4,382 research outputs found
On the entropy of plasmas described with regularized -distributions
In classical thermodynamics the entropy is an extensive quantity, i.e.\ the
sum of the entropies of two subsystems in equilibrium with each other is equal
to the entropy of the full system consisting of the two subsystems. The
extensitivity of entropy has been questioned in the context of a theoretical
foundation for the so-called -distributions, which describe plasma
constituents with power-law velocity distributions. We demonstrate here, by
employing the recently introduced {\it regularized -distributions},
that entropy can be defined as an extensive quantity even for such
power-law-like distributions that truncate exponentially.Comment: Preprint accepted for publication in Phys. Rev.
Quasilinear approach of the cumulative whistler instability in fast solar winds: Constraints of electron temperature anisotropy
Context. Solar outflows are a considerable source of free energy which
accumulates in multiple forms like beaming (or drifting) components and/or
temperature anisotropies. However, kinetic anisotropies of plasma particles do
not grow indefinitely and particle-particle collisions are not efficient enough
to explain the observed limits of these anisotropies. Instead, the
self-generated wave instabilities can efficiently act to constrain kinetic
anisotropies, but the existing approaches are simplified and do not provide
satisfactory explanations. Thus, small deviations from isotropy shown by the
electron temperature () in fast solar winds are not explained yet.
Aims. This paper provides an advanced quasilinear description of the whistler
instability driven by the anisotropic electrons in conditions typical for the
fast solar winds. The enhanced whistler-like fluctuations may constrain the
upper limits of temperature anisotropy ,
where are defined with respect to the magnetic field
direction.
Methods. Studied are the self-generated whistler instabilities, cumulatively
driven by the temperature anisotropy and the relative (counter)drift of the
electron populations, e.g., core and halo electrons. Recent studies have shown
that quasi-stable states are not bounded by the linear instability thresholds
but an extended quasilinear approach is necessary to describe them in this
case.
Results. Marginal conditions of stability are obtained from a quasilinear
theory of the cumulative whistler instability, and approach the quasi-stable
states of electron populations reported by the observations.The instability
saturation is determined by the relaxation of both the temperature anisotropy
and the relative drift of electron populations.Comment: Accepted for publication in A&
Dual Maxwellian-Kappa modelling of the solar wind electrons: new clues on the temperature of Kappa populations
Context. Recent studies on Kappa distribution functions invoked in space
plasma applications have emphasized two alternative approaches which may assume
the temperature parameter either dependent or independent of the power-index
. Each of them can obtain justification in different scenarios
involving Kappa-distributed plasmas, but direct evidences supporting any of
these two alternatives with measurements from laboratory or natural plasmas are
not available yet. Aims. This paper aims to provide more facts on this
intriguing issue from direct fitting measurements of suprathermal electron
populations present in the solar wind, as well as from their destabilizing
effects predicted by these two alternating approaches. Methods. Two fitting
models are contrasted, namely, the global Kappa and the dual Maxwellian-Kappa
models, which are currently invoked in theory and observations. The
destabilizing effects of suprathermal electrons are characterized on the basis
of a kinetic approach which accounts for the microscopic details of the
velocity distribution. Results. In order to be relevant, the model is chosen to
accurately reproduce the observed distributions and this is achieved by a dual
Maxwellian-Kappa distribution function. A statistical survey indicates a
-dependent temperature of the suprathermal (halo) electrons for any
heliocentric distance. Only for this approach the instabilities driven by the
temperature anisotropy are found to be systematically stimulated by the
abundance of suprathermal populations, i.e., lowering the values of
-index.Comment: Submitted to A&
Cartan's spiral staircase in physics and, in particular, in the gauge theory of dislocations
In 1922, Cartan introduced in differential geometry, besides the Riemannian
curvature, the new concept of torsion. He visualized a homogeneous and
isotropic distribution of torsion in three dimensions (3d) by the "helical
staircase", which he constructed by starting from a 3d Euclidean space and by
defining a new connection via helical motions. We describe this geometric
procedure in detail and define the corresponding connection and the torsion.
The interdisciplinary nature of this subject is already evident from Cartan's
discussion, since he argued - but never proved - that the helical staircase
should correspond to a continuum with constant pressure and constant internal
torque. We discuss where in physics the helical staircase is realized: (i) In
the continuum mechanics of Cosserat media, (ii) in (fairly speculative) 3d
theories of gravity, namely a) in 3d Einstein-Cartan gravity - this is Cartan's
case of constant pressure and constant intrinsic torque - and b) in 3d Poincare
gauge theory with the Mielke-Baekler Lagrangian, and, eventually, (iii) in the
gauge field theory of dislocations of Lazar et al., as we prove for the first
time by arranging a suitable distribution of screw dislocations. Our main
emphasis is on the discussion of dislocation field theory.Comment: 31 pages, 8 figure
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