33,446 research outputs found
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&
Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films
We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial
thin films, resulting in a giant flexoelectric effect. Using grazing-incidence
in-plane X-ray diffraction, we measured strain gradients in the films, which
were 6 or 7 orders of magnitude larger than typical values reported for bulk
oxides. The combination of transmission electron microscopy, electrical
measurements, and electrostatic calculations showed that flexoelectricity
provides a means of tuning the physical properties of ferroelectric epitaxial
thin films, such as domain configurations and hysteresis curves.Comment: Accepted by Phys. Rev. Let
Macromolecular separation through a porous surface
A new technique for the separation of macromolecules is proposed and
investigated. A thin mesh with pores comparable to the radius of gyration of a
free chain is used to filter chains according to their length. Without a field
it has previously been shown that the permeability decays as a power law with
chain length. However by applying particular configurations of pulsed fields,
it is possible to have a permeability that decays as an exponential. This
faster decay gives much higher resolution of separation. We also propose a
modified screen containing an array of holes with barb-like protrusions running
parallel to the surface. When static friction is present between the
macromolecule and the protrusion, some of the chains get trapped for long
durations of time. By using this and a periodic modulation of an applied
electric field, high resolution can be attained.Comment: 18 pages latex, 6 postscript figures, using psfi
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