55 research outputs found
Anisotropic diffusion of galactic cosmic ray protons and their steady-state azimuthal distribution
Galactic transport models for cosmic rays involve the diffusive motion of
these particles in the interstellar medium. Due to the large-scale structured
galactic magnetic field this diffusion is anisotropic with respect to the local
field direction. We included this transport effect along with continuous loss
processes in a quantitative model of galactic propagation for cosmic ray
protons which is based on stochastic differential equations. We calculated
energy spectra at different positions along the Sun's galactic orbit and
compared them to the isotropic diffusion case. The results show that a larger
amplitude of variation as well as different spectral shapes are obtained in the
introduced anisotropic diffusion scenario and emphasize the need for accurate
galactic magnetic field models.Comment: 7 pages, 5 figures, accepted for publication in A&
The interaction of multiple stellar winds in stellar clusters: potential flow
While several studies have investigated large-scale cluster winds resulting
from an intra-cluster interaction of multiple stellar winds, as yet they have
not provided details of the bordering flows inside a given cluster. The present
work explores the principal structure of the combined flow resulting from the
interaction of multiple stellar winds inside stellar clusters. The theory of
complex potentials is applied to analytically investigate stagnation points,
boundaries between individual outflows, and the hydrodynamic structure of the
asymptotic large-scale cluster wind. In a second part, these planar
considerations are extended to fully three-dimensional, asymmetric
configurations of wind-driving stars.
We find (i) that one can distinguish regions in the large-scale cluster wind
that are determined by the individual stellar winds, (ii) that there are
comparatively narrow outflow channels, and (iii) that the large-scale cluster
wind asymptotically approaches spherical symmetry at large distances. The
combined flow inside a stellar cluster resulting from the interaction of
multiple stellar winds is highly structured.Comment: 8 pages, 8 Figure
Electron holes in a regularized kappa background
The pseudopotential method is used to derive electron hole structures in a suprathermal plasma with a regularized κ probability distribution function background. The regularized character allows the exploration of small κ values beyond the standard suprathermal case for which κ > 3/2 is a necessary condition. We found the nonlinear dispersion relation yielding the amplitude of the electrostatic potential in terms of the remaining parameters, in particular the drift velocity, the wavenumber and the spectral index. Periodic, solitary wave, drifting and non-drifting solutions have been identified. In the linear limit, the dispersion relation yields generalized Langmuir and electron acoustic plasma modes. Standard electron hole structures are regained in the κ 1 limit
Electron Holes in a Regularized Kappa Background
The pseudopotential method is used to derive electron hole structures in a
suprathermal plasma having a regularized probability distribution
function background. The regularized character allows the exploration of small
values beyond the standard suprathermal case, for which
is a necessary condition. We have found the nonlinear dispersion relation
yielding the amplitude of the electrostatic potential in terms of the remaining
parameters, in particular the drift velocity, the wavenumber and the spectral
index. Periodic, solitary wave, drifting and non-drifting solutions have been
identified. In the linear limit, the dispersion relation yields generalized
Langmuir and electron acoustic plasma modes. Standard electron hole structures
are regained in the limit
Revisiting the Ulysses electron data with a triple fit of velocity distributions
Given their uniqueness, the Ulysses data can still provide us with valuable
new clues about the properties of plasma populations in the solar wind, and,
especially, about their variations with heliographic coordinates. We revisit
the electron data reported by by the SWOOPS instrument on-board of the Ulysses
spacecraft between 1990 to early 2008. These observations reveal velocity
distributions out of thermal equilibrium, with anisotropies (e.g., parallel
drifts or/and different temperatures, parallel and perpendicular to the
background magnetic field), and quasi-thermal and suprathermal populations with
different properties. We apply a 2D nonlinear least square fitting procedure,
using the Levenberg-Marquardt algorithm, to simultaneously fit the velocity
electron data (up to a few keV) with a triple model combining three distinct
populations: the more central quasi-thermal core and suprathermal halo, and a
second suprathermal population consisting mainly of the electron strahl (or
beaming population with a major field-aligned drift). The recently introduced
-cookbook is used to describe each component with the following
anisotropic distribution functions (recipes): Maxwellian, regularized kappa-,
and generalized kappa-distributions. The temperature anisotropy quantified by
the best fits is considered as a case study of the main parameters
characterizing electron populations. By comparison to the core, both
suprathermal populations exhibit higher temperature anisotropies, which
slightly increase with the energy of electrons.Comment: 11 pages, 11 Figure
Linear Dispersion Theory of Parallel Electromagnetic Modes for Regularized Kappa-Distributions
The velocity particle distributions measured in-situ in space plasmas deviate
from Maxwellian (thermal) equilibrium, showing enhanced suprathermal tails
which are well described by the standard Kappa-distribution (SKD). Despite its
successful application, the SKD is frequently disputed due to a series of
unphysical implications like diverging velocity moments, preventing a
macroscopic description of the plasma. The regularized Kappa-distribution (RKD)
has been introduced to overcome these limitations, but the dispersion
properties of RKD-plasmas are not explored yet. In the present paper we compute
the wavenumber dispersion of the frequency and damping or growth rates for the
electromagnetic modes in plasmas characterized by the RKD. This task is
accomplished by using the grid-based kinetic dispersion solver LEOPARD
developed for arbitrary gyrotropic distributions [P. Astfalk and F. Jenko, J.
Geophys. Res. 122, 89 (2017)]. By reproducing previous results obtained for the
SKD and Maxwellian, we validate the functionality of the code. Furthermore, we
apply the isotropic as well as the anisotropic RKDs to investigate stable
electromagnetic electron-cyclotron (EMEC) and ion-cyclotron (EMIC) modes as
well as temperature-anisotropy-driven instabilities, both for the case (EMEC and EMIC instabilities) and for the case (proton and electron firehose instabilities), where
and denote directions parallel and perpendicular to the
local time-averaged magnetic field. Provided that the cutoff parameter
is small enough, the results show that the RKDs reproduce the dispersion curves
of the SKD plasmas at both qualitative and quantitative levels. For higher
values, however, physically significant deviation occurs.Comment: 15 pages, 9 figures, 3 table
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