4,216 research outputs found
Astrophysical gyrokinetics: Turbulence in pressure-anisotropic plasmas at ion scales and beyond
We present a theoretical framework for describing electromagnetic kinetic
turbulence in a multi-species, magnetized, pressure-anisotropic plasma.
Turbulent fluctuations are assumed to be small compared to the mean field, to
be spatially anisotropic with respect to it, and to have frequencies small
compared to the ion cyclotron frequency. At scales above the ion Larmor radius,
the theory reduces to the pressure-anisotropic generalization of kinetic
reduced magnetohydrodynamics (KRMHD) formulated by Kunz et al. (2015). At
scales at and below the ion Larmor radius, three main objectives are achieved.
First, we analyse the linear response of the pressure-anisotropic gyrokinetic
system, and show it to be a generalisation of previously explored limits. The
effects of pressure anisotropy on the stability and collisionless damping of
Alfvenic and compressive fluctuations are highlighted, with attention paid to
the spectral location and width of the frequency jump that occurs as Alfven
waves transition into kinetic Alfven waves. Secondly, we derive and discuss a
general free-energy conservation law, which captures both the KRMHD free-energy
conservation at long wavelengths and dual cascades of kinetic Alfven waves and
ion entropy at sub-ion-Larmor scales. We show that non-Maxwellian features in
the distribution function change the amount of phase mixing and the efficiency
of magnetic stresses, and thus influence the partitioning of free energy
amongst the cascade channels. Thirdly, a simple model is used to show that
pressure anisotropy can cause large variations in the ion-to-electron heating
ratio due to the dissipation of Alfvenic turbulence. Our theory provides a
foundation for determining how pressure anisotropy affects the turbulent
fluctuation spectra, the differential heating of particle species, and the
ratio of parallel and perpendicular phase mixing in space and astrophysical
plasmas.Comment: 59 pages, 6 figures, accepted for publication in Journal of Plasma
Physics (original 28 Nov 2017); abstract abridge
Über einen energieunabhängigen austausch der alkali-ionen Li+, Na+, K+, Rb+ und Cs+ GEGEN H+ in der membran intakter und desintegrierter rattenlebermitochondrien
Abstract1.1. The energy-independent exchange of alkali metal cations against protons was investigated in intact and disintegrated mitochondria. The exchange is not specific for different alkali metal ions.2.2. In intact mitochondria the apparent Km for the energy-independent exchange range from 5 to 25 mM (Cs+, Rb+, K+ 5–10 mM; Na+ 15 mM; Li+ 20 mM). At all ions studied the maximum release of protons was 15–20 nval/mg protein.3.3. In disintegrated mitochondria there are no differences between the alkali metal ions (apparent Km 30 mM; maximum H+-release 50 nval/mg protein).4.4. Directly accessible binding sites seem to exist in the outer membrane. Binding sites in the inner membrane are accessible only after disintegration, but not after addition of valinomycin + rotenone
The GTC exoplanet transit spectroscopy survey. VI. A spectrally-resolved Rayleigh scattering slope in GJ 3470b
Aims. As a sub-Uranus-mass low-density planet, GJ 3470b has been found to
show a flat featureless transmission spectrum in the infrared and a tentative
Rayleigh scattering slope in the optical. We conducted an optical transmission
spectroscopy project to assess the impacts of stellar activity and to determine
whether or not GJ 3470b hosts a hydrogen-rich gas envelop. Methods. We observed
three transits with the low-resolution OSIRIS spectrograph at the 10.4 m Gran
Telescopio Canarias, and one transit with the high-resolution UVES spectrograph
at the 8.2 m Very Large Telescope. Results. From the high-resolution data, we
find that the difference of the Ca II H+K lines in- and out-of-transit is only
0.67 +/- 0.22%, and determine a magnetic filling factor of about 10-15%. From
the low-resolution data, we present the first optical transmission spectrum in
the 435-755 nm band, which shows a slope consistent with Rayleigh scattering.
