8,336 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
Determination of step rate thresholds corresponding to physical activity intensity classifications in adults
Current recommendations call for adults to be physically active at moderate and/or vigorous intensities. Given the popularity of walking and running, the use of step rates may provide a practical and inexpensive means to evaluate ambulatory intensity. Thus, the purpose of this study was to identify step rate thresholds that correspond to various intensity classifications. Methods: Oxygen consumption was measured at rest and during 10 minute treadmill walking and running trials at 6 standardized speeds (54, 80, 107, 134,
161, and 188 m∙min-1) in 9 men and 10 women (28.8 ± 6.8 yrs). Two observers counted the participants’ steps
at each treadmill speed. Linear and nonlinear regression analyses were used to develop prediction equations
to ascertain step rate thresholds at various intensities. Results: Nonlinear regression analysis of the metabolic
cost versus step rates across all treadmill speeds yielded the highest R2 values for men (R2 = .91) and women
(R2 = .79). For men, the nonlinear analysis yielded 94 and 125 step∙min-1 for moderate and vigorous intensities,
respectively. For women, 99 and 135 step∙min-1 corresponded with moderate and vigorous intensities, respectively. Conclusions: Promoting a step rate of 100 step∙min-1 may serve as a practical public health recommendation to exercise at moderate intensity
The H II Region/PDR Connection: Self-Consistent Calculations of Physical Conditions in Star-Forming Regions
We have performed a series of calculations designed to reproduce infrared
diagnostics used to determine physical conditions in star forming regions. We
self-consistently calculate the thermal and chemical structure of an H II
region and photodissociation region (PDR) that are in pressure equilibrium.
This differs from previous work, which used separate calculations for each gas
phase. Our calculations span a wide range of stellar temperatures, gas
densities, and ionization parameters. We describe improvements made to the
spectral synthesis code Cloudy that made these calculations possible. These
include the addition of a molecular network with ~1000 reactions involving 68
molecular species and improved treatment of the grain physics. Data from the
Spitzer First Look Survey, along with other archives, are used to derive
important physical characteristics of the H II region and PDR. These include
stellar temperatures, electron densities, ionization parameters, UV radiation
flux, and PDR density. Finally, we calculate the contribution of the H II
region to PDR emission line diagnostics, which allows for a more accurate
determination of physical conditions in the PDR.Comment: 60 pages, 35 figures, to be published in the Astrophysical Journal.
Version with full resolution is available at
http://www.pa.uky.edu/~nicholas/hii_pdr_high_res.pd
Effect of FSH on testicular morphology and spermatogenesis in gonadotrophin-deficient hypogonadal mice lacking androgen receptors
Follicle stimulating hormone (FSH) and androgen act to stimulate and maintain spermatogenesis. FSH acts directly on the Sertoli cells to stimulate germ cell number and acts indirectly to increase androgen production by the Leydig cells. In order to differentiate between the direct effects of FSH on spermatogenesis and those mediated indirectly through androgen action we have crossed hypogonadal (hpg) mice which lack gonadotrophins with mice lacking androgen receptors (AR) either ubiquitously (ARKO) or specifically on the Sertoli cells (SCARKO). These hpg.ARKO and hpg.SCARKO mice were treated with recombinant FSH for 7 days and testicular morphology and cell numbers assessed. In untreated hpg and hpg.SCARKO mice germ cell development was limited and did not progress beyond the pachytene stage. In hpg.ARKO mice testes were smaller with fewer Sertoli cells and germ cells compared to hpg mice. Treatment with FSH had no effect on Sertoli cell number but significantly increased germ cell numbers in all groups. In hpg mice FSH increased numbers of spermatogonia and spermatocytes and induced round spermatid formation. In hpg.SCARKO and hpg.ARKO mice, in contrast, only spermatogonial and spermatocyte numbers were increased with no formation of spermatids. Leydig cell numbers were increased by FSH in hpg and hpg.SCARKO mice but not in hpg.ARKO mice. Results show that in rodents 1) FSH acts to stimulate spermatogenesis through an increase in spermatogonial number and subsequent entry of these cells into meiosis, 2) FSH has no direct effect on the completion of meiosis and 3) FSH effects on Leydig cell number are mediated through interstitial ARs
Multiscale Gyrokinetics for Rotating Tokamak Plasmas: Fluctuations, Transport and Energy Flows
This paper presents a complete theoretical framework for plasma turbulence
and transport in tokamak plasmas. The fundamental scale separations present in
plasma turbulence are codified as an asymptotic expansion in the ratio of the
gyroradius to the equilibrium scale length. Proceeding order-by-order in this
expansion, a framework for plasma turbulence is developed. It comprises an
instantaneous equilibrium, the fluctuations driven by gradients in the
equilibrium quantities, and the transport-timescale evolution of mean profiles
of these quantities driven by the fluctuations. The equilibrium distribution
functions are local Maxwellians with each flux surface rotating toroidally as a
rigid body. The magnetic equillibrium is obtained from the Grad-Shafranov
equation for a rotating plasma and the slow (resistive) evolution of the
magnetic field is given by an evolution equation for the safety factor q.
