-
Hellinger, P., L. Matteini, S. Stverak,
P. M. Travnicek, and E. Marsch (2011), Heating and cooling of protons in the fast solar wind between 0.3 and 1 AU:
Helios revisited, J. Geophys. Res., 116, A09105.
reprint. paper.
Abstract:
The proton thermal energetics in the fast solar wind between 0.3 and 1 AU is
re-investigated using the Helios 1 and 2 data. Closer to the Sun, it is
estimated that, to account for the observed radial profiles of the proton
parallel and perpendicular temperature, non-negligible parallel cooling and
perpendicular heating are necessary. Around 1 AU heating is needed in both
directions. We also calculate the corresponding rates and find that in total
significant interplanetary heating is necessary, in agreement with previous
results. The possible influence that deceleration of fast solar wind streams
due to interaction with slow ones has on the proton thermodynamics is
evaluated.
-
Genot, V., L. Broussillou, E. Budnik, P. Hellinger, P. M. Travnicek, E. Lucek,
and I. Dandouras (2011), Timing mirror structures observed by Cluster with a
magnetosheath flow model, Ann. Geophys., 29, 1849–1860.
paper.
Abstract:
The evolution of structures associated with mirror modes during their flow in
the Earth's magnetosheath is studied. The fact that the related magnetic fluctuations can
take distinct shapes, from deep holes to high peaks, has been assessed in previous works
on the observational, modeling and numerical points of view. In this paper we present
an analytical model for the flow lines and velocity magnitude inside the magnetosheath.
This model is used to interpret almost 10 years of Cluster observations of mirror structures:
by back tracking each isolated observation to the shock, the “age”, or flow time, of these
structures is determined together with the geometry of the shock. Using this flow time the
evolutionary path of the structures may be studied with respect to different quantities: the
distance to mirror threshold, the amplitude of mirror fluctuations and the skewness of the
magnetic amplitude distribution as a marker of the shape of the structures. These behaviours
are confronted to numerical simulations which confirm the dynamical perspective gained
from the association of the statistical analysis and the analytical model: magnetic peaks are
mostly formed just behind the shock and are quickly overwhelmed by magnetic holes as the
plasma conditions get more mirror stable. The amplitude of the fluctuations are found to
saturate before the skewness vanishes, i.e. when both structures quantitatively balance each
other, which typically occurs after a flow time of 100-200s in the Earth's magnetosheath.
Comparison with other astrophysical contexts is discussed.
-
Hellinger, P., and P. M. Travnicek (2011),
Proton core-beam system in the expanding solar wind:
Hybrid simulations, J. Geophys. Res., 116, A11101.
reprint. paper.
Abstract:
Results of a two-dimensional hybrid expanding box simulation
of a proton beam-core system in the solar wind are presented.
The expansion with a strictly radial magnetic field leads to a decrease
of the ratio between the proton perpendicular and parallel temperatures as well
as to an increase of the ratio between the beam-core differential velocity
and the local Alfvén velocity
creating a free energy for many different instabilities.
The system
is indeed most of the time marginally stable with respect to the
parallel magnetosonic, oblique Alfvén, proton
cyclotron and parallel fire hose instabilities which determine the system evolution
counteracting some effects of the expansion and interacting with each other.
Nonlinear evolution of these instabilities leads to
large modifications of the proton velocity distribution function.
The beam and core protons are slowed with respect to each
other and heated, and at later stages of the evolution
the two populations are not clearly distinguishable.
On the macroscopic level the instabilities cause large departures
from the double adiabatic prediction leading
to an efficient isotropization of effective
proton temperatures in agreement with Helios observations.
-
Schriver, D., et al. (2011),
Electron transport and precipitation at Mercury during the MESSENGER flybys: Implications for electron stimulated desorption,
Planet. Space Sci., 59, 2026–2036.
paper.
Abstract:
To examine electron transport, energization, and precipitation in Mercury's magnetosphere, a
simulation study has been carried out that follows electron trajectories within the global
magnetospheric electric and magnetic field configuration of Mercury. We report analysis for
two solar-wind parameter conditions corresponding to the first two MESSENGER Mercury
flybys on January 14, 2008, and October 6, 2008, which occurred for similar solar wind speed
and density, but contrasting interplanetary magnetic field (IMF) directions. During the first flyby
the IMF had a northward component, while during the second flyby the IMF was southward.
Electron trajectories are traced in the fields of global hybrid simulations for the two flybys. Some
solar wind electrons follow complex trajectories at or near where dayside reconnection occurs
and enter the magnetosphere at these locations. The entry locations depend on the IMF
orientation (north or south). As the electrons move through the entry regions they can be
energized as they execute non-adiabatic (demagnetized) motion. Some electrons become
magnetically trapped and drift around the planet with energies the order of 1-10 keV. The highest
energy of electrons anywhere in the magnetosphere is about 25 keV, consistent with the absence
of high-energy (> 35 keV) electrons observed during either MESSENGER flyby. Once within
the magnetosphere, a fraction of the electrons precipitate at the planetary surface with fluxes the
order of 109 cm-2s-1 and with energies hundreds of eV. This finding has important implications
for the viability of electron-stimulated desorption (ESD) as a mechanism for contributing to the
formation of the exosphere and heavy ion cloud around Mercury. Using laboratory estimates of
ESD ion yields from McLain et al. (2010) an ion production rate due to ESD at Mercury is
calculated and it is found to be on a par with ion sputtering yields.
-
Schriver, D., et al. (2011), Quasi-trapped ion and electron populations at Mercury,
Geophys. Res. Lett., 38, L23103.
paper.
Abstract:
Mariner 10 and MESSENGER spacecraft observations have established that Mercury has an intrinsic magnetic field
and magnetosphere. Following the March 2011 insertion of MESSENGER into orbit around Mercury,
measurements show that ions and electrons with typical energies of about 1-10 keV
form an equatorially
centered distribution of plasma at 1.4 RM radial distance (where RM is Mercury's radius)
around a substantial portion of the planet in local time from morning through night and into the afternoon sector.
Coincident with the detection of plasma around Mercury, an observed drop in the total magnetic pressure is attributable
to the ion and electron thermal pressure. Additionally, intense waves near the ion cyclotron frequency were observed
at the same location as the quasi-trapped particle population, which are likely a result of anisotropic distributions
created by the large loss cone (> 30o) at these radial distances.
-
Sibeck, D. G., et al. (2011), ARTEMIS Science Objectives, Space Sci. Rev.,
165, 59–91.
paper.
