5,570 research outputs found
Comparative Analysis of Non-thermal Emissions and Study of Electron Transport in a Solar Flare
We study the non-thermal emissions in a solar flare occurring on 2003 May 29
by using RHESSI hard X-ray (HXR) and Nobeyama microwave observations. This
flare shows several typical behaviors of the HXR and microwave emissions: time
delay of microwave peaks relative to HXR peaks, loop-top microwave and
footpoint HXR sources, and a harder electron energy distribution inferred from
the microwave spectrum than from the HXR spectrum. In addition, we found that
the time profile of the spectral index of the higher-energy (\gsim 100 keV)
HXRs is similar to that of the microwaves, and is delayed from that of the
lower-energy (\lsim 100 keV) HXRs. We interpret these observations in terms
of an electron transport model called {\TPP}. We numerically solved the
spatially-homogeneous {\FP} equation to determine electron evolution in energy
and pitch-angle space. By comparing the behaviors of the HXR and microwave
emissions predicted by the model with the observations, we discuss the
pitch-angle distribution of the electrons injected into the flare site. We
found that the observed spectral variations can qualitatively be explained if
the injected electrons have a pitch-angle distribution concentrated
perpendicular to the magnetic field lines rather than isotropic distribution.Comment: 32 pages, 12 figures, accepted for publication in The Astronomical
Journa
Temporal evolution of an energetic electron population in an inhomogeneous medium: Application to solar hard X-ray bursts
Energetic electrons accelerated during solar flares can be studied through the hard X-ray emission they produce when interacting with the solar ambient atmosphere. In the case of the non thermal hard X-ray emission, the instanteous X-ray flux emitted at one point of the atmosphere is related to the instantaneous fast electron spectrum at that point. A hard X-ray source model then requires the understanding of the evolution in space and time of the fast particle distribution. The physical processes involved here are energy losses due to Coulomb collisions and pitch angle scattering due to both collisions and magnetic field gradients
Metal-insulator transition in three dimensional Anderson model: universal scaling of higher Lyapunov exponents
Numerical studies of the Anderson transition are based on the finite-size
scaling analysis of the smallest positive Lyapunov exponent. We prove
numerically that the same scaling holds also for higher Lyapunov exponents.
This scaling supports the hypothesis of the one-parameter scaling of the
conductance distribution. From the collected numerical data for quasi one
dimensional systems up to the system size 24 x 24 x infinity we found the
critical disorder 16.50 < Wc < 16.53 and the critical exponent 1.50 < \nu <
1.54. Finite-size effects and the role of irrelevant scaling parameters are
discussed.Comment: 4 pages, 2 figure
Numerical simulations of chromospheric hard X-ray source sizes in solar flares
X-ray observations are a powerful diagnostic tool for transport,
acceleration, and heating of electrons in solar flares. Height and size
measurements of X-ray footpoints sources can be used to determine the
chromospheric density and constrain the parameters of magnetic field
convergence and electron pitch-angle evolution. We investigate the influence of
the chromospheric density, magnetic mirroring and collisional pitch-angle
scattering on the size of X-ray sources. The time-independent Fokker-Planck
equation for electron transport is solved numerically and analytically to find
the electron distribution as a function of height above the photosphere. From
this distribution, the expected X-ray flux as a function of height, its peak
height and full width at half maximum are calculated and compared with RHESSI
observations. A purely instrumental explanation for the observed source size
was ruled out by using simulated RHESSI images. We find that magnetic mirroring
and collisional pitch-angle scattering tend to change the electron flux such
that electrons are stopped higher in the atmosphere compared with the simple
case with collisional energy loss only. However, the resulting X-ray flux is
dominated by the density structure in the chromosphere and only marginal
increases in source width are found. Very high loop densities (>10^{11}
cm^{-3}) could explain the observed sizes at higher energies, but are
unrealistic and would result in no footpoint emission below about 40 keV,
contrary to observations. We conclude that within a monolithic density model
the vertical sizes are given mostly by the density scale-height and are
predicted smaller than the RHESSI results show.Comment: 19 pages, 9 figures, accepted for publication in Ap
Critical regime of two dimensional Ando model: relation between critical conductance and fractal dimension of electronic eigenstates
The critical two-terminal conductance and the spatial fluctuations of
critical eigenstates are investigated for a disordered two dimensional model of
non-interacting electrons subject to spin-orbit scattering (Ando model). For
square samples, we verify numerically the relation between critical conductivity and
the fractal information dimension of the electron wave function, . Through a detailed numerical scaling analysis of the two-terminal
conductance we also estimate the critical exponent that
governs the quantum phase transition.Comment: IOP Latex, 7 figure
Multifractal properties of critical eigenstates in two-dimensional systems with symplectic symmetry
The multifractal properties of electronic eigenstates at the metal-insulator
transition of a two-dimensional disordered tight-binding model with spin-orbit
interaction are investigated numerically. The correlation dimensions of the
spectral measure and of the fractal eigenstate are
calculated and shown to be related by . The exponent
describing the energy correlations of the critical
eigenstates is found to satisfy the relation .Comment: 6 pages RevTeX; 3 uuencoded, gzipped ps-figures to appear in J. Phys.
