Publications
101 results found
Papini E, Franci L, Landi S, et al., 2019, Can Hall Magnetohydrodynamics Explain Plasma Turbulence at Sub-ion Scales?, ASTROPHYSICAL JOURNAL, Vol: 870, ISSN: 0004-637X
Matteini L, Stansby D, Horbury TS, et al., 2018, On the 1/f spectrum in the solar wind and its connection with magnetic compressibility, Letters of the Astrophysical Journal, Vol: 869, ISSN: 2041-8205
We discuss properties of Alfvénic fluctuations with large amplitude in plasmas characterized by low magnetic field compression. We note that in such systems power laws cannot develop with arbitrarily steep slopes at large scales, i.e., when $| \delta {\boldsymbol{B}}| $ becomes of the order of the background field $| {\boldsymbol{B}}| $. In such systems there is a scale l 0 at which the spectrum has to break due to the condition of weak compressibility. A very good example of this dynamics is offered by solar wind fluctuations in Alfvénic fast streams, characterized by the property of constant field magnitude. We show here that the distribution of $\delta B=| \delta {\boldsymbol{B}}| $ in the fast wind displays a strong cutoff at $\delta B/| {\boldsymbol{B}}| \lesssim 2$, as expected for fluctuations bounded on a sphere of radius $B=| {\boldsymbol{B}}| $. This is also associated with a saturation of the rms of the fluctuations at large scales and introduces a specific length l 0, above which the amplitude of the fluctuations becomes independent on the scale l. Consistent with that, the power spectrum at l > l 0 is characterized by a −1 spectral slope, as expected for fluctuations that are scale-independent. Moreover, we show that the spectral break between the 1/f and inertial range in solar wind spectra indeed corresponds to the scale l 0 at which $\langle \delta B/B\rangle \sim 1$. Such a simple model provides a possible alternative explanation of magnetic spectra observed in interplanetary space, also pointing out the inconsistency for a plasma to simultaneously maintain $| {\boldsymbol{B}}| \sim \mathrm{const}.$ at arbitrarily large scales and satisfy a Kolmogorov scaling.
Stansby D, Salem C, Matteini L, et al., 2018, A new inner heliosphere proton parameter dataset from the Helios mission, Solar Physics, Vol: 293, ISSN: 0038-0938
In the near future, Parker Solar Probe and Solar Orbiter will provide the first comprehensive in-situ measurements of the solar wind in the inner heliosphere since the Helios mission in the 1970s. We describe a reprocessing of the original Helios ion distribution functions to provide reliable and reproducible data to characterise the proton core population of the solar wind in the inner heliosphere. A systematic fitting of bi-Maxwellian distribution functions was performed to the raw Helios ion distribution function data to extract the proton core number density, velocity, and temperatures parallel and perpendicular to the magnetic field. We present radial trends of these derived proton parameters, forming a benchmark to which new measurements in the inner heliosphere will be compared. The new dataset has been made openly available for other researchers to use, along with the source code used to generate it.
Horbury TS, Matteini L, Stansby D, 2018, Short, large-amplitude speed enhancements in the near-Sun fast solar wind, Monthly Notices of the Royal Astronomical Society, Vol: 478, Pages: 1980-1986, ISSN: 0035-8711
We report the presence of intermittent, short discrete enhancements in plasma speed in the near-Sun high speed solar wind. Lasting tens of seconds to minutes in spacecraft measurements at 0.3 AU, speeds inside these enhancements can reach 1000 km/s, corresponding to a kinetic energy up to twice that of the bulk high speed solar wind. These events, which occur around 5% of the time, are Alfvenic in nature with large magnetic field deflections and are the same temperature as the surrounding plasma, in contrast to the bulk fast wind which has a well-established positive speed-temperature correlation. The origin of these speed enhancements is unclear but they may be signatures of discrete jets associated with transient events in the chromosphere or corona. Such large short velocity changes represent a measurement and analysis challenge for the upcoming Parker Solar Probe and Solar Orbiter missions.