Abstract:
NASA's two spacecraft ARTEMIS mission will address both heliospheric and planetary research questions, first while in orbit about the Earth with the Moon and subsequently while in orbit about the Moon. Heliospheric topics include the structure of the Earth's magnetotail; reconnection, particle acceleration, and turbulence in the Earth's magnetosphere, at the bow shock, and in the solar wind; and the formation and structure of the lunar wake. Planetary topics include the lunar exosphere and its relationship to the composition of the lunar surface, the effects of electric fields on dust in the exosphere, internal structure of the Moon, and the lunar crustal magnetic field. This paper describes the expected contributions of ARTEMIS to these baseline scientific objectives.
-
Hellinger, P., and P. M. Travnicek (2015), Proton temperature-anisotropy-driven instabilities in weakly collisional plasmas:
Hybrid simulations, J. Plasma Phys., 81, 305810103.
arXiv, paper.
Abstract:
Kinetic instabilities in weakly collisional, high beta plasmas are investigated
using two-dimensional hybrid expanding box simulations with Coulomb collisions modeled
through the Langevin equation (corresponding to the Fokker-Planck one).
The expansion drives a parallel or perpendicular temperature anisotropy (depending on the orientation
of the ambient magnetic field). For the chosen parameters the Coulomb collisions are important
with respect to the driver but are not
strong enough to keep the system stable with respect to instabilities driven by the proton temperature
anisotropy. In the case of the
parallel temperature anisotropy the dominant oblique fire hose
instability efficiently reduces the anisotropy in a quasilinear
manner. In the case of the perpendicular temperature anisotropy the dominant mirror instability generates
coherent compressive structures which scatter protons and reduce the temperature
anisotropy. For both the cases the instabilities generate temporarily enough wave energy
so that the corresponding (anomalous) transport coefficients dominate over
the collisional ones and their properties are similar to those in
collisionless plasmas.
-
Verdini, A., R. Grappin, P. Hellinger, S. Landi, and W. C. Mueller (2015),
Anisotropy of the third-order structure functions in MHD turbulence,
Astrophys. J., 804, 119.
arXiv, paper.
Abstract:
The measure of the third-order structure function, Y, is employed in the solar wind to compute
the cascade rate of turbulence. In the absence of a mean field B0=0,
Y is expected to be isotropic (radial) and independent of the direction of increments,
so its measure yields directly the cascade rate. For turbulence with mean field, as in the solar wind,
Y is expected to become more two dimensional (2D), that is, to have larger perpendicular
components, loosing the above simple symmetry. To get the cascade rate one should compute the flux of
Y, which is not feasible with single-spacecraft data, thus measurements rely upon
assumptions about the unknown symmetry. We use direct numerical simulations (DNS) of magneto-hydrodynamic
(MHD) turbulence to characterize the anisotropy of Y. We find that for strong guide field
B0=5 the degree of two-dimensionalization depends on the relative importance
of shear and pseudo polarizations (the two components of an Alfvén mode in incompressible MHD).
The anisotropy also shows up in the inertial range. The more Y is 2D, the more the inertial
range extent differs along parallel and perpendicular directions. We finally test the two methods
employed in observations and find that the so-obtained cascade rate may depend on the angle between
B0 and the direction of increments. Both methods yield a vanishing cascade
rate along the parallel direction, contrary to observations, suggesting a weaker anisotropy of solar
wind turbulence compared to our DNS. This could be due to a weaker mean field and/or to solar wind expansion.
-
Franci, L., A. Verdini, L. Matteini, S. Landi, and P. Hellinger (2015),
Solar wind turbulence from MHD to sub-ion scales: high-resolution hybrid simulations,
Astrophys. J. Lett., 804, L39.
arXiv, paper.
Abstract:
We present results from a high-resolution and large-scale hybrid (fluid electrons and particle-in-cell protons)
two-dimensional numerical simulation of decaying turbulence.
Two distinct spectral regions (separated by a smooth break at proton scales) develop with clear power-law scaling, each one occupying about a decade in wave numbers.
The simulation results exhibit simultaneously several properties of the observed solar wind fluctuations:
spectral indices of the magnetic, kinetic, and residual energy spectra in the magneto-hydrodynamic
(MHD) inertial range along with a flattening of the electric field spectrum,
an increase in magnetic compressibility, and a strong coupling of the cascade with the density and the parallel component of the
magnetic fluctuations at sub-proton scales. Our findings support the
interpretation that in the solar wind large-scale MHD fluctuations naturally
evolve beyond proton scales into a turbulent regime that is governed by the
generalized Ohm's law.
-
Vandas, M., and P. Hellinger (2015),
Linear dispersion properties of ring velocity distribution functions,
Phys. Plasmas, 22, 062107.
reprint, paper.
Abstract:
Linear properties of ring velocity distribution functions are investigated.
The dispersion tensor in a form similar to the case of a Maxwellian distribution
function, but for a general distribution function separable in velocities,
is presented. Analytical forms of the dispersion tensor are derived
for two cases of ring velocity distribution functions: one obtained from
physical arguments and one for the usual, ad hoc ring distribution.
The analytical expressions involve generalized hypergeometric, Kampe de Feriet
functions of two arguments. For a set of plasma parameters the two ring distribution functions
are compared. At the parallel propagation with respect to the ambient
magnetic field the two ring distributions give the same results identical
to the corresponding bi-Maxwellian distribution. At oblique propagation
the two ring distributions give similar results only for strong instabilities
whereas for weak growth rates their predictions are significantly different;
the two ring distributions have different marginal stability conditions.
-
Hellinger, P., L. Matteini, S. Landi, A. Verdini, L. Franci, and P. M. Travnicek (2015),
Plasma turbulence and kinetic instabilities at ion scales in the expanding solar wind,
Astrophys. J. Lett., 811, L32.
arXiv,
paper.
Abstract:
The relationship between a decaying strong turbulence and kinetic instabilities in a slowly expanding plasma
is investigated using
two-dimensional (2-D) 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 which have energy equipartition between kinetic and magnetic
fluctuations and 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 ion scales.
The turbulent cascade leads to an overall anisotropic proton heating, protons
are heated in the perpendicular direction, and, initially, also in the parallel
direction. The imposed expansion leads to generation of
a large parallel proton temperature anisotropy which is at later stages partly reduced by turbulence.
The turbulent heating is not sufficient to overcome the expansion-driven perpendicular cooling
and the system eventually drives the oblique firehose instability
in a form of localized nonlinear wave packets which
efficiently reduce the parallel temperature anisotropy.
This work demonstrates that kinetic instabilities may coexist
with strong plasma turbulence even in a constrained 2-D regime.
