843 research outputs found
Suppression of energetic electron transport in flares by double layers
During flares and coronal mass ejections, energetic electrons from coronal
sources typically have very long lifetimes compared to the transit times across
the systems, suggesting confinement in the source region. Particle-in-cell
simulations are carried out to explore the mechanisms of energetic electron
transport from the corona to the chromosphere and possible confinement. We set
up an initial system of pre-accelerated hot electrons in contact with ambient
cold electrons along the local magnetic field, and let it evolve over time.
Suppression of transport by a nonlinear, highly localized electrostatic
electric field (in the form of a double layer) is observed after a short phase
of free-streaming by hot electrons. The double layer (DL) emerges at the
contact of the two electron populations. It is driven by an ion-electron
streaming instability due to the drift of the back-streaming return current
electrons interacting with the ions. The DL grows over time and supports a
significant drop in temperature and hence reduces heat flux between the two
regions that is sustained for the duration of the simulation. This study shows
transport suppression begins when the energetic electrons start to propagate
away from a coronal acceleration site. It also implies confinement of energetic
electrons with kinetic energies less than the electrostatic energy of the DL
for the DL lifetime, which is much longer than the electron transit time
through the source region
The flux induced switch in the mechanism of catalytic decomposition of CH3OH on Ni foil as studied by MBRS and time resolved PES
Undamped electrostatic plasma waves
Electrostatic waves in a collision-free unmagnetized plasma of electrons with
fixed ions are investigated for electron equilibrium velocity distribution
functions that deviate slightly from Maxwellian. Of interest are undamped waves
that are the small amplitude limit of nonlinear excitations, such as electron
acoustic waves (EAWs). A deviation consisting of a small plateau, a region with
zero velocity derivative over a width that is a very small fraction of the
electron thermal speed, is shown to give rise to new undamped modes, which here
are named {\it corner modes}. The presence of the plateau turns off Landau
damping and allows oscillations with phase speeds within the plateau. These
undamped waves are obtained in a wide region of the plane
( being the real part of the wave frequency and the
wavenumber), away from the well-known `thumb curve' for Langmuir waves and EAWs
based on the Maxwellian. Results of nonlinear Vlasov-Poisson simulations that
corroborate the existence of these modes are described. It is also shown that
deviations caused by fattening the tail of the distribution shift roots off of
the thumb curve toward lower -values and chopping the tail shifts them
toward higher -values. In addition, a rule of thumb is obtained for
assessing how the existence of a plateau shifts roots off of the thumb curve.
Suggestions are made for interpreting experimental observations of
electrostatic waves, such as recent ones in nonneutral plasmas.Comment: 11 pages, 10 figure
Connection Conditions and the Spectral Family under Singular Potentials
To describe a quantum system whose potential is divergent at one point, one
must provide proper connection conditions for the wave functions at the
singularity. Generalizing the scheme used for point interactions in one
dimension, we present a set of connection conditions which are well-defined
even if the wave functions and/or their derivatives are divergent at the
singularity. Our generalized scheme covers the entire U(2) family of
quantizations (self-adjoint Hamiltonians) admitted for the singular system. We
use this scheme to examine the spectra of the Coulomb potential and the harmonic oscillator with square inverse potential , and thereby provide a general perspective for these
models which have previously been treated with restrictive connection
conditions resulting in conflicting spectra. We further show that, for any
parity invariant singular potentials , the spectrum is determined
solely by the eigenvalues of the characteristic matrix .Comment: TeX, 18 page
Space-filter techniques for quasi-neutral hybrid-kinetic models
The space-filter approach has proved a fundamental tool in studying
turbulence in neutral fluids, providing the ability to analyze scale-to-scale
energy transfer in configuration space. It is well known that turbulence in
plasma presents challenges different from neutral fluids, especially when the
scale of interests include kinetic effects. The space-filter approach is still
largely unexplored for kinetic plasma. Here we derive the space-filtered (or,
equivalently "coarse-grained") equations in configuration space for a
