24,536 research outputs found
The Impulsive Phase in Solar Flares: Recent Multi-wavelength Results and their Implications for Microwave Modeling and Observations
This short paper reviews several recent key observations of the processes
occurring in the lower atmosphere (chromosphere and photosphere) during flares.
These are: evidence for compact and fragmentary structure in the flare
chromosphere, the conditions in optical flare footpoints, step-like variations
in the magnetic field during the flare impulsive phase, and hot, dense
'chromospheric' footpoints. The implications of these observations for
microwaves are also discussed.Comment: 6 pages, 5 figures, presented at 'Solar Physics with Radio
Observations' Symposium, November 2012, Nagoya, Japa
Effect of blade geometry on the aerodynamic loads produced by vertical-axis wind turbines
Accurate aerodynamic modelling of vertical-axis wind turbines poses a significant challenge. The rotation of the turbine induces large variations in the angle of attack of its blades that can manifest as dynamic stall. In addition, interactions between the blades of the turbine and the wake that they produce can result in impulsive changes to the aerodynamic loading. The Vorticity Transport Model has been used to simulate the aerodynamic performance and wake dynamics of three different vertical-axis wind turbine configurations. It is known that vertical-axis turbines with either straight or curved blades deliver torque to their shaft that fluctuates at the blade passage frequency of the rotor. In contrast, a turbine with helically twisted blades delivers a relatively steady torque to the shaft. In this article, the interactions between helically twisted blades and the vortices within their wake are shown to result in localized perturbations to the aerodynamic loading on the rotor that can disrupt the otherwise relatively smooth power output that is predicted by simplistic aerodynamic tools that do not model the wake to sufficient fidelity. Furthermore, vertical-axis wind turbines with curved blades are shown to be somewhat more susceptible to local dynamic stall than turbines with straight blades
Forces and atomic relaxations in the pSIC approach with ultrasoft pseudopotentials
We present the scheme that allows for efficient calculations of forces in the
framework of pseudopotential self-interaction corrected (pSIC) formulation of
the density functional theory. The scheme works with norm conserving and also
with ultrasoft pseudopotentials and has been implemented in the plane-wave
basis code {\sc quantum espresso}. We have performed tests of the internal
consistency of the derived expressions for forces considering ZnO and CeO
crystals. Further, we have performed calculations of equilibrium geometry for
LaTiO, YTiO, and LaMnO perovskites and also for Re and Mn pairs in
silicon. Comparison with standard DFT and DFT+U approaches shows that in the
cases where spurious self-interaction matters, the pSIC approach predicts
different geometry, very often closer to the experimental data.Comment: 11 pages, 2 figure
Impulsive Heating of Solar Flare Ribbons Above 10 MK
The chromospheric response to the input of flare energy is marked by extended
extreme ultraviolet (EUV) ribbons and hard X-ray (HXR) footpoints. These are
usually explained as the result of heating and bremsstrahlung emission from
accelerated electrons colliding in the dense chromospheric plasma. We present
evidence of impulsive heating of flare ribbons above 10 MK in a two-ribbon
flare. We analyse the impulsive phase of SOL2013-11-09T06:38, a C2.6 class
event using data from Atmospheric Imaging Assembly (AIA) on board of Solar
Dynamics Observatory (SDO) and the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) to derive the temperature, emission measure and
differential emission measure of the flaring regions and investigate the
evolution of the plasma in the flaring ribbons. The ribbons were visible at all
SDO/AIA EUV/UV wavelengths, in particular, at 94 and 131 \AA\ filters,
sensitive to temperatures of 8 MK and 12 MK. Time evolution of the emission
measure of the plasma above 10 MK at the ribbons has a peak near the HXR peak
time. The presence of hot plasma in the lower atmosphere is further confirmed
by RHESSI imaging spectroscopy analysis, which shows resolved sources at 11-13
MK associated with at least one ribbon. We found that collisional beam heating
can only marginally explain the necessary power to heat the 10 MK plasma at the
ribbons.Comment: 21 pages, 15 figure
The supernova-regulated ISM. I. The multi-phase structure
We simulate the multi-phase interstellar medium randomly heated and stirred
by supernovae, with gravity, differential rotation and other parameters of the
solar neighbourhood. Here we describe in detail both numerical and physical
aspects of the model, including injection of thermal and kinetic energy by SN
explosions, radiative cooling, photoelectric heating and various transport
processes. With 3D domain extending 1 kpc^2 horizontally and 2 kpc vertically,
the model routinely spans gas number densities 10^-5 - 10^2 cm^-3, temperatures
10-10^8 K, local velocities up to 10^3 km s^-1 (with Mach number up to 25).