-
Matteini, L., P. Hellinger, S. J. Schwartz, and S. Landi (2015),
Firehose instability driven by alpha particle temperature anisotropy,
Astrophys. J., 812, 13.
arXiv, paper.
Abstract:
We investigate linear and nonlinear properties of a solar wind-like plasma consisting of electrons,
dominant protons, and a secondary alpha particle population exhibiting a parallel temperature
anisotropy with respect to the background magnetic field, using Vlasov linear and quasi-linear predictions
and by means of one-dimensional hybrid simulations. We show that anisotropic alpha particles
can drive a parallel fire hose instability with properties analogous to those of the more studied parallel
fire hose instability generated by protons, but that, remarkably, the instability can be triggered also
when the parallel plasma beta of alpha particles is below unity. The wave activity generated by the
alpha particle anisotropy affects the evolution of the more abundant protons, leading to their anisotropic
heating. When both protons and alpha particles have sufficient parallel temperature anisotropies both
of them can drive the instability, and we observe generation of two distinct peaks in the spectra of
the fluctuations, with longer wavelengths associated to alphas and shorter ones to protons. For no
relative drift between alphas and protons the generated waves properties are symmetric along and
against the ambient magnetic field. If a non-zero relative drift is present, the symmetry is broken
and modes propagate preferentially in the direction of the drift associated with the unstable species.
The generated waves scatter particles and reduce their temperature anisotropy to marginally stable
state, and, moreover, they significantly reduce the relative drift between the two ion populations. The
coexistence of modes excited by both ion species leads to saturation of the plasma in distinct regions
of the plasma beta/anisotropy parameter space for protons and alpha particles, in good agreement
with in situ solar wind observations. Our results confirm that fire hose instabilities are likely at work
in the solar wind and limit the anisotropy of different ion species in the plasma.
-
Franci, L., S. Landi, L. Matteini, A. Verdini, and P. Hellinger (2015),
Hybrid high-resolution simulations of kinetic turbulence at proton scales,
Astrophys. J., 812, 21.
arXiv, paper.
Abstract:
We investigate properties of turbulence from large, magneto-hydrodynamic (MHD) to ion scales 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 we add a spectrum of large-scale
linearly polarized fluctuations, which have energy equipartition and vanishing correlation between
kinetic and magnetic fluctuations. Once turbulence is fully developed, we observe a MHD inertial
range characterized by a spectrum of perpendicular magnetic field fluctuations following a power-law
with a Kolmogorov spectral index of -5/3, while the proton bulk velocity fluctuations exhibit a
less steep Iroshnikov-Kraichnan scaling with index close to -3/2. Both these trends hold over a full
decade of wave-vectors. A transition is observed at a scale of the order of the proton inertial length,
after which both spectra steepen, with the perpendicular magnetic field exhibiting a power law with
spectral index of approximatively -3 over another full decade at sub-ion scales. A similar -2.8
slope is observed also in density and parallel component of magnetic field fluctuations, highlighting
the presence of compressive effects in the cascade at kinetic scales. The spectrum of perpendicular
electric fluctuations follows that of the proton bulk velocity at MHD scales and flattens at small scales.
The turbulent nature of our data is also supported by the presence of intermittency. This is revealed
by the non-Gaussian probability distribution functions of MHD primitive variables which increase
when approaching kinetic scales. All these features, which we carefully tested against variations in
the numerical resolution, are in good agreement with solar wind observations. The turbulent cascade
leads to on overall proton energization with similar heating rates in the parallel and perpendicular
directions. While the parallel proton heating is found to be independent on the resistivity, the number
of particles per cell and the resolution employed, the perpendicular proton temperature strongly
depends on these parameters.
-
Stverak, S., P. M. Travnicek, and P. Hellinger (2015),
Electron energetics in the expanding solar wind via
Helios observations,
J. Geophys. Res., 120, 8177–8193.
reprint, paper.
Abstract:
We present an observational analysis of electron cooling/heating
rates in the fast and slow solar wind between 0.3 and 1 AU. We fit electron
velocity distribution functions acquired in situ by Helios 1 and 2 spacecraft
by a three component (core-halo-strahl) analytical model. The resulting radial
profiles of macroscopic characteristics (density, temperatures and heat
fluxes) are employed to examine properties of theoretical energy balance equations
and to estimate external cooling/heating terms. Our analysis indicates
that in contrast to solar wind protons the electrons do not require
important heating mechanisms to explain the observed temperature gradients. The
electron heating rates are actually found to be negative for both the slow and
fast solar wind, namely due to the significant degradation of the electron heat
flux with increasing radial distance from the Sun. Cooling mechanisms acting
on electrons are found to be significantly stronger in the slow wind than
in the fast wind streams.
-
Matteini, L., S. J. Schwartz, and P. Hellinger (2015),
Cometary ion instabilities in the solar wind, Planet. Space. Sci., 119, 3–12.
paper.
Abstract:
We review some of the processes that characterize the interaction of the solar wind with the newborn cometary ions.
Instabilities generated by the typical drifting-ring velocity-space configuration of the pick-up ions in the solar wind
frame are studied by mean of 1D and 2D hybrid numerical simulations. In agreement with previous studies, we
find that instabilities generated by the cometary ions play an important role in shaping the properties of the plasma.
The resulting ion distributions are in good agreement with observations, showing the presence of energy shells.
Bi-spherical shells for the heavy ions are also observed in the late phase of the simulations. Moreover, we also investigate
some new aspects of the dynamics, such as the generation of turbulent cascade from the initial spectra of unstable
waves, and the related heating and back reaction of the solar wind plasma. We also consider the case of initial
non-gyrotropic pick-up ion distributions, and we focus on the polarization of the associated waves, suggesting that linear
polarization can be a signature of this configuration, possibly observed by the Rosetta spacecraft in orbit around comet
67P/CG.
-
Hercik, D., P. M. Travnicek, S. Stverak, and P. Hellinger (2016),
Properties of Hermean plasma belt: numerical simulations and comparison with MESSENGER data,
J. Geophys. Res., 121, 413–431.
reprint, paper.
Abstract:
Using a global hybrid model and test particle simulations we present a detailed analysis of the Hermean plasma belt structure. We
investigate characteristic properties of quasi-trapped particle population characteristics and its behavior under different orientations of the interplanetary magnetic field. The plasma belt region is constantly supplied with solar wind protons via magnetospheric flanks and tail current sheet region. Protons inside the plasma belt region are quasi-trapped in the magnetic field of Mercury and perform westward drift along the planet. This region is well separated by a magnetic shell and has higher average temperatures and lower bulk proton current densities than the surrounding area. On the dayside the population exhibits loss cone distribution function matching the theoretical loss cone angle. The simulation results are in a good agreement with in situ observations of MESSENGER's MAG and FIPS instruments.