quasi-neutral hybrid-kinetic plasma model, in which ions are fully kinetic and
electrons are a neutralizing fluid. Different models and closures for the
electron fluid are considered, including finite electron-inertia effects and
full electrons' pressure-tensor dynamics. Implications for the cascade of
turbulent fluctuations in real space depending on different approximations are
discussed.Comment: 43 pages, 2 figure
Measuring air–sea gas exchange velocities in a large scale annular wind-wave tank
In this study we present gas-exchange measurements conducted in a large-scale wind–wave tank. Fourteen chemical species spanning a wide range of solubility (dimensionless solubility, α = 0.4 to 5470) and diffusivity (Schmidt number in water, Scw = 594 to 1194) were examined under various turbulent (u10 = 0.73 to 13.2 m s−1) conditions. Additional experiments were performed under different surfactant modulated (two different concentration levels of Triton X-100) surface states. This paper details the complete methodology, experimental procedure and instrumentation used to derive the total transfer velocity for all examined tracers. The results presented here demonstrate the efficacy of the proposed method, and the derived gas-exchange velocities are shown to be comparable to previous investigations. The gas transfer behaviour is exemplified by contrasting two species at the two solubility extremes, namely nitrous oxide (N2O) and methanol (CH3OH). Interestingly, a strong transfer velocity reduction (up to a factor of 3) was observed for the relatively insoluble N2O under a surfactant covered water surface. In contrast, the surfactant effect for CH3OH, the high solubility tracer, was significantly weake
Forskolin refractoriness. Exposure to the diterpene alters guanine nucleotide-dependent adenylate cyclase and calcium-uptake activity of cells cultured from the rat aorta
Free streaming in mixed dark matter
Free streaming in a \emph{mixture} of collisionless non-relativistic dark
matter (DM) particles is studied by implementing methods from the theory of
multicomponent plasmas. The mixture includes Fermionic, condensed and non
condensed Bosonic particles decoupling in equilibrium while relativistic, heavy
non-relativistic thermal relics (WIMPs), and sterile neutrinos that decouple
\emph{out of equilibrium} when they are relativistic. The free-streaming length
is obtained from the marginal zero of the gravitational
polarization function, which separates short wavelength Landau-damped from long
wavelength Jeans-unstable \emph{collective} modes. At redshift we find ,where are the \emph{fractions} of the respective DM components of mass
that decouple when the effective number of ultrarelativistic degrees of
freedom is , and only depend on the distribution functions at
decoupling, given explicitly in all cases. If sterile neutrinos produced either
resonantly or non-resonantly that decouple near the QCD scale are the
\emph{only} DM component,we find (non-resonant), (resonant).If WIMPs with
decoupling at are present in the mixture with
, is \emph{dominated} by CDM. If a Bose Einstein condensate is a DM
component its free streaming length is consistent with CDM because of the
infrared enhancement of the distribution function.Comment: 19 pages, 2 figures. More discussions same conclusions and results.
Version to appear in Phys. Rev.
Phase separating binary fluids under oscillatory shear
We apply lattice Boltzmann methods to study the segregation of binary fluid
mixtures under oscillatory shear flow in two dimensions. The algorithm allows
to simulate systems whose dynamics is described by the Navier-Stokes and the
convection-diffusion equations. The interplay between several time scales
produces a rich and complex phenomenology. We investigate the effects of
different oscillation frequencies and viscosities on the morphology of the
phase separating domains. We find that at high frequencies the evolution is
almost isotropic with growth exponents 2/3 and 1/3 in the inertial (low
viscosity) and diffusive (high viscosity) regimes, respectively. When the
period of the applied shear flow becomes of the same order of the relaxation
time of the shear velocity profile, anisotropic effects are clearly
observable. In correspondence with non-linear patterns for the velocity
profiles, we find configurations where lamellar order close to the walls
coexists with isotropic domains in the middle of the system. For particular
values of frequency and viscosity it can also happen that the convective
effects induced by the oscillations cause an interruption or a slowing of the
segregation process, as found in some experiments. Finally, at very low
frequencies, the morphology of domains is characterized by lamellar order
everywhere in the system resembling what happens in the case with steady shear.Comment: 1 table and 12 figures in .gif forma
- …