Breuillard H, Matteini L, Argall MR, et al., 2018, New Insights into the Nature of Turbulence in the Earth's Magnetosheath Using Magnetospheric MultiScale Mission Data, ASTROPHYSICAL JOURNAL, Vol: 859, ISSN: 0004-637X
The Earth's magnetosheath, which is characterized by highly turbulent fluctuations, is usually divided into two regions of different properties as a function of the angle between the interplanetary magnetic field and the shock normal. In this study, we make use of high-time resolution instruments on board the Magnetospheric MultiScale spacecraft to determine and compare the properties of subsolar magnetosheath turbulence in both regions, i.e., downstream of the quasi-parallel and quasi-perpendicular bow shocks. In particular, we take advantage of the unprecedented temporal resolution of the Fast Plasma Investigation instrument to show the density fluctuations down to sub-ion scales for the first time. We show that the nature of turbulence is highly compressible down to electron scales, particularly in the quasi-parallel magnetosheath. In this region, the magnetic turbulence also shows an inertial (Kolmogorov-like) range, indicating that the fluctuations are not formed locally, in contrast with the quasi-perpendicular magnetosheath. We also show that the electromagnetic turbulence is dominated by electric fluctuations at sub-ion scales (f > 1 Hz) and that magnetic and electric spectra steepen at the largest-electron scale. The latter indicates a change in the nature of turbulence at electron scales. Finally, we show that the electric fluctuations around the electron gyrofrequency are mostly parallel in the quasi-perpendicular magnetosheath, where intense whistlers are observed. This result suggests that energy dissipation, plasma heating, and acceleration might be driven by intense electrostatic parallel structures/waves, which can be linked to whistler waves.
Hellinger P, Verdini A, Landi S, et al., 2018, von Karman-Howarth Equation for Hall Magnetohydrodynamics: Hybrid Simulations, ASTROPHYSICAL JOURNAL LETTERS, Vol: 857, ISSN: 2041-8205
A dynamical vectorial equation for homogeneous incompressible Hall-magnetohydrodynamic (MHD) turbulence together with the exact scaling law for third-order correlation tensors, analogous to that for the incompressible MHD, is rederived and applied to the results of two-dimensional hybrid simulations of plasma turbulence. At large (MHD) scales the simulations exhibit a clear inertial range where the MHD dynamic law is valid. In the sub-ion range the cascade continues via the Hall term, but the dynamic law derived in the framework of incompressible Hall-MHD equations is obtained only in a low plasma beta simulation. For a higher beta plasma the cascade rate decreases in the sub-ion range and the change becomes more pronounced as the plasma beta increases. This break in the cascade flux can be ascribed to nonthermal (kinetic) features or to others terms in the dynamical equation that are not included in the Hall-MHD incompressible approximation.
Franci L, Landi S, Verdini A, et al., 2018, Solar Wind Turbulent Cascade from MHD to Sub-ion Scales: Large-size 3D Hybrid Particle-in-cell Simulations, ASTROPHYSICAL JOURNAL, Vol: 853, ISSN: 0004-637X
Franci L, Cerri SS, Califano F, et al., 2017, Magnetic Reconnection as a Driver for a Sub-ion-scale Cascade in Plasma Turbulence, ASTROPHYSICAL JOURNAL LETTERS, Vol: 850, ISSN: 2041-8205
A new path for the generation of a sub-ion-scale cascade in collisionless space and astrophysical plasma turbulence, triggered by magnetic reconnection, is uncovered by means of high-resolution two-dimensional hybrid-kinetic simulations employing two complementary approaches, Lagrangian and Eulerian, and different driving mechanisms. The simulation results provide clear numerical evidence that the development of power-law energy spectra below the so-called ion break occurs as soon as the first magnetic reconnection events take place, regardless of the actual state of the turbulent cascade at MHD scales. In both simulations, the reconnection-mediated small-scale energy spectrum of parallel magnetic fluctuations exhibits a very stable spectral slope of $\sim -2.8$, whether or not a large-scale turbulent cascade has already fully developed. Once a quasi-stationary turbulent state is achieved, the spectrum of the total magnetic fluctuations settles toward a spectral index of $-5/3$ in the MHD range and of $\sim -3$ at sub-ion scales.