-
Franci, L., P. Hellinger, L. Matteini, A. Verdini, and S. Landi (2016),
Two-dimensional hybrid simulations of kinetic plasma turbulence: Current and vorticity vs proton temperature,
AIP Conf. Proc., 1720, 040003.
arXiv, paper.
Abstract:
Proton temperature anisotropies between the directions parallel and perpendicular to the mean magnetic field are usually
observed in the solar wind plasma. Here, we employ a high-resolution hybrid particle-in-cell simulation in order to investigate
the relation between spatial properties of the proton temperature and the peaks in the current density and in the flow vorticity. Our
results indicate that, although regions where the proton temperature is enhanced and temperature anisotropies are larger correspond
approximately to regions where many thin current sheets form, no firm quantitative evidence supports the idea of a direct causality
between the two phenomena. On the other hand, quite a clear correlation between the behavior of the proton temperature and the
out-of-plane vorticity is obtained.
-
Hellinger, P. (2016), Ion collisional transport coefficients in the solar wind at 1 au,
Astrophys. J., 825, 120.
arXiv, paper.
Abstract:
Proton and alpha particle collisional transport coefficients (isotropization, relative deceleration
frequencies and heating rates)
at 1 au are quantified using the WIND/SWE data. In agreement with
previous studies the ion-ion Coulomb collisions are generally important for
slow solar wind streams and
tend to reduce the temperature anisotropies,
the differential streaming and the differences between proton and alpha particle
temperatures.
In slow solar wind streams the Coulomb collisions between protons and alpha
particles are important for the overall proton energetics
as well as for
the relative deceleration between the two species.
It is also shown
that ion temperature anisotropies and differential streaming
need to be generally taken into account for evaluation of the collisional transport coefficients.
-
Burgess, P., P. Hellinger, I. Gingell, and P. M. Travnicek (2016),
Microstructure in two- and three-dimensional hybrid simulations of perpendicular collisionless shocks,
J. Plasma Phys., 82, 905820401.
arXiv, paper.
Abstract:
Supercritical collisionless perpendicular shocks have an average macrostructure determined
primarily by the dynamics of ions specularly reflected at the magnetic ramp.
Within the overall macrostructure, instabilities, both linear and nonlinear, generate
fluctuations and microstructure. To identify the sources of such microstructure, high-resolution
two- and three-dimensional simulations have been carried out using the hybrid method,
wherein the ions are treated as particles and the electron response is modelled
as a massless fluid. We confirm the results of earlier 2-D simulations showing both
field-parallel aligned propagating fluctuations and fluctuations carried by the
reflected-gyrating ions. In addition, it is shown that, for 2-D simulations of the shock coplanarity
plane, the presence of short-wavelength fluctuations in all magnetic components is associated
with the ion Weibel instability driven at the upstream edge of the foot by the
reflected-gyrating ions. In 3-D simulations we show for the first time that the dominant
microstructure is due to a coupling between field-parallel propagating fluctuations in
the ramp and the motion of the reflected ions. This results in a pattern of fluctuations
counter-propagating across the surface of the shock at an angle inclined to the magnetic
field direction, due to a combination of field-parallel motion at the Alfvén speed of the
ramp, and motion in the sense of gyration of the reflected ions.
-
Sebek, O., P. M. Travnicek, R. J. Walker, and P. Hellinger (2016),
Ion cyclotron instability at Io: Hybrid simulation results compared to in situ observations,
J. Geophys. Res., 121, 7514–7534.
paper.
Abstract:
We present analysis of global 3-dimensional hybrid simulations of Io's interaction with Jovian magnetospheric plasma. We apply a single-species model with simplified neutral-plasma chemistry and down-scale Io in order to resolve the ion kinetic scales. We consider charge exchange, electron impact ionization and photoionization by using variable rates of these processes to investigate their impact. Our results are in a good qualitative agreement with the in situ magnetic field measurements for five Galileo flybys around Io. The hybrid model describes ion kinetics self-consistently. This allows us to assess the distribution of temperature anisotropies around Io and thereby determine the possible triggering mechanism for waves observed near Io. We compare simulated dynamic spectra of magnetic fluctuations with in situ observations made by Galileo. Our results are consistent with both the spatial distribution and local amplitude of magnetic fluctuations found in the observations. Cyclotron waves, triggered probably by the growth of ion cyclotron instability, are observed mainly downstream of Io and on the flanks in regions further from Io where the ion pick-up rate is relatively low. Growth of the ion cyclotron instability is governed mainly by the charge exchange rate.
-
Hellinger, P., and P. M. Travnicek (2016),
Proton heating by pick-up ion driven cyclotron waves in the outer heliosphere: Hybrid
expanding box simulations, Astrophys. J., 832, 32.
paper, arXiv.
Abstract:
Using one-dimensional hybrid expanding box model we investigate
properties of the solar wind in the outer heliosphere. We assume a proton-electron plasma
with a strictly transverse ambient magnetic field and, beside the expansion, we take into account
influence of a continuous injection of cold pick-up protons through the charge-exchange process
between the solar wind protons and hydrogen of interstellar origin.
The injected cold pick-up protons form a ring distribution function that
rapidly becomes unstable and generate Alfvén cyclotron waves.
The Alfvén cyclotron waves scatter pick-up protons to a spherical shell
distribution function that thickens over that time owing to the
expansion-driven cooling. The Alfvén cyclotron waves heat
solar wind protons in the perpendicular direction (with respect to the ambient magnetic field)
through the cyclotron resonance. At later
times, the Alfvén cyclotron waves become parametrically unstable
and the generated ion acoustic waves heat protons in the parallel direction through the
Landau resonance. The resulting heating of
the solar wind protons is efficient on the expansion time scale.
-
Franci, L., S. Landi, L. Matteini, A. Verdini, and P. Hellinger (2016),
Plasma beta dependence of the ion-scale spectral break of solar wind turbulence: high-resolution 2D hybrid simulations,
Astrophys. J., 833, 91.
paper,
arXiv.
Abstract:
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 the 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 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, di, for β ≪ 1 and to the ion
gyroradius, ρi = diβ½, for β ≫ 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.
