190 research outputs found
Apparent temperature anisotropies due to wave activity in the solar wind
The fast solar wind is a collisionless plasma permeated by plasma waves on
many different scales. A plasma wave represents the natural interplay between
the periodic changes of the electromagnetic field and the associated coherent
motions of the plasma particles. In this paper, a model velocity distribution
function is derived for a plasma in a single, coherent, large-amplitude wave.
This model allows one to study the kinetic effects of wave motions on particle
distributions. They are by in-situ spacecraft measured by counting, over a
certain sampling time, the particles coming from various directions and having
different energies. We compare our results with the measurements by the Helios
spacecraft, and thus find that by assuming high wave activity we are able to
explain key observed features of the measured distributions within the
framework of our model. We also address the recent discussions on nonresonant
wave--particle interactions and apparent heating. The applied time-averaging
procedure leads to an apparent ion temperature anisotropy which is connected
but not identical to the intrinsic temperature of the underlying distribution
function.Comment: 9 pages, 4 figures, publisher version under
http://www.ann-geophys.net/29/909/2011/angeo-29-909-2011.htm
NHDS: The New Hampshire Dispersion Relation Solver
NHDS is the New Hampshire Dispersion Relation Solver. This article describes
the numerics of the solver and its capabilities. The code is available for
download on https://github.com/danielver02/NHDS.Comment: 3 pages, 1 figur
Instabilities Driven by the Drift and Temperature Anisotropy of Alpha Particles in the Solar Wind
We investigate the conditions under which parallel-propagating
Alfv\'en/ion-cyclotron (A/IC) waves and fast-magnetosonic/whistler (FM/W) waves
are driven unstable by the differential flow and temperature anisotropy of
alpha particles in the solar wind. We focus on the limit in which , where is the
parallel alpha-particle thermal speed and is the Alfv\'en
speed. We derive analytic expressions for the instability thresholds of these
waves, which show, e.g., how the minimum unstable alpha-particle beam speed
depends upon , the degree of alpha-particle
temperature anisotropy, and the alpha-to-proton temperature ratio. We validate
our analytical results using numerical solutions to the full hot-plasma
dispersion relation. Consistent with previous work, we find that temperature
anisotropy allows A/IC waves and FM/W waves to become unstable at significantly
lower values of the alpha-particle beam speed than in the
isotropic-temperature case. Likewise, differential flow lowers the minimum
temperature anisotropy needed to excite A/IC or FM/W waves relative to the case
in which . We discuss the relevance of our results to alpha
particles in the solar wind near 1 AU.Comment: 13 pages, 13 figure
The electron distribution function downstream of the solar-wind termination shock: Where are the hot electrons?
In the majority of the literature on plasma shock waves, electrons play the
role of "ghost particles," since their contribution to mass and momentum flows
is negligible, and they have been treated as only taking care of the electric
plasma neutrality. In some more recent papers, however, electrons play a new
important role in the shock dynamics and thermodynamics, especially at the
solar-wind termination shock. They react on the shock electric field in a very
specific way, leading to suprathermal nonequilibrium distributions of the
downstream electrons, which can be represented by a kappa distribution
function. In this paper, we discuss why this anticipated hot electron
population has not been seen by the plasma detectors of the Voyager spacecraft
downstream of the solar-wind termination shock. We show that hot nonequilibrium
electrons induce a strong negative electric charge-up of any spacecraft
cruising through this downstream plasma environment. This charge reduces
electron fluxes at the spacecraft detectors to nondetectable intensities.