Lacombe C, Alexandrova O, Matteini L, 2017, Anisotropies of the Magnetic Field Fluctuations at Kinetic Scales in the Solar Wind: Cluster Observations, ASTROPHYSICAL JOURNAL, Vol: 848, ISSN: 0004-637X
We present the first statistical study of the anisotropy of the magnetic field turbulence in the solar wind between 1 and 200 Hz, i.e., from proton to sub-electron scales. We consider 93 ten-minute intervals of the Cluster/STAFF measurements. We find that the fluctuations $\delta {B}_{\perp }^{2}$ are not gyrotropic at a given frequency f, a property already observed at larger scales ($\parallel /\perp $ means parallel/perpendicular to the average magnetic ${{\boldsymbol{B}}}_{0}$). This non-gyrotropy gives indications of the angular distribution of the wave vectors ${\boldsymbol{k}}$: at $f\lt $ 10 Hz, we find that ${k}_{\perp }\gg {k}_{\parallel }$, mainly in the fast wind; at $f\,\gt $ 10 Hz, fluctuations with a non-negligible k ∥ are also present. We then consider the anisotropy ratio $\delta {B}_{\parallel }^{2}/\delta {B}_{\perp }^{2}$, which is a measure of the magnetic compressibility of the fluctuations. This ratio, always smaller than 1, increases with f. It reaches a value showing that the fluctuations are more or less isotropic at electron scales, for $f\geqslant 50\,\mathrm{Hz}$. From 1 to 15–20 Hz, there is a strong correlation between the observed compressibility and the one expected for the kinetic Alfvén waves (KAWs), which only depends on the total plasma β. For $f\gt 15\mbox{--}20\,\mathrm{Hz}$, the observed compressibility is larger than expected for KAWs, and it is stronger in the slow wind: this could be an indication of the presence of a slow-ion acoustic mode of fluctuations, which is more compressive and is favored by the larger values of the electron to proton temperature ratio generally observed in the slow wind.
Dudik J, Dzifcakova E, Meyer-Vernet N, et al., 2017, Nonequilibrium Processes in the Solar Corona, Transition Region, Flares, and SolarWind (Invited Review), SOLAR PHYSICS, Vol: 292, ISSN: 0038-0938
Hellinger P, Landi S, Matteini L, et al., 2017, Mirror Instability in the Turbulent Solar Wind, ASTROPHYSICAL JOURNAL, Vol: 838, ISSN: 0004-637X
The relationship between a decaying strong turbulence and the mirror instability in a slowly expanding plasma is investigated using two-dimensional hybrid expanding box simulations. We impose an initial ambient magnetic field perpendicular to the simulation box, and we start with a spectrum of large-scale, linearly polarized, random-phase Alfvénic fluctuations that have energy equipartition between kinetic and magnetic fluctuations and a vanishing correlation between the two fields. A turbulent cascade rapidly develops, magnetic field fluctuations exhibit a Kolmogorov-like power-law spectrum at large scales and a steeper spectrum at sub-ion scales. The imposed expansion (taking a strictly transverse ambient magnetic field) leads to the generation of an important perpendicular proton temperature anisotropy that eventually drives the mirror instability. This instability generates large-amplitude, nonpropagating, compressible, pressure-balanced magnetic structures in a form of magnetic enhancements/humps that reduce the perpendicular temperature anisotropy.