-
Franci, L., S. Landi, A. Verdini, L. Matteini, and P. Hellinger (2018),
Solar wind turbulent cascade from MHD to sub-ion scales:
large-size 3D hybrid particle-in-cell simulations,
Astrophys. J., 1, 26.
arXiv, paper.
Abstract:
Spectral properties of the turbulent cascade from fluid to kinetic scales in collisionless plasmas are investigated by means of large-size three-dimensional (3D) hybrid (fluid electrons, kinetic protons) particle-in-cell simulations. Initially isotropic Alfvénic fluctuations rapidly develop a strongly anisotropic turbulent cascade, mainly in the direction perpendicular to the ambient magnetic field. The omnidirectional magnetic field spectrum shows a double power-law behavior over almost two decades in wavenumber, with a Kolmogorov-like index at large scales, a spectral break around ion scales, and a steepening at sub-ion scales. Power laws are also observed in the spectra of the ion bulk velocity, density, and electric field, both at magnetohydrodynamic (MHD) and at kinetic scales. Despite the complex structure, the omnidirectional spectra of all fields at ion and sub-ion scales are in remarkable quantitative agreement with those of a two-dimensional (2D) simulation with similar physical parameters. This provides a partial, a-posteriori validation of the 2D approximation at kinetic scales. Conversely, at MHD scales, the spectra of the density and of the velocity (and, consequently, of the electric field) exhibit differences between the 2D and 3D cases. Although they can be partly ascribed to the lower spatial resolution, the main reason is likely the larger importance of compressible effects in a full geometry. Our findings are also in remarkable quantitative agreement with solar wind observations.
-
Bowen, T. A., S. Badman, P. Hellinger, and S. D. Bale (2018),
Density fluctuations in the solar wind driven by Alfven wave parametric decay,
Astrophys. J. Lett., 854, L33.
arXiv, paper.
Abstract:
Measurements and simulations of inertial compressive turbulence in the solar wind are characterized by anti-correlated magnetic fluctuations parallel to the mean field and density structures. This signature has been interpreted as observational evidence for non-propagating pressure balanced structures (PBS), kinetic ion acoustic waves, as well as the MHD slow-mode. Given the high damping rates of parallel propagating compressive fluctuations, their ubiquity in satellite observations is surprising, and suggestive of a local driving process. One possible candidate for the generation of compressive fluctuations in the solar wind is Alfven wave parametric instability. Here we test the parametric decay process as a source of compressive waves in the solar wind by comparing the collisionless damping rates of compressive fluctuations with the growth rates of the parametric decay instability daughter waves. Our results suggest that generation of compressive waves through parametric decay is overdamped at 1 AU, but that the presence of slow-mode like density fluctuations is correlated with the parametric decay of Alfven waves.
-
Hellinger, P., A. Verdini, S. Landi, L. Franci, and L. Matteini (2018),
von Kármán-Howarth equation for Hall magnetohydrodynamics: Hybrid simulations,
Astrophys. J. Lett., 857, L19.
paper,
arXiv.
Abstract:
A dynamical vectorial equation for homogeneous incompressible Hall-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 exhibits 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 non thermal (kinetic) features or to others terms in the dynamical equation that are not included in the Hall-MHD incompressible approximation.
-
Tsurutani, B. T., G. S. Lakhina, A. Sen, P. Hellinger,
K.-H. Glassmeier, and A. J. Mannucci (2018),
A review of Alfvénic turbulence in high speed solar wind streams:
Hints from cometary plasma turbulence,
J. Geophys. Res., 123, 2458–2492.
paper.
Abstract:
Solar wind turbulence within high speed streams (HSSs) is reviewed from the
point of view of embedded single nonlinear Alfvén wave cycles, discontinuities,
magnetic decreases (MDs) and shocks. For comparison and guidance, cometary
plasma turbulence is reviewed. It is demonstrated that cometary nonlinear
magnetosonic waves phase-steepen, with a right-hand circular polarized
foreshortened front and an elongated, compressive trailing edge. The former
part is a form of "wave breaking" and the latter that of "period doubling".
Interplanetary nonlinear Alfvén waves have similar features. The Alfvén waves
are arc polarized, with a ~180° foreshortened front and with an elongated
trailing, thus also exhibiting "wave breaking" and "period doubling" features.
Alfvén waves have different polarizations from those of cometary magnetosonic
waves, indicating that helicity is a durable feature of turbulence.
Interplanetary Alfvén waves are noted to be spherical waves, suggesting the
possibility of additional local generation. Alfvén wave kinetically dissipate,
forming MDs, thus the solar wind is partially "compressive" and static. ~2 MeV
protons can nonresonantly interact with MDs leading to rapid cross-field (~5.5%
Bohm) diffusion. The possibility of local (~1 AU) generation of Alfvén waves
may make it difficult to forecast HILDCAAs and relativistic magnetospheric
electrons with great accuracy. The future Solar Orbiter and Solar Probe Plus
missions should be able to not only test these ideas but to extend our
knowledge of plasma turbulence evolution.
-
Franci, L., P. Hellinger, M. Guarrasi, C. H. K. Chen, E. Papini, A. Verdini,
L. Matteini, and S. Landi (2018), Three-dimensional simulations of solar wind turbulence
with the hybrid code CAMELIA, J. Phys.: Conf. Ser.,
1031, 012002.
paper.
Abstract:
We investigate the spectral properties of plasma turbulence from fluid to sub-ion scales by
means of high-resolution three-dimensional (3D) numerical simulations performed with the hybrid particle-
in-cell (HPIC) code CAMELIA. We produce extended turbulent spectra with well-defined power laws
for the magnetic, ion bulk velocity, density, and electric fluctuations. The present results are in good
agreement with previous two-dimensional (2D) HPIC simulations, especially in the kinetic range of scales,
and reproduce several features observed in solar wind spectra. By providing scaling tests on many different
architectures and convergence studies, we prove CAMELIA to represent a very efficient, accurate and
reliable tool for investigating the development of the turbulent cascade in the solar wind, being able to
cover simultaneously several decades in wavenumber, also in 3D.
-
Hellinger, P., and S. Stverak (2018), Electron mirror instability: Particle-in-cell simulations,
J. Plasma Phys., 84, 905840402.
arXiv, paper.
Abstract:
Properties of the electron mirror instability
and its competition with the usually dominant whistler (electron cyclotron)
instability driven by the electron perpendicular temperature anisotropy
are investigated on the linear level using a Vlasov linear solver and
on the nonlinear level using a two-dimensional full particle code.
The simulation results show that the linearly subdominant
electron mirror instability may compete on the nonlinear level
with the whistler instability and may even become eventually the dominant mode
that generates robust non-propagating sub-ion-scale coherent structures in the form of magnetic peaks.