Large-scale deviations of the distribution function from a Maxwellian are given
by neoclassical theory. The fluctuations are determined by the high-flow
gyrokinetic equation, from which we derive the governing principle for
gyrokinetic turbulence in tokamaks: the conservation and local cascade of free
energy. Transport equations for the evolution of the mean density, temperature
and flow velocity profiles are derived. These transport equations show how the
neoclassical corrections and the fluctuations act back upon the mean profiles
through fluxes and heating. The energy and entropy conservation laws for the
mean profiles are derived. Total energy is conserved and there is no net
turbulent heating. Entropy is produced by the action of fluxes flattening
gradients, Ohmic heating, and the equilibration of mean temperatures. Finally,
this framework is condensed, in the low-Mach-number limit, to a concise set of
equations suitable for numerical implementation.Comment: 113 pages, 3 figure
Noncommutativity from the string perspective: modification of gravity at a mm without mm sized extra dimensions
We explore how the IR pathologies of noncommutative field theory are resolved
when the theory is realized as open strings in background B-fields:
essentially, since the IR singularities are induced by UV/IR mixing, string
theory brings them under control in much the same way as it does the UV
singularities. We show that at intermediate scales (where the Seiberg-Witten
limit is a good approximation) the theory reproduces the noncommutative field
theory with all the (un)usual features such as UV/IR mixing, but that outside
this regime, in the deep infra-red, the theory flows continuously to the
commutative theory and normal Wilsonian behaviour is restored. The resulting
low energy physics resembles normal commutative physics, but with additional
suppressed Lorentz violating operators. We also show that the phenomenon of
UV/IR mixing occurs for the graviton as well, with the result that, in
configurations where Planck's constant receives a significant one-loop
correction (for example brane-induced gravity), the distance scale below which
gravity becomes non-Newtonian can be much greater than any compact dimensions.Comment: 30 pages. Slight revision: clarified some points and added a
referenc
Rare kaon decays in SUSY with non-universal A terms
We study the rare kaon decays in the framework of general SUSY models. Unlike
the results in the literature, we find the contributions from the gluino
exchange to the branching ratio of can reach the
central value () of the new E787 data while the
predicted value of standard model is less than . We also find that
the same effects also enhance the decays of ,
and .Comment: 9 pages, references added, revised version to appear in J. Phys.
Instanton induced charged fermion and neutrino masses in a minimal Standard Model scenario from intersecting D-branes
String instanton Yukawa corrections from Euclidean D-branes are investigated
in an effective Standard Model theory obtained from the minimal U(3)xU(2)xU(1)
D-brane configuration. In the case of the minimal chiral and Higgs spectrum, it
is found that superpotential contributions are induced by string instantons for
the perturbatively forbidden entries of the up and down quark mass matrices.
Analogous non-perturbative effects generate heavy Majorana neutrino masses and
a Dirac neutrino texture with factorizable Yukawa couplings. For this latter
case, a specific example is worked out where it is shown how this texture can
reconcile the neutrino data.Comment: 17 pages, 3 figure
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