Matteini L, Alexandrova O, Chen CHK, et al., 2017, Electric and magnetic spectra from MHD to electron scales in the magnetosheath, Monthly Notices of the Royal Astronomical Society, Vol: 466, Pages: 945-951, ISSN: 0035-8711
We investigate the transition of the turbulence from large to kinetic scales using Cluster observations. Simultaneous spectra of magnetic and electric fields in the Earth's magnetosheath from magnetohydrodynamic (MHD) to electron scales are presented for the first time. While the two spectra have approximatively similar behaviour in the fluid-MHD regime, they show different trends in the kinetic range. As the magnetic field spectrum steepens at ion scales, the electric field spectrum is characterized by a shallower power law continuing down to electron scales. Such an evolution is consistent with theoretical expectations, assuming that the turbulence is dominated by highly oblique k-vectors and that between ion and electron scales the electric field is governed by the non-ideal terms in the generalized Ohm's law. This leads to an expected linear increase of the electric-to-magnetic ratio of fluctuations, consistent with observations presented here. The influence of local whistler wave activity on electron-scale spectra is also discussed.
Gingell IL, Sorriso-Valvo L, Burgess D, et al., 2017, Three dimensional simulations of sheared current sheets: transition to turbulence?, Journal of Plasma Physics, Vol: 83, ISSN: 1469-7807
Systems of multiple current sheets arise in various situations in natural plasmas, such asat the heliospheric current sheet in the solar wind and in the outer heliosphere in theheliosheath. Previous three-dimensional simulations have shown that such systems candevelop turbulent-like fluctuations resulting from forward and inverse cascade in wavevector space. We present a study of the transition to turbulenceof such multiple currentsheet systems, including the effects of adding a magnetic guide field and velocity shearsacross the current sheets. Three-dimensional hybrid simulationsare performed of systemswith eight narrow current sheets in a triply-periodic geometry. We carry out a numberof different analyses of the evolution of the fluctuations as the initially highly orderedstate relaxes to one which resembles turbulence. Despite the evidence of forward andinverse cascade in the fluctuation power spectra, we find that none of the simulated caseshave evidence of intermittency after the initial period of fast reconnection associatedwith the ion tearing instability at the current sheets. Cancellation analysis confirms thatthe simulations have not evolved to a state which can be identified as fully developedturbulence. The addition of velocity shears across the current sheets slows the evolutionin the properties of the fluctuations, but by the end of the simulation they are broadlysimilar. However, if the simulation is constrained to be two-dimensional, differences arefound, indicating that fully three-dimensional simulations are important when studyingthe evolution of an ordered equilibrium towards a turbulent-like state.
Franci L, Landi S, Matteini L, et al., 2016, Plasma beta dependence of the ion-scale spectral break of solar wind turbulence: high-resolution 2D hybrid simulations, Astrophysical Journal, Vol: 833, ISSN: 0004-637X
We investigate properties of the ion-scale spectral break of solar wind turbulence by means of two-dimensional high-resolution hybrid particle-in-cell simulations. We impose an initial ambient magnetic field perpendicular to the simulation box and add a spectrum of in-plane, large-scale, magnetic and kinetic fluctuations. We perform a set of simulations with different values of the plasma β, distributed over three orders of magnitude, from 0.01 to 10. In all cases, once turbulence is fully developed, we observe a power-law spectrum of the fluctuating magnetic field on large scales (in the inertial range) with a spectral index close to −5/3, while in the sub-ion range we observe another power-law spectrum with a spectral index systematically varying with β (from around −3.6 for small values to around −2.9 for large ones). The two ranges are separated by a spectral break around ion scales. The length scale at which this transition occurs is found to be proportional to the ion inertial length, d i , for β Lt 1 and to the ion gyroradius, ${\rho }_{i}={d}_{i}\sqrt{\beta }$, for β Gt 1, i.e., to the larger between the two scales in both the extreme regimes. For intermediate cases, i.e., β ~ 1, a combination of the two scales is involved. We infer an empiric relation for the dependency of the spectral break on β that provides a good fit over the whole range of values. We compare our results with in situ observations in the solar wind and suggest possible explanations for such a behavior.
