361 research outputs found
Fire Hose instability driven by alpha particle temperature anisotropy
We investigate properties of a solar wind-like plasma including a secondary
alpha particle population exhibiting a parallel temperature anisotropy with
respect to the background magnetic field, using 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
analogous to that 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 anisotropy affects the
evolution of the more abundant protons, leading to their anisotropic heating.
When both ion species have sufficient parallel 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. If a non-zero relative drift is present, the unstable
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 species leads to saturation of the plasma
in distinct regions of the 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.Comment: 10 pages, 9 figures, Accepted for publication in The Astrophysical
Journa
Solar wind turbulence from MHD to sub-ion scales: high-resolution hybrid simulations
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.Comment: 5 pages, 5 figures; introduction and conclusions changed, references
updated, accepted for publication in ApJ
Two-dimensional Hybrid Simulations of Kinetic Plasma Turbulence: Current and Vorticity vs Proton Temperature
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.Comment: 4 pages, 2 figures, Proceedings of the Fourteenth International Solar
Wind Conferenc
Three-dimensional evolution of magnetic and velocity shear driven instabilities in a compressible magnetized jet
The problem of three-dimensional combined magnetic and velocity shear driven
instabilities of a compressible magnetized jet modeled with a plane
neutral/current double vortex sheet in the framework of the resistive
magnetohydrodynamics is addressed. The resulting dynamics given by the
stream+current sheet interaction is analyzed and the effects of a variable
geometry of the basic fields are considered. Depending on the basic asymptotic
magnetic field configuration, a selection rule of the linear instability modes
can be obtained. Hence, the system follows a two-stage path developing either
through a fully three-dimensional dynamics with a rapid evolution of kink modes
leading to a final turbulent state, or rather through a driving two-dimensional
instability pattern that develops on parallel planes on which a
reconnection+coalescence process takes place.Comment: 33 pages, 15 figures, accepted for publication in Physics of Plasma
High-resolution hybrid simulations of kinetic plasma turbulence at proton scales
We investigate properties of plasma turbulence from magneto-hydrodynamic
(MHD) to sub-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
magnetic and kinetic fluctuations, with energy equipartition and vanishing
correlation. Once the turbulence is fully developed, we observe a MHD inertial
range, where the spectra of the perpendicular magnetic field and the
perpendicular proton bulk velocity fluctuations exhibit power-law scaling with
spectral indices of -5/3 and -3/2, respectively. This behavior is extended over
a full decade in wavevectors and is very stable in time. A transition is
observed around proton scales. At sub-ion scales, both spectra steepen, with
the former still following a power law with a spectral index of ~-3. A -2.8
slope is observed in the density and parallel magnetic fluctuations,
highlighting the presence of compressive effects at kinetic scales. The
spectrum of the perpendicular electric fluctuations follows that of the proton
bulk velocity at MHD scales, and flattens at small scales. All these features,
which we carefully tested against variations of many parameters, 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.Comment: 15 pages, 13 figures, submitted to Ap
Anisotropy of third-order structure functions in MHD turbulence
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 Alfv\'en 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.Comment: Some text editing and typos corrected, 13 pages, 6 figures, to be
published in Ap
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