Furthermore, we show that the Debye length
grows to values of about compared to the classical value in this
hot-electron environment. This unusual condition allows for the propagation of
a certain type of electrostatic plasma waves that, at very large wavelengths,
allow us to determine the effective temperature of the suprathermal electrons
directly by means of the phase velocity of these waves. At moderate
wavelengths, the electron-acoustic dispersion relation leads to nonpropagating
oscillations with the ion-plasma frequency , instead of
the traditional electron plasma frequency.Comment: 6 pages, 2 figure
Spectral evolution of two-dimensional kinetic plasma turbulence in the wavenumber-frequency domain
We present a method for studying the evolution of plasma turbulence by
tracking dispersion relations in the energy spectrum in the
wavenumber-frequency domain. We apply hybrid plasma simulations in a simplified
two-dimensional geometry to demonstrate our method and its applicability to
plasma turbulence in the ion kinetic regime. We identify four dispersion
relations: ion-Bernstein waves, oblique whistler waves, oblique
Alfv\'en/ion-cyclotron waves, and a zero-frequency mode. The energy partition
and frequency broadening are evaluated for these modes. The method allows us to
determine the evolution of decaying plasma turbulence in our restricted
geometry and shows that it cascades along the dispersion relations during the
early phase with an increasing broadening around the dispersion relations.Comment: 11 pages, 4 figure
Magnetohydrodynamic Slow Mode with Drifting He: Implications for Coronal Seismology and the Solar Wind
The MHD slow mode wave has application to coronal seismology, MHD turbulence,
and the solar wind where it can be produced by parametric instabilities. We
consider analytically how a drifting ion species (e.g. He) affects the
linear slow mode wave in a mainly electron-proton plasma, with potential
consequences for the aforementioned applications. Our main conclusions are: 1.
For wavevectors highly oblique to the magnetic field, we find solutions that
are characterized by very small perturbations of total pressure. Thus, our
results may help to distinguish the MHD slow mode from kinetic Alfv\'en waves
and non-propagating pressure-balanced structures, which can also have very
small total pressure perturbations. 2. For small ion concentrations, there are
solutions that are similar to the usual slow mode in an electron-proton plasma,
and solutions that are dominated by the drifting ions, but for small drifts the
wave modes cannot be simply characterized. 3. Even with zero ion drift, the
standard dispersion relation for the highly oblique slow mode cannot be used
with the Alfv\'en speed computed using the summed proton and ion densities, and
with the sound speed computed from the summed pressures and densities of all
species. 4. The ions can drive a non-resonant instability under certain
circumstances. For low plasma beta, the threshold drift can be less than that
required to destabilize electromagnetic modes, but damping from the Landau
resonance can eliminate this instability altogether, unless .Comment: 35 pages, 5 figures, accepted for publication in Astrophys.
On kinetic slow modes, fluid slow modes, and pressure-balanced structures in the solar wind
Observations in the solar wind suggest that the compressive component of inertial-range solar-wind turbulence is dominated by slow modes. The low collisionality of the solar wind allows for nonthermal features to survive, which suggests the requirement of a kinetic plasma description. The least-damped kinetic slow mode is associated with the ion-acoustic (IA) wave and a nonpropagating (NP) mode. We derive analytical expressions for the IA-wave dispersion relation in an anisotropic plasma in the framework of gyrokinetics and then compare them to fully kinetic numerical calculations, results from two-fluid theory, and magnetohydrodynamics (MHD). This comparison shows major discrepancies in the predicted wave phase speeds from MHD and kinetic theory at moderate to high β. MHD and kinetic theory also dictate that all plasma normal modes exhibit a unique signature in terms of their polarization. We quantify the relative amplitude of fluctuations in the three lowest particle velocity moments associated with IA and NP modes in the gyrokinetic limit and compare these predictions with MHD results and in situ observations of the solar-wind turbulence. The agreement between the observations of the wave polarization and our MHD predictions is better than the kinetic predictions, which suggests that the plasma behaves more like a fluid in the solar wind than expected
Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities
Compressive fluctuations are a minor yet significant component of
astrophysical plasma turbulence. In the solar wind, long-wavelength compressive
slow-mode fluctuations lead to changes in and in , where and are the perpendicular and parallel
temperatures of the protons, is the magnetic field strength, and
is the proton density. If the amplitude of the compressive
fluctuations is large enough, crosses one or more instability
thresholds for anisotropy-driven microinstabilities. The enhanced field
fluctuations from these microinstabilities scatter the protons so as to reduce
the anisotropy of the pressure tensor. We propose that this scattering drives
the average value of away from the marginal stability boundary
until the fluctuating value of stops crossing the boundary. We
model this "fluctuating-anisotropy effect" using linear Vlasov--Maxwell theory
to describe the large-scale compressive fluctuations. We argue that this effect
can explain why, in the nearly collisionless solar wind, the average value of
is close to unity.Comment: 11 pages, published in Ap
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