Stansby D, Horbury TS, Chen CHK, et al., 2016, Experimental determination of whistler wave dispersion relation in the solar wind, Astrophysical Journal Letters, Vol: 829, ISSN: 2041-8213
The origins and properties of large-amplitude whistler wavepackets in the solar wind are still unclear. In this Letter, we utilize single spacecraft electric and magnetic field waveform measurements from the ARTEMIS mission to calculate the plasma frame frequency and wavevector of individual wavepackets over multiple intervals. This allows direct comparison of experimental measurements with theoretical dispersion relations to identify the observed waves as whistler waves. The whistlers are right-hand circularly polarized, travel anti-sunward, and are aligned with the background magnetic field. Their dispersion is strongly affected by the local electron parallel beta in agreement with linear theory. The properties measured are consistent with the electron heat flux instability acting in the solar wind to generate these waves.
Chen CHK, Matteini L, Schekochihin AA, et al., 2016, Multi-species measurements of the firehose and mirror instability thresholds in the solar wind, Astrophysical Journal Letters, Vol: 825, ISSN: 2041-8213
The firehose and mirror instabilities are thought to arise in a variety of space and astrophysical plasmas, constraining the pressure anisotropies and drifts between particle species. The plasma stability depends on all species simultaneously, meaning that a combined analysis is required. Here, we present the first such analysis in the solar wind, using the long-wavelength stability parameters to combine the anisotropies and drifts of all major species (core and beam protons, alphas, and electrons). At the threshold, the firehose parameter was found to be dominated by protons (67%), but also to have significant contributions from electrons (18%) and alphas (15%). Drifts were also found to be important, contributing 57% in the presence of a proton beam. A similar situation was found for the mirror, with contributions of 61%, 28%, and 11% for protons, electrons, and alphas, respectively. The parallel electric field contribution, however, was found to be small at 9%. Overall, the long-wavelength thresholds constrain the data well (${\text{}}\lt 1 \% $ unstable), and the implications of this are discussed.
Burgess D, Gingell PW, Matteini L, 2016, Multiple current sheet systems in the outer heliosphere: energy release and turbulence, Astrophysical Journal, Vol: 822, ISSN: 1538-4357
In the outer heliosphere, beyond the solar wind termination shock, it isexpected that the warped heliospheric current sheet forms a region of closelypacked,multiple, thin current sheets. Such a system may be subject to theion-kinetic tearing instability, and hence generate magnetic islands and hot populationsof ions associated with magnetic reconnection. Reconnection processesin this environment have important implications for local particle transport, andfor particle acceleration at reconnection sites and in turbulence. We study thiscomplex environment by means of three-dimensional hybrid simulations over longtime scales, in order to capture the evolution from linear growth of the tearinginstability to a fully developed turbulent state at late times. The final state developsfrom the highly ordered initial state via both forward and inverse cascades.Component and spectral anisotropy in the magnetic fluctuations is present whena guide field is included. The inclusion of a population of new-born interstellarpickup protons does not strongly affect these results. Finally, we conclude thatreconnection between multiple current sheets can act as an important sourceof turbulence in the outer heliosphere, with implications for energetic particleacceleration and propagation.