The thermal structure of the modelled ISM is classified by inspection of the
joint probability density of the gas number density and temperature. We confirm
that most of the complexity can be captured in terms of just three phases,
separated by temperature borderlines at about 10^3 K and 5x10^5 K. The
probability distribution of gas density within each phase is approximately
lognormal. We clarify the connection between the fractional volume of a phase
and its various proxies, and derive an exact relation between the fractional
volume and the filling factors defined in terms of the volume and probabilistic
averages. These results are discussed in both observational and computational
contexts. The correlation scale of the random flows is calculated from the
velocity autocorrelation function; it is of order 100 pc and tends to grow with
distance from the mid-plane. We use two distinct parameterizations of radiative
cooling to show that the multi-phase structure of the gas is robust, as it does
not depend significantly on this choice.Comment: 28 pages, 22 figures and 8 table
On the Spatial Distribution of Hard X-Rays from Solar Flare Loops
The aim of this paper is to investigate the spatial structure of the
impulsive phase hard X-ray emission from solar flares. This work is motivated
by the YOHKOH and the forthcoming HESSI observations. Summarizing past results,
it is shown that the transport effects can account for the observations by
inhomogeneous loops where there is a strong field convergence and/or density
enhancement at the top of the flaring loop. Scattering by plasma turbulence at
the acceleration site or pancake type pitch angle distribution of the
accelerated electrons can also give rise to enhanced emission at the loop tops.
These could be a natural consequence of acceleration by plasma waves. This
paper considers a general case of stochastic scattering and acceleration that
leads to an isotropic pitch angle distribution and an enhanced emission from
the loop tops or the acceleration site.
Following the formalism developed in earlier papers the strength and the
spectrum of the radiation expected from the acceleration site and the foot
points are evaluated and their dependence on the parameters describing the
acceleration process and the flare plasma are determined. The theoretical ratio
of these two intensities and relative values of their spectral indices are
compared with the YOHKOH observations, demonstrating that the above mentioned
parameters can be constrained with such observations. It is shown that future
high spatial and spectral resolution observations, for example those expected
from HESSI, can begin to distinguish between different models and constrain
their parameters.Comment: 37 pages with 20 figures. Accepted for publication in ApJ
http://www.astronomy.stanford.ed
Omega time transmissions and receiving requirements
A short history is given of the development of dual VLF time transmission techniques. The theory of time recovery from the relative phase of the dual frequency transmission is presented. The transmission and receiving requirements for cycle identification and cycle ambiguity resolution are described. Finally, an experiment to test the capability of time transmission of the OMEGA system is propose
Temperature dependent anisotropy of the penetration depth and coherence length in MgB$_2
We report measurements of the temperature dependent anisotropies
( and ) of both the London penetration depth
and the upper critical field of MgB. Data for
was obtained from measurements of
and on a single crystal sample using a tunnel diode
oscillator technique. was
deduced from field dependent specific heat measurements on the same sample.
and have opposite temperature dependencies, but
close to tend to a common value (). These results are in good agreement with theories
accounting for the two gap nature of MgBComment: 4 pages with figures (New version
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