113 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&
Toward more realistic analytic models of the heliotail: Incorporating magnetic flattening via distortion flows
Both physical arguments and simulations of the global heliosphere indicate
that the tailward heliopause is flattened considerably in the direction
perpendicular to both the incoming flow and the large-scale interstellar
magnetic field. Despite this fact, all of the existing global analytical models
of the outer heliosheath's magnetic field assume a circular cross section of
the heliotail. To eliminate this inconsistency, we introduce a mathematical
procedure by which any analytically or numerically given magnetic field can be
deformed in such a way that the cross sections along the heliotail axis attain
freely prescribed, spatially dependent values for their total area and aspect
ratio. The distorting transformation of this method honors both the
solenoidality condition and the stationary induction equation with respect to
an accompanying flow field, provided that both constraints were already
satisfied for the original magnetic and flow fields prior to the
transformation. In order to obtain realistic values for the above parameters,
we present the first quantitative analysis of the heliotail's overall
distortion as seen in state-of-the-art three-dimensional hybrid MHD-kinetic
simulations.Comment: 15 pages, 8 figures. Published in The Astrophysical Journa
Parameter estimation of superdiffusive motion of energetic particles upstream of heliospheric shocks
In-situ spacecraft observations recently suggested that the transport of
energetic particles accelerated at heliospheric shocks can be anomalous, i.e.
the mean square displacement can grow non-linearly in time. In particular, a
new analysis technique has permitted the study of particle transport properties
from energetic particle time profiles upstream of interplanetary shocks.
Indeed, the time/spatial power laws of the differential intensity upstream of
several shocks are indicative of superdiffusion. A complete determination of
the key parameters of superdiffusive transport comprises the power-law index,
the superdiffusion coefficient, the related transition scale at which the
energetic particle profiles turn to decay as power laws, and the energy
spectral index of the shock accelerated particles. Assuming large-scale spatial
homogeneity of the background plasma, the power-law behaviour can been derived
from both a (microscopic) propagator formalism and a (macroscopic) fractional
transport equation. We compare the two approaches and find a relation between
the diffusion coefficients used in the two formalisms. Based on the assumption
of superdiffusive transport, we quantitatively derive these parameters by
studying energetic particle profiles observed by the Ulysses and Voyager 2
spacecraft upstream of shocks in the heliosphere, for which a superdiffusive
particle transport has previously been observed. Further, we have jointly
studied the electron energy spectra, comparing the values of the spectral
indices observed with those predicted by the standard diffusive shock
acceleration theory and by a model based on superdiffusive transport. For a
number of interplanetary shocks and for the solar wind termination shock, for
the first time we obtain the anomalous diffusion constants and the scale at
which the probability of particle free paths changes to a power-law...Comment: 5 Figure
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
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