Franci L, Hellinger P, Matteini L, et al., 2016, Two-dimensional Hybrid Simulations of Kinetic Plasma Turbulence: Current and Vorticity vs Proton Temperature, 14th International Solar Wind Conference (Solar Wind), Publisher: AMER INST PHYSICS, ISSN: 0094-243X
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- Citations: 33
Franci L, Landi S, Matteini L, et al., 2015, High-resolution hybrid simulations of kinetic plasma turbulence at proton scales, Astrophysical Journal, Vol: 812, ISSN: 1538-4357
We investigate properties of plasma turbulence from magnetohydrodynamic (MHD) to sub-ion scales by means oftwo-dimensional, high-resolution hybrid particle-in-cell simulations. We impose an initial ambient magneticfield perpendicular to the simulation box, and we add a spectrum of large-scale magnetic and kineticfluctuations with energy equipartition and vanishing correlation. Once the turbulence is fully developed, weobserve an MHD inertial range, where the spectra of the perpendicular magnetic field and the perpendicular protonbulk velocity fluctuations exhibit power-law scaling with spectral indices of -5 3 and -3 2, respectively. Thisbehavior is extended over a full decade in wavevectors and is very stable in time. A transition is observed aroundproton scales. At sub-ion scales, both spectra steepen, with the former still following a power law with a spectralindex of ~-3. A-2.8 slope is observed in the density and parallel magnetic fluctuations, highlighting the presenceof compressive effects at kinetic scales. The spectrum of the perpendicular electric fluctuations follows that of theproton bulk velocity at MHD scales, and flattens at small scales. All these features, which we carefully testedagainst variations of many parameters, are in good agreement with solar wind observations. The turbulent cascadeleads to on overall proton energization with similar heating rates in the parallel and perpendicular directions. Whilethe parallel proton heating is found to be independent on the resistivity, the number of particles per cell, and theresolution employed, the perpendicular proton temperature strongly depends on these parameters.
Matteini L, Hellinger P, Schwartz SJ, et al., 2015, FIRE HOSE INSTABILITY DRIVEN BY ALPHA PARTICLE TEMPERATURE ANISOTROPY, Astrophysical Journal, Vol: 812, ISSN: 1538-4357
We investigate properties of a solar wind-like plasma, including a secondary alpha particle population exhibiting aparallel temperature anisotropy with respect to the background magnetic field, using linear and quasi-linearpredictions and by means of one-dimensional hybrid simulations. We show that anisotropic alpha particles candrive a parallel fire hose instability analogous to that generated by protons, but that, remarkably, can also betriggered when the parallel plasma beta of alpha particles is below unity. The wave activity generated by the alphaanisotropy affects the evolution of the more abundant protons, leading to their anisotropic heating. When both ionspecies have sufficient parallel anisotropies, both of them can drive the instability, and we observe the generationof two distinct peaks in the spectra of the fluctuations, with longer wavelengths associated to alphas and shorterones to protons. If a non-zero relative drift is present, the unstable modes propagate preferentially in the directionof the drift associated with the unstable species. The generated waves scatter particles and reduce their temperatureanisotropy to a marginally stable state, and, moreover, they significantly reduce the relative drift between the twoion populations. The coexistence of modes excited by both species leads to saturation of the plasma in distinctregions of the beta/anisotropy parameter space for protons and alpha particles, in good agreement with in situ solarwind observations. Our results confirm that fire hose instabilities are likely at work in the solar wind and limit theanisotropy of different ion species in the plasma.
Hellinger P, Matteini L, Landi S, et al., 2015, Plasma turbulence and kinetic instabilities at ion scales in the expanding solar wind, Astrophysical Journal Letters, Vol: 811, ISSN: 2041-8213
The relationship between a decaying strong turbulence and kinetic instabilities in a slowly expanding plasma isinvestigated using two-dimensional (2D) hybrid expanding box simulations. We impose an initial ambientmagnetic field perpendicular to the simulation box, and we start with a spectrum of large-scale, linearly polarized,random-phase Alfvénic fluctuations that have energy equipartition between kinetic and magnetic fluctuations andvanishing correlation between the two fields. A turbulent cascade rapidly develops; magnetic field fluctuationsexhibit a power-law spectrum at large scales and a steeper spectrum at ion scales. The turbulent cascade leads to anoverall anisotropic proton heating, protons are heated in the perpendicular direction, and, initially, also in theparallel direction. The imposed expansion leads to generation of a large parallel proton temperature anisotropywhich is at later stages partly reduced by turbulence. The turbulent heating is not sufficient to overcome theexpansion-driven perpendicular cooling and the system eventually drives the oblique firehose instability in a formof localized nonlinear wave packets which efficiently reduce the parallel temperature anisotropy. This workdemonstrates that kinetic instabilities may coexist with strong plasma turbulence even in a constrained 2D regime.