Conclusions. After exploring the potential impacts of stellar activity in our
observations, we confirm that Rayleigh scattering in an extended
hydrogen/helium atmosphere is currently the best explanation. Further
high-precision observations that simultaneously cover optical and infrared
bands are required to answer whether or not clouds and hazes exist at
high-altitude.Comment: 12 pages, 11 figures, accepted for publication in A&
The Sphaleron Barrier in the Presence of Fermions
We calculate the minimal energy path over the sphaleron barrier in the
pre\-sen\-ce of fermions, assuming that the fermions of a doublet are
degenerate in mass. This allows for spherically symmetric ans\"atze for the
fields, when the mixing angle dependence is neglected. While light fermions
have little influence on the barrier, the presence of heavy fermions ( TeV) strongly deforms the barrier, giving rise to additional sphalerons
for very heavy fermions ( 10 TeV). Heavy fermions form
non-topological solitons in the vacuum sector.Comment: 19 pages, latex, 18 figures in 3 seperate uuencoded postscript files
THU-93/1
General approach for studying first-order phase transitions at low temperatures
By combining different ideas, a general and efficient protocol to deal with
discontinuous phase transitions at low temperatures is proposed. For small
's, it is possible to derive a generic analytic expression for appropriate
order parameters, whose coefficients are obtained from simple simulations. Once
in such regimes simulations by standard algorithms are not reliable, an
enhanced tempering method, the parallel tempering -- accurate for small and
intermediate system sizes with rather low computational cost -- is used.
Finally, from finite size analysis, one can obtain the thermodynamic limit. The
procedure is illustrated for four distinct models, demonstrating its power,
e.g., to locate coexistence lines and the phases density at the coexistence.Comment: 5 page
Level Crossing Along Sphaleron Barriers
In the electroweak sector of the standard model topologically inequivalent
vacua are separated by finite energy barriers, whose height is given by the
sphale\-ron. For large values of the Higgs mass there exist several sphaleron
solutions and the barriers are no longer symmetric. We construct paths of
classical configurations from one vacuum to a neighbouring one and solve the
fermion equations in the background field configurations along such paths,
choosing the fermions of a doublet degenerate in mass. As in the case of light
Higgs masses we observe the level crossing phenomenon also for large Higgs
masses.Comment: 17 pages, latex, 10 figures in uuencoded postscript files. THU-94/0
Nondegenerate Fermions in the Background of the Sphaleron Barrier
We consider level crossing in the background of the sphaleron barrier for
nondegenerate fermions. The mass splitting within the fermion doublets allows
only for an axially symmetric ansatz for the fermion fields. In the background
of the sphaleron we solve the partial differential equations for the fermion
functions. We find little angular dependence for our choice of ansatz. We
therefore propose a good approximate ansatz with radial functions only. We
generalize this approximate ansatz with radial functions only to fermions in
the background of the sphaleron barrier and argue, that it is a good
approximation there, too.Comment: LATEX, 20 pages, 11 figure
Static black hole solutions with axial symmetry
We construct a new class of asymptotically flat black hole solutions in
Einstein-Yang-Mills and Einstein-Yang-Mills-dilaton theory. These black hole
solutions are static, and they have a regular event horizon. However, they
possess only axial symmetry. Like their regular counterparts, the black hole
solutions are characterized by two integers, the winding number and the
node number of the gauge field functions.Comment: 14 pages, including 4 postscript figures, LaTe
Comparison between two methods of solution of coupled equations for low-energy scattering
Cross sections from low-energy neutron-nucleus scattering have been evaluated
using a coupled channel theory of scattering. Both a coordinate-space and a
momentum-space formalism of that coupled-channel theory are considered.A simple
rotational model of the channel interaction potentials is used to find results
using two relevant codes, ECIS97 and MCAS, so that they may be compared. The
very same model is then used in the MCAS approach to quantify the changes that
occur when allowance is made for effects of the Pauli principle.Comment: 6 pages, 3 figure
- …