Condensed Matte
Effectiveness of an inlet flow turbulence control device to simulate flight noise fan in an anechoic chamber
A hemispherical inlet flow control device was tested on a 50.8 cm. (20-inch) diameter fan stage in the NASA-Lewis anechoic chamber. The control device used honeycomb and wire mesh to reduce turbulence intensities entering the fan. Far field acoustic power level results show about a 5 db reduction in blade passing tone and about 10 dB reduction in multiple pure tone sound power at 90% design fan speed with the inlet device in place. Hot film cross probes were inserted in the inlet to obtain data for two components of the turbulence at 65 and 90% design fan speed. Without the flow control device, the axial intensities were below 1.0%, while the circumferential intensities were almost twice this value. The inflow control device significantly reduced the circumferential turbulence intensities and also reduced the axial length scale
Failure of single-parameter scaling of wave functions in Anderson localization
We show how to use properties of the vectors which are iterated in the
transfer-matrix approach to Anderson localization, in order to generate the
statistical distribution of electronic wavefunction amplitudes at arbitary
distances from the origin of disordered systems. For
our approach is shown to reproduce exact diagonalization results
available in the literature. In , where strips of width sites
were used, attempted fits of gaussian (log-normal) forms to the wavefunction
amplitude distributions result in effective localization lengths growing with
distance, contrary to the prediction from single-parameter scaling theory. We
also show that the distributions possess a negative skewness , which is
invariant under the usual histogram-collapse rescaling, and whose absolute
value increases with distance. We find for the
range of parameters used in our study, .Comment: RevTeX 4, 6 pages, 4 eps figures. Phys. Rev. B (final version, to be
published
Time-dependent quantum transport in a resonant tunnel junction coupled to a nanomechanical oscillator
We present a theoretical study of time-dependent quantum transport in a
resonant tunnel junction coupled to a nanomechanical oscillator within the
non-equilibrium Green's function technique. An arbitrary voltage is applied to
the tunnel junction and electrons in the leads are considered to be at zero
temperature. The transient and the steady state behavior of the system is
considered here in order to explore the quantum dynamics of the oscillator as a
function of time. The properties of the phonon distribution of the
nanomechnical oscillator strongly coupled to the electrons on the dot are
investigated using a non-perturbative approach. We consider both the energy
transferred from the electrons to the oscillator and the Fano factor as a
function of time. We discuss the quantum dynamics of the nanomechanical
oscillator in terms of pure and mixed states. We have found a significant
difference between a quantum and a classical oscillator. In particular, the
energy of a classical oscillator will always be dissipated by the electrons
whereas the quantum oscillator remains in an excited state. This will provide
useful insight for the design of experiments aimed at studying the quantum
behavior of an oscillator.Comment: 24 pages, 10 figure
Pregnancy and levels of depression and anxiety: a prospective cohort study of Australian women
OBJECTIVES:The effects of pregnancy on depression and anxiety remain unclear. Previous research is predominantly cross-sectional, not representative of the general community, and does not include data on mental health prior to pregnancy. This study used longitudinal Australian population-based data to examine whether pregnancy is associated with increases in women's anxiety and depression levels (from pre-pregnancy).METHOD:
A community sample of Australian women aged 20-24 years were recruited prospectively and assessed in 1999, 2003 and 2007. At the follow-up assessments 76 women were pregnant (with no prior children) and 542 remained nulliparous. Mixed models repeated measures analyses of variance were undertaken to compare change in levels of anxiety and depression (Goldberg Anxiety and Depression scales) between those who became pregnant and those who remained non-pregnant.
RESULTS:
Pregnancy was not associated with increased symptoms of depression or anxiety. No association was found with depression, while pregnancy was associated with a decrease in anxiety. Including somatic items in the measures of depression and anxiety resulted in higher symptom levels in pregnancy, suggesting possible item bias.
CONCLUSIONS:
This study is one of the first to follow a community sample of women from pre-pregnancy to pregnancy. The findings suggest that pregnancy is not typically detrimental to women's mental health. The current study offers a starting point for future prospective studies to follow women from pre-pregnancy to postpartum. Study limitations to be improved upon in follow-up research include expanding the sample size, and including both pregnancy-specific measures and trimester-specific data. Future research should continue to identify those women who are most (and least) at risk during pregnancy in order to target resources and assistance most effectively.This research was funded through a grant from the National Health and Medical Research Council nhmrc ID:1035803
- …