- Papini, E., L. Franci, S. Landi, A. Verdini, L. Matteini, and P. Hellinger (2019),
Can Hall Magnetohydrodynamics explain plasma turbulence at sub-ion scales?,
Astrophys. J., 870, 52.
arXiv, paper
Abstract:
We investigate the properties of plasma turbulence by means of two-dimensional Hall-magnetohydrodynamic (HMHD) and hybrid particle-in-cell (HPIC) numerical simulations. We find that the HMHD simulations exhibit spectral properties that are in most cases in agreement with the results of the HPIC simulations and with solar wind observations. The energy spectra of magnetic fluctuations exhibit a double power law with spectral index -5/3 at MHD scales and -3 at kinetic scales, while for velocity fluctuations the spectral index is -3/2 at MHD scales. The break between the MHD and the kinetic scales occurs at the same scale in both simulations. In the MHD range the slopes of the total energy and residual energy spectra satisfy a fast Alfvén-dynamo balance. The development of a turbulent cascade is concurrently characterized by magnetic reconnection events taking place in thin current sheets that form between large eddies. A statistical analysis reveals that reconnection is qualitatively the same and fast in both the HMHD and HPIC models, characterized by inverse reconnection rates much smaller than the characteristic large-eddy nonlinear time. The agreement extends to other statistical properties, such us the kurtosis of the magnetic field. Moreover, the observation of a direct energy transfer from the large vortices to the small sub-ion scales, triggered by magnetic reconnection, further supports the existence of a reconnection-mediated turbulent regime at kinetic scales. We conclude that the HMHD fluid description captures to a large extent the transition of the turbulent cascade between the large MHD scales and the sub-ion scales.
- Sebek, O., P. M. Travnicek, R. J. Walker, and P. Hellinger (2019),
Dynamic plasma interaction at Io: Multi-species hybrid simulations,
J. Geophys. Res., 124, 313–341.
paper.
Abstract:
The interaction between the Io plasma torus and Io exhibits significant
dynamics resulting from temporal variations of both external conditions in the
plasma torus and local conditions in Io's environment. We present analysis of
an extensive simulation campaign of the Io plasma interaction under varying
interaction conditions performed by using a hybrid multi-species simulation
model. We test two models of electron impact ionization, one resulting in
plasma production constrained to the upstream hemisphere of Io and the other
producing a more symmetric distribution of the plasma production with
non-negligible plasma production on the downstream side of Io. In the latter
case, the simulation model provides unprecedented level of agreement with
Galileo measurements of magnetic field obtained during the I0 flyby, where the
model captures even smaller scale features of the magnetic field profile. Our
simulated results provide strong support for the existence of Io's induced
dipole field. In this model we assume induced dipole field moment is
anti-parallel to the direction of the inducing magnetic field, equivalent to
induction in a highly conductive Io. Our results further support the idea that
Io's atmosphere collapsed during the Galileo I27 flyby, which manifests itself
in the lack of ion heating due to pick-up processes in the extended atmosphere.
Our results show that the dominant torus species, O+ and S++, are
depleted around Io and may exhibit temperature anisotropy,
T⊥/T|| < 1,
resulting from merging of ion populations coming from opposite sides of Io
along the magnetic field.
- Hellinger, P., L. Matteini, S. Landi, L. Franci, A. Verdini, and E. Papini (2019),
Turbulence vs. fire hose instabilities: 3-D hybrid expanding box simulations,
Astrophys. J., 883, 178.
arXiv, paper.
Abstract:
The relationship between a decaying plasma turbulence and proton fire hose instabilities in a slowly expanding plasma
is investigated using three-dimensional (3-D) hybrid expanding box simulations.
We impose an initial ambient magnetic
field along the radial direction, and we start with an isotropic spectrum of
large-scale, linearly-polarized, random-phase Alfvnéic fluctuations with zero cross-helicity.
A turbulent cascade rapidly develops and
leads to a weak proton heating that is not sufficient to overcome the expansion-driven perpendicular cooling.
The plasma system eventually drives the parallel and oblique fire hose instabilities
that generate quasi-monochromatic wave packets that reduce the proton temperature anisotropy.
The fire hose wave activity has a low amplitude with wave vectors quasi-parallel/oblique with
respect to the ambient magnetic field outside of
the region dominated by the turbulent cascade and
is discernible in one-dimensional power spectra taken only in the direction quasi-parallel/oblique
with respect to the ambient magnetic field; at quasi-perpendicular angles
the wave activity is hidden by the turbulent background.
These waves are partly reabsorbed by protons
and partly couple to and participate in the turbulent cascade. Their presence
reduces kurtosis, a measure of intermittency, and the Shannon entropy
but increases the Jensen-Shannon complexity of magnetic fluctuations;
these changes are weak and anisotropic with respect
to the ambient magnetic field and it's not clear if they can be used
to indirectly discern the presence of instability-driven waves.
- Landi, S., L. Franci, E. Papini, A. Verdini, L. Matteini, and P. Hellinger (2019),
Spectral anisotropies and intermittency of plasma turbulence at ion kinetic scales.
arXiv.
- Trotta, D., L. Franci, D. Burgess, and P. Hellinger (2020),
Fast acceleration of transrelativistic electrons in astrophysical turbulence,
Astrophys. J., 894, 136.
arXiv, paper.
Abstract:
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
Alfvenic 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.
- Maksimovic, M., et al. (2020), The Solar Orbiter Radio and Plasma Waves (RPW) instrument,
A&A, 642, A12.
paper.
Abstract:
The Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter mission is described in this paper. This instrument is
designed to measure in-situ magnetic and electric fields and waves from 'DC' to a few hundreds of kHz. RPW will also observe solar
radio emissions up to 16 MHz. The RPW instrument is of primary importance to the Solar Orbiter mission and science requirements,
since it is essential to answer three of the four mission overarching science objectives. In addition RPW will exchange on-board data with the other in-situ instruments, in order to process algorithms for interplanetary shocks and type III langmuir waves detections.
- Bandyopadhyay, R., et al. (2020), In-situ observation of Hall magnetohydrodynamic cascade in space
plasma, Phys. Rev. Lett., 124, 225101.
arXiv, paper.
Abstract:
We present estimates of the turbulent energy cascade rate, derived from a
Hall-Magnetohydrodynamic (MHD) third-order law. We compute the contr ibution from the Hall term
and the MHD term to the energy flux. Magnetospheric MultiScale (MMS) data, accumulated in
the magnetosheath and the solar wind, are compared with previously established simulation results.