Chen CHK, Matteini L, Burgess D, et al., 2015, Magnetic field rotations in the solar wind at kinetic scales, Monthly Notices of the Royal Astronomical Society: Letters, Vol: 453, Pages: L64-L68, ISSN: 1745-3933
The solar wind magnetic field contains rotations at a broad range of scales, which have been extensively studied in the magnetohydrodynamics range. Here, we present an extension of this analysis to the range between ion and electron kinetic scales. The distribution of rotation angles was found to be approximately lognormal, shifting to smaller angles at smaller scales almost self-similarly, but with small, statistically significant changes of shape. The fraction of energy in fluctuations with angles larger than α was found to drop approximately exponentially with α, with e-folding angle 9.8° at ion scales and 0.66° at electron scales, showing that large angles (α > 30°) do not contain a significant amount of energy at kinetic scales. Implications for kinetic turbulence theory and the dissipation of solar wind turbulence are discussed.
Matteini L, Schwartz SJ, Hellinger P, 2015, Cometary ion instabilities in the solar wind, Planetary and Space Science, ISSN: 1873-5088
Franci L, Verdini A, Matteini L, et al., 2015, SOLAR WIND TURBULENCE FROM MHD TO SUB-ION SCALES: HIGH-RESOLUTION HYBRID SIMULATIONS, Astrophysical Journal Letters, Vol: 804, ISSN: 2041-8213
We present results from a high-resolution and large-scale hybrid (fluid electrons and particle-in-cell protons) twodimensionalnumerical simulation of decaying turbulence. Two distinct spectral regions (separated by a smoothbreak at proton scales) develop with clear power-law scaling, each one occupying about a decade in wavenumbers.The simulation results simultaneously exhibit several properties of the observed solar wind fluctuations: spectralindices of the magnetic, kinetic, and residual energy spectra in the magnetohydrodynamic (MHD) inertial rangealong with a flattening of the electric field spectrum, an increase in magnetic compressibility, and a strong couplingof the cascade with the density and the parallel component of the magnetic fluctuations at sub-proton scales. Ourfindings support the interpretation that in the solar wind, large-scale MHD fluctuations naturally evolve beyondproton scales into a turbulent regime that is governed by the generalized Ohm’s law
Gingell PW, Burgess D, Matteini L, 2015, THE THREE-DIMENSIONAL EVOLUTION OF ION-SCALE CURRENT SHEETS: TEARING AND DRIFT-KINK INSTABILITIES IN THE PRESENCE OF PROTON TEMPERATURE ANISOTROPY, ASTROPHYSICAL JOURNAL, Vol: 802, ISSN: 0004-637X
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- Citations: 17
Matteini L, Horbury TS, Pantellini F, et al., 2015, Ion kinetic energy conservation and magnetic field strength constancy in multi-fluid solar wind alfvénic turbulence, Astrophysical Journal, Vol: 802, ISSN: 1538-4357
We investigate the properties of plasma fluid motion in the large-amplitude, low-frequency fluctuations of highlyAlfvénic fast solar wind. We show that protons locally conserve total kinetic energy when observed from aneffective frame of reference comoving with the fluctuations. For typical properties of the fast wind, this frame canbe reasonably identified by alpha particles which, due to their drift with respect to protons at about the Alfvénspeed along the magnetic field, do not partake in the fluid low-frequency fluctuations. Using their velocity totransform the proton velocity into the frame of Alfvénic turbulence, we demonstrate that the resulting plasmamotion is characterized by a constant absolute value of the velocity, zero electric fields, and aligned velocity andmagnetic field vectors as expected for unidirectional Alfvénic fluctuations in equilibrium. We propose that thisconstraint, via the correlation between velocity and magnetic field in Alfvénic turbulence, is the origin of theobserved constancy of the magnetic field; while the constant velocity corresponding to constant energy can only beobserved in the frame of the fluctuations, the corresponding constant total magnetic field, invariant for Galileantransformations, remains the observational signature in the spacecraft frame of the constant total energy in theAlfvén turbulence frame
