections, namely switchbacks, during its first perihelion where it was embedded in a highly Alfvénic slow stream. Here, we study the turbulent magnetic fluctuations around ion scales in three intervals characterized by a different switchback activity, identified by the behaviour of the magnetic field radial component, $B_r$. \textit{Quiet} ($B_r$ does not show significant fluctuations), \textit{weak} ($B_r$ has strong fluctuations but no reversals) and \textit{strong} ($B_r$ has full reversals) periods show a different behaviour also for ion quantities and Alfvénicity. However, the spectral analysis shows that each stream is characterized by the typical Kolmogorov/Kraichnan power law in the inertial range, followed by a break around the characteristic ion scales. This frequency range is characterized by strong intermittent activity, with the presence of non-compressive coherent structures, such as current sheets and vortex-like structures, and wave packets, identified as ion cyclotron modes. Although, all these intermittent events have been detected in the three periods, they have a different influence in each of them. Current sheets are dominant in the \textit{strong} period, wave packets are the most common in the \textit{quiet} interval; while, in the \textit{weak} period, a mixture of vortices and wave packets is observed. This work provides an insight into the heating problem in collisionless plasmas, fitting in the context of the new solar missions, and, especially for \textit{Solar Orbiter}, which will allow an accurate magnetic connectivity analysis, to link the presence of different intermittent events to the source region.

astro-ph.SR↗

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

On the deviation from Maxwellian of the ion velocity distribution functions in the turbulent magnetosheath

The degree of deviation from the thermodynamic equilibrium in the ion velocity distribution functions (VDFs), measured by the Magnetospheric Multiscale (MMS) mission in the Earth's turbulent magnetosheath, is quantitatively investigated. Taking advantage of MMS ion data, having a resolution never reached before in space missions, and of the comparison with Vlasov-Maxwell simulations, this analysis aims at relating any deviation from Maxwellian equilibrium to typical plasma parameters. Correlations of the non-Maxwellian features with plasma quantities such as electric fields, ion temperature, current density and ion vorticity are very similar in both magnetosheath data and numerical experiments, and suggest that distortions in the ion VDFs occur close to (but not exactly at) peaks in current density and ion temperature. Similar results have also been found during a magnetopause crossing by MMS. This work could help clarifying the origin of distortion of the ion VDFs in space plasmas.

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↗

arXiv2017

On the statistical properties of turbulent energy transfer rate in the inner Heliosphere

The transfer of energy from large to small scales in solar wind turbulence is an important ingredient of the longstanding question about the mechanism of the interplanetary plasma heating. Previous studies have shown that magnetohydrodynamic (MHD) turbulence is statistically compatible with the observed solar wind heating as it expands in the heliosphere. However, in order to understand which processes contribute to the plasma heating, it is necessary to have a local description of the energy flux across scales. To this aim, it is customary to use indicators such as the magnetic field partial variance of increments (PVI), which is associated with the local, relative, scale-dependent magnetic energy. A more complete evaluation of the energy transfer should also include other terms, related to velocity and cross-helicity. This is achieved here by introducing a proxy for the local, scale dependent turbulent energy transfer rate epsilon_Dt(t), based on the third-order moment scaling law for MHD turbulence. Data from Helios 2 are used to determine the statistical properties of such a proxy in comparison with the magnetic and velocity fields PVI, and the correlation with local solar wind heating is computed. PVI and epsilon_Dt(t) are generally well correlated, however epsilon_Dt(t) is a very sensitive proxy that can exhibit large amplitude values, both positive and negative, even for low amplitude peaks in the PVI. Furthermore, epsilon_Dt(t) is very well correlated with local increases of temperature when large amplitude bursts of energy transfer are localized, thus suggesting an important role played by this proxy in the study of plasma energy dissipation.

physics.space-ph↗

arXiv2017

On the estimation of the current density in space plasmas: multi versus single-point techniques

Thanks to multi-spacecraft mission, it has recently been possible to directly estimate the current density in space plasmas, by using magnetic field time series from four satellites flying in a quasi perfect tetrahedron configuration. The technique developed, commonly called 'curlometer' permits a good estimation of the current density when the magnetic field time series vary linearly in space. This approximation is generally valid for small spacecraft separation. The recent space missions Cluster and Magnetospheric Multiscale (MMS) have provided high resolution measurements with inter-spacecraft separation up to 100 km and 10 km, respectively. The former scale corresponds to the proton gyroradius/ion skin depth in 'typical' solar wind conditions, while the latter to sub-proton scale. However, some works have highlighted an underestimation of the current density via the curlometer technique with respect to the current computed directly from the velocity distribution functions, measured at sub-proton scales resolution with MMS. In this paper we explore the limit of the curlometer technique studying synthetic data sets associated to a cluster of four artificial satellites allowed to fly in a static turbulent field, spanning a wide range of relative separation. This study tries to address the relative importance of measuring plasma moments at very high resolution from a single spacecraft with respect to the multi-spacecraft missions in the current density evaluation.

physics.space-ph↗

arXiv2015

Parameter estimation of superdiffusive motion of energetic particles upstream of heliospheric shocks

In-situ spacecraft observations recently suggested that the transport of energetic particles accelerated at heliospheric shocks can be anomalous, i.e. the mean square displacement can grow non-linearly in time. In particular, a new analysis technique has permitted the study of particle transport properties from energetic particle time profiles upstream of interplanetary shocks. Indeed, the time/spatial power laws of the differential intensity upstream of several shocks are indicative of superdiffusion. A complete determination of the key parameters of superdiffusive transport comprises the power-law index, the superdiffusion coefficient, the related transition scale at which the energetic particle profiles turn to decay as power laws, and the energy spectral index of the shock accelerated particles. Assuming large-scale spatial homogeneity of the background plasma, the power-law behaviour can been derived from both a (microscopic) propagator formalism and a (macroscopic) fractional transport equation. We compare the two approaches and find a relation between the diffusion coefficients used in the two formalisms. Based on the assumption of superdiffusive transport, we quantitatively derive these parameters by studying energetic particle profiles observed by the Ulysses and Voyager 2 spacecraft upstream of shocks in the heliosphere, for which a superdiffusive particle transport has previously been observed. Further, we have jointly studied the electron energy spectra, comparing the values of the spectral indices observed with those predicted by the standard diffusive shock acceleration theory and by a model based on superdiffusive transport. For a number of interplanetary shocks and for the solar wind termination shock, for the first time we obtain the anomalous diffusion constants and the scale at which the probability of particle free paths changes to a power-law...

astro-ph.SR↗

arXiv2007

Position and velocity space diffusion of test particles in stochastic electromagnetic fields

The two--dimensional diffusive dynamics of test particles in a random electromagnetic field is studied. The synthetic electromagnetic fluctuations are generated through randomly placed magnetised ``clouds'' oscillating with a frequency $ω$. We investigate the mean square displacements of particles in both position and velocity spaces. As $ω$ increases the particles undergo standard (Brownian--like) motion, anomalous diffusion and ballistic motion in position space. Although in general the diffusion properties in velocity space are not trivially related to those in position space, we find that energization is present only when particles display anomalous diffusion in position space. The anomalous character of the diffusion is only in the non--standard values of the scaling exponents while the process is Gaussian.

cond-mat.stat-mech↗