Consistent with the simulations, we find that at large (MHD) scales the MMS observations exhibit
a clear inertial range, dominated by the MHD flux. In the sub-ion range the cascade continues at a
diminished level via the Hall term, and the change becomes more pronounced as the plasma beta
increases. Additionally, the MHD contribution to interscale energy transfer remains important at
smaller scales than previously thought. Possible reasons are offered for this unanticipated result.
- Franci, L., J. Stawarz, E. Papini, P. Hellinger, T. Nakamura, D. Burgess,
S. Landi, A. Verdini, L. Matteini, R. E. Ergun, O. Le Contel, and P.-A. Lindqvist (2020),
Modeling Kelvin-Helmholtz instability-driven turbulence with hybrid simulations of Alfvenic turbulence,
Astrophys. J., 898, 175.
arXiv, paper.
Abstract:
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 Alfvenic 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 fact that our results are compatible with the validity of the
Taylor hypothesis, in the whole range of scales investigated numerically, suggests that the
fluctuations at sub-ion scales might have predominantly low frequencies. This would be
consistent with a kinetic Alfven wave-like nature and/or with the presence of quasi-static
structures. Finally, the third-order structure function analysis indicates that the cascade rate of the
turbulence generated by a KH event at the magnetopause is an order of magnitude larger than in the
ambient magnetosheath.
- Papini, E., A. Cicone, M. Piersanti, L. Franci,
P. Hellinger, S. Landi, and A. Verdini (2020),
Multidimensional Iterative Filtering: a new approach for investigating plasma turbulence
in numerical simulations,
J. Plasma Phys., 86, 871860501.
arXiv, paper.
Abstract:
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
non-stationary 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
multi-scale 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.
- Matteini, L., L. Franci, O. Alexandrova, C. Lacombe, S. Landi, P. Hellinger, E. Papini, and A. Verdini
(2020), Magnetic field turbulence in the solar wind at sub-ion scales: in situ observations
and numerical simulations, Front. Astron. Space Sci., 7, 563075.
arXiv,
paper.
Abstract:
We investigate the transition of the solar wind turbulent cascade from MHD to sub-ion range by means of a detail comparison between in situ observations and hybrid numerical simulations. In particular we focus on the properties of the magnetic field and its component anisotropy in Cluster measurements and hybrid 2D simulations. First, we address the angular distribution of wave-vectors in the kinetic range between ion and electron scales by studying the variance anisotropy of the magnetic field components. When taking into account the single-direction sampling performed by spacecraft in the solar wind, the main properties of the fluctuations observed in situ are also recovered in our numerical description. This result confirms that solar wind turbulence in the sub-ion range is characterized by a quasi-2D gyrotropic distribution of k-vectors around the mean field. We then consider the magnetic compressibility associated with the turbulent cascade and its evolution from large-MHD to sub-ion scales. The ratio of field-aligned to perpendicular fluctuations, typically low in the MHD inertial range, increases significantly when crossing ion scales and its value in the sub-ion range is a function of the total plasma beta only, as expected from theoretical predictions, with higher magnetic compressibility for higher beta. Moreover, we observe that this increase has a gradual trend from low to high beta values in the in situ data; this behaviour is well captured by the numerical simulations. The level of magnetic field compressibility that is observed in situ and in the simulations is in fairly good agreement with theoretical predictions, especially at high beta, suggesting that in the kinetic range explored the turbulence is supported by low-frequency and highly-oblique fluctuations in pressure balance, like kinetic Alfvén waves or other slowly evolving coherent structures.
- González, C. A., A. Tenerani, M. Velli, and P. Hellinger (2020),
The role of parametric instabilities in turbulence generation and proton heating: Hybrid simulations of parallel propagating Alfvén waves
Astrophys. J., 904, 81.
arXiv, paper
Abstract:
Large amplitude Alfvén waves tend to be unstable to parametric instabilities which result in a
decay process of the initial wave into different daughter waves depending upon the amplitude of the
fluctuations and the plasma beta. The propagation angle with respect to the mean magnetic field of
the daughter waves plays an important role in determining the type of decay. In this paper, we revisit
this problem by means of multi-dimensional hybrid simulations. In particular, we study the decay and
the subsequent nonlinear evolution of large-amplitude Alfvén waves by investigating the saturation
mechanism of the instability and its final nonlinear state reached for different wave amplitudes and
plasma beta conditions. As opposed to one-dimensional simulations where the Decay instability is
suppressed for increasing plasma beta values, we find that the decay process in multi-dimensions
persists at large values of the plasma beta via the filamentation/magnetosonic decay instabilities. In
general, the decay process acts as a trigger both to develop a perpendicular turbulent cascade and
to enhance mean field-aligned wave-particle interactions. We find indeed that the saturated state is
characterized by a turbulent plasma displaying a field-aligned beam at the Alfvén speed and increased
temperatures that we ascribe to the Landau resonance and pitch angle scattering in phase space.
- Hellinger, P., A. Verdini, S. Landi, L. Franci, E. Papini, and L. Matteini (2020),
On cascade of kinetic energy in compressible hydrodynamic turbulence.
arXiv
- Alexandrova, O., V. K. Jagarlamudi, C. Rossi, M. Maksimovic, P. Hellinger, Y. Shprits,
and A. Mangeney (2020), Kinetic turbulence in space plasmas observed in the near-Earth
and near-Sun solar wind.
arXiv
- Hellinger, P., A. Verdini, S. Landi, E. Papini, L. Franci, and L. Matteini (2021),
Scale dependence and cross-scale transfer of kinetic
energy in compressible hydrodynamic turbulence at moderate Reynolds numbers,
Phys. Rev. Fluids, 6, 044607.
paper,
arXiv
Abstract:
We investigate properties of the scale dependence and cross-scale transfer of kinetic
energy in compressible three-dimensional hydrodynamic turbulence, by means of two direct
numerical simulations of decaying turbulence with initial Mach numbers M = 1/3 and M = 1,
and with moderate Reynolds numbers, Rλ ∼ 100.
The turbulent dynamics is analyzed using compressible and incompressible versions of
the dynamic spectral transfer (ST) and the Karman-Howarth-Monin (KHM) equations.
We find that the nonlinear coupling leads to a flux of the kinetic energy
to small scales where it is dissipated; at the same time, the reversible
pressure-dilatation mechanism causes oscillatory exchanges between the kinetic and internal
energies with an average zero net energy transfer.