Del Zanna L, Matteini L, Landi S, et al., 2015, Parametric decay of parallel and oblique Alfvén waves in the expanding solar wind, Journal of Plasma Physics, Vol: 81, ISSN: 0022-3778
<jats:p>The long-term evolution of large-amplitude Alfvén waves propagating in the solar wind is investigated by performing two-dimensional MHD simulations within the expanding box model. The linear and nonlinear phases of the parametric decay instability are studied for both circularly polarized waves in parallel propagation and for arc-polarized waves in oblique propagation. The non-monochromatic case is also considered. In the oblique case, the direct excitation of daughter modes transverse to the local background field is found for the first time in an expanding environment, and this transverse cascade seems to be favored for monochromatic mother waves. The expansion effect reduces the instability growth rate, and it can even suppress its onset for the lowest frequency modes considered here, possibly explaining the persistence of these outgoing waves in the solar wind.</jats:p>
Lacombe C, Alexandrova O, Matteini L, et al., 2014, Whistler mode waves and the electron heat flux in the Solar wind: cluster observations, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 796, Pages: 1-11, ISSN: 0004-637X
The nature of the magnetic field fluctuations in the solar wind between the ion and electron scales is still under debate. Using the Cluster/STAFF instrument, we make a survey of the power spectral density and of the polarization of these fluctuations at frequencies f in [1, 400] Hz, during five years (2001-2005), when Cluster was in the free solar wind. In ~10% of the selected data, we observe narrowband, right-handed, circularly polarized fluctuations, with wave vectors quasi-parallel to the mean magnetic field, superimposed on the spectrum of the permanent background turbulence. We interpret these coherent fluctuations as whistler mode waves. The lifetime of these waves varies between a few seconds and several hours. Here, we present, for the first time, an analysis of long-lived whistler waves, i.e., lasting more than five minutes. We find several necessary (but not sufficient) conditions for the observation of whistler waves, mainly a low level of background turbulence, a slow wind, a relatively large electron heat flux, and a low electron collision frequency. When the electron parallel beta factor β e∥ is larger than 3, the whistler waves are seen along the heat flux threshold of the whistler heat flux instability. The presence of such whistler waves confirms that the whistler heat flux instability contributes to the regulation of the solar wind heat flux, at least for β e∥ ≥ 3, in slow wind at 1 AU.
Landi S, Matteini L, Pantellini F, 2014, ELECTRON HEAT FLUX IN THE SOLAR WIND: ARE WE OBSERVING THE COLLISIONAL LIMIT IN THE 1 AU DATA?, ASTROPHYSICAL JOURNAL LETTERS, Vol: 790, ISSN: 2041-8205
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- Citations: 26
Matteini L, Horbury TS, Neugebauer M, et al., 2014, Dependence of solar wind speed on the local magnetic field orientation: Role of Alfvenic fluctuations, Geophysical Research Letters, Vol: 41, Pages: 259-265, ISSN: 0094-8276
We report an analysis of correlations between magnetic field and velocity fluctuations in the fast solar wind beyond 1 AU at high latitudes. We have found that on scales shorter than the microstream structures, there exists a well‐defined dependence of the flow speed on the angle between the magnetic field vector and the radial direction. Solar wind is found to be slightly faster when the measured magnetic field vector is transverse to the velocity, while it is always slower when the magnetic field is parallel, or antiparallel, to the radial direction. We show that this correlation is a direct consequence of the high Alfvénicity of fast wind fluctuations and that it can be reasonably described by a simple model taking into account the main properties of the low‐frequency antisunward Alfvén fluctuations as observed in the solar wind plasma. We also discuss how switchbacks, short periods of magnetic field reversals, naturally fit in this new observed correlation.
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