While the incompressible KHM and ST equations are not generally valid in the simulations,
their compressible counterparts are well satisfied and describe,
in a quantitatively similar way, the decay of the kinetic energy on large scales,
the cross-scale energy transfer/cascade, the pressure dilatation, and the dissipation.
There exists a simple relationship between the KHM and ST results through the inverse
proportionality between the wave vector k and the spatial separation length l
as k l ∼ 31/2. For a given time
the dissipation and pressure-dilatation terms are strong
on large scales in the KHM approach whereas the ST terms
become dominant on small scales; this is owing to the complementary cumulative
behavior of the two methods. The effect of pressure dilatation is weak
when averaged over a period of its oscillations
and may lead to a transfer of the kinetic energy from large to small scales
without a net exchange between the kinetic and internal energies.
Our results suggest that for large-enough systems there exists an inertial range
for the kinetic energy cascade. This transfer is partly owing to the classical,
nonlinear advection-driven cascade and partly owing to the pressure dilatation-induced energy
transfer. We also use the ST and KHM approaches to investigate
properties of the internal energy. The dynamic ST and KHM equations for the internal energy
are well satisfied in the simulations but behave very differently with respect
to the viscous dissipation. We conclude that ST and KHM approaches should better
be used for the kinetic and internal energies separately.
- Alexandrova, O., V. K. Jagarlamudi, P. Hellinger, M. Maksimovic,
Y. Shprits, and A. Mangeney (2021), Spectrum of kinetic plasma turbulence at 0.3-0.9 AU from the Sun,
Phys. Rev. E, 103, 063202.
paper.
Abstract:
We investigate the spectral properties of the turbulence in the solar wind which is a weakly collisional astrophysical plasma, accessible to in-situ observations.
Using the Helios search coil magnetometer measurements in the fast solar wind, in the inner heliosphere, we focus on properties of the turbulent magnetic fluctuations at scales smaller than the ion characteristic scales, the so-called kinetic plasma turbulence.
At such small scales, we show that the magnetic power spectra between 0.3 and 0.9 au from the Sun have a generic shape ∼ f-8/3 exp(-f/fd) where the dissipation frequency fd is correlated with the Doppler shifted frequency fρ e
of the electron Larmor radius. This behavior is statistically significant: all the observed kinetic spectra are well described by this model, with fd=fρ e/1.8.
Our results indicate that the electron gyroradius plays the role of the
dissipation scale and marks the end of the electromagnetic cascade in the solar wind.
- González, C. A., A. Tenerani, L. Matteini, P. Hellinger, and M. Velli (2021),
Proton energization by phase-steepening of parallel propagating Alfvénic fluctuations,
Astrophys. J. Lett., 914, L36.
arXiv, paper
Abstract:
Proton energization at magnetic discontinuities generated by phase-steepened fronts of parallel propagating, large-amplitude Alfvénic fluctuation is studied using hybrid simulations. We find that dispersive effects yield to the collapse of the wave via phase steepening and the subsequent generation of compressible fluctuations that mediate an efficient local energy transfer from the wave to the protons. Proton scattering at the steepened edges causes non-adiabatic proton perpendicular heating. Furthermore, the parallel electric field at the propagating fronts mediates the acceleration of protons along the mean field. A steady-state is achieved where proton distribution function displays a field-aligned beam at the Alfvén speed, and compressible fluctuations are largely damped. We discuss the implications of our results in the context of Alfvénic solar wind.
- Papini, E., A. Cicone, L. Franci, M. Piersanti, S. Landi,
P. Hellinger, and A. Verdini (2021),
Spacetime Hall-MHD turbulence at sub-ion scales: structures or waves?,
Astrophys. J. Lett., 917, L12.
arXiv,
paper.
Abstract:
Spatiotemporal properties of two-dimensional (2D) Hall-magnetohydrodynamic turbulence at
intermediate plasma β = 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 Alfvén 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.
- Hellinger, P., E. Papini, A. Verdini, S. Landi, L. Franci,
L. Matteini, and V. Montagud-Camps (2021), Spectral transfer and Kármán-Howarth-Monin
equations for compressible Hall magnetohydrodynamics,
Astrophys. J., 917, 101.
arXiv, paper.
Abstract:
Kármán-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.
- Papini, E., P. Hellinger, A. Verdini, S. Landi, L. Franci, V. Montagud-Camps, and
L. Matteini (2021), Properties of Hall-MHD turbulence at sub-ion scales: Spectral
transfer analysis, Atmosphere, 12, 1632.
paper.
Abstract:
We present results of a multiscale study of Hall-magnetohydrodynamic (MHD) turbulence, carried out on a dataset of compressible nonlinear 2D Hall-MHD numerical simulations of decaying Alfvénic turbulence. For the first time, we identify two distinct regimes of fully developed turbulence. In the first one, the power spectrum of the turbulent magnetic fluctuations at sub-ion scales exhibits a power law with a slope of ~ -2.9, typically observed both in solar wind and in magnetosheath turbulence. The second regime, instead, shows a slope of -7/3, in agreement with classical theoretical models of Hall-MHD turbulence. A spectral-transfer analysis reveals that the latter regime occurs when the energy transfer rate at sub-ion scales is dominated by the Hall term, whereas in the former regime, the governing process is the dissipation (and the system exhibits large intermittency). Results of this work are relevant to the space plasma community, as they may potentially reconcile predictions from theoretical models with results from numerical simulations and spacecraft observations.
- Maksimovic, M., et al. (2021), First observations and performance of the RPW
instrument onboard the Solar Orbiter mission, A&A, 656, A41.
paper.
Abstract:
The Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter mission is designed
to measure in-situ magnetic and electric fields and waves from the continuum to a few hundreds
of kHz. RPW also observes solar and heliospheric radio emissions up to 16 MHz. RPW was
switched on and its antennae were deployed successfully two days after the launch of Solar
Orbiter on February 10, 2020. Since then, the instrument has acquired enough data so that we
can assess its performance and the electromagnetic disturbances it experiences. In this article we
present the first RPW observations and assess its scientific performance. In particular we focuss
on a statistical analysis of the first observations of interplanetary dust by the Thermal Noise
Receiver of the instrument. We review also the electro-magnetic disturbances that RPW suffers
from and that potential users of the instrument data should be aware of, before starting their
research work.
- Franci, L., D. Del Sarto, E. Papini, A. Giroul, J. E. Stawarz, D. Burgess, P. Hellinger,
S. Landi, and S. D. Bale (2021), A new regime of ion-scale plasma turbulence in the inner heliosphere: simulations and PSP observations, Astrophys. J. Lett., submitted.
arXiv