48,279 research outputs found
Temperature dependence of emission measure in solar X-ray plasmas. 1: Non-flaring active regions
X-ray and ultraviolet line emission from hot, optically thin material forming coronal active regions on the sun may be described in terms of an emission measure distribution function, Phi (T). A relationship is developed between line flux and Phi (T), a theory which assumes that the electron density is a single-valued function of temperature. The sources of error involved in deriving Phi (T) from a set of line fluxes are examined in some detail. These include errors in atomic data (collisional excitation rates, assessment of other mechanisms for populating excited states of transitions, element abundances, ion concentrations, oscillator strengths) and errors in observed line fluxes arising from poorly - known instrumental responses. Two previous analyses are discussed in which Phi (T) for a non-flaring active region is derived. A least squares method of Batstone uses X-ray data of low statistical significance, a fact which appears to influence the results considerably. Two methods for finding Phi (T) ab initio are developed. The coefficients are evaluated by least squares. These two methods should have application not only to active-region plasmas, but also to hot, flare-produced plasmas
Internal Stresses and Formation of Switchable Nanowires at Thin Silica Film Edge
At vertical edges, thin films of silicon oxide (SiO_{2-x}) contain
semiconductive c-Si layered nanocrystals (Si NC) embedded in and supported by
an insulating g-SiO2 matrix. Tour et al. have shown that a trenched thin film
geometry enables the NC to form switchable nanowires (SNW) when trained by an
applied field. The field required to form SNW decreases rapidly within a few
cycles, or by annealing at 600 C in even fewer cycles, and is stable to 700C.
Here we describe the intrinsic evolution of Si NC and SNW in terms of the
competition between internal stresses and electro-osmosis. The analysis relies
heavily on experimental data from a wide range of thin film studies, and it
explains why a vertical edge across the planar Si-SiOx interface is necessary
to form SNW. The discussion also shows that the formation mechanisms of Si NC
and Si/SiO_{2-x} SNW are intrinsic and result from optimization of nanowire
conductivity in the presence of residual host misfit stresses
Characteristic polynomials in real Ginibre ensembles
We calculate the average of two characteristic polynomials for the real Ginibre ensemble of asymmetric random matrices, and its chiral counterpart. Considered as quadratic forms they determine a skew-symmetric kernel from which all
complex eigenvalue correlations can be derived. Our results are obtained in a very simple fashion without going to an eigenvalue representation, and are completely new in the chiral case. They hold for Gaussian ensembles which are partly symmetric, with kernels given in terms of Hermite and Laguerre polynomials respectively, depending on an asymmetry parameter. This allows us to interpolate between the maximally asymmetric real Ginibre and the Gaussian Orthogonal Ensemble, as well as their chiral counterparts
Flexurally-resisted uplift of the Tharsis Province, Mars
The tectonic style of Mars is dominated by vertical motion, perhaps more than any of the terrestrial planets. The imprint of this tectonic activity has left a surface widely faulted even though younger volcanism has masked the expression of tectonism in many places. Geological activity associated with the Tharsis and, to a lesser extent, Elysium provinces is responsible for a significant portion of this faulting, while the origins of the remaining features are enigmatic in many cases. The origin and evolution of the Tharsis and Elysium provinces, in terms of their great elevation, volcanic activity, and tectonic style, has sparked intense debate over the last fifteen years. Central to these discussions are the relative roles of structural uplift and volcanic construction in the creation of immense topographic relief. For example, it is argued that the presence of very old and cratered terrain high on the Tharsis rise, in the vicinity of Claritas Fossae, points to structural uplift of an ancient crust. Others have pointed out, however, that there is no reason that this terrain could not be of volcanic origin and thus part of the constructional mechanism
RESIK observations of He-like Ar X-ray line emission in solar flares
The Ar XVII X-ray line group principally due to transitions 1s2 - 1s2l (l=s,
p) near 4 Anstroms was observed in numerous flares by the RESIK bent crystal
spectrometer aboard CORONAS-F between 2001 and 2003. The three line features
include the Ar XVII w (resonance line), a blend of x and y (intercombination
lines), and z (forbidden line), all of which are blended with Ar XVI
dielectronic satellites. The ratio G, equal to [I(x+y) + I(z)]/I(w), varies
with electron temperature Te mostly because of unresolved dielectronic
satellites. With temperatures estimated from GOES X-ray emission, the observed
G ratios agree fairly well with those calculated from CHIANTI and other data.
With a two-component emission measure, better agreement is achieved. Some S XV
and S XVI lines blend with the Ar lines, the effect of which occurs at
temperatures greater than 8MK, allowing the S/Ar abundance ratio to be
determined. This is found to agree with coronal values. A nonthermal
contribution is indicated for some spectra in the repeating-pulse flare of 2003
February 6.Comment: Latex file and 3 ps files. Astrophysical Journal Letters (accepted,
June 2008
Silicon abundance from RESIK solar flare observations
The RESIK instrument on the CORONAS-F spacecraft obtained solar flare and
active region X-ray spectra in four channels covering the wavelength range 3.8
-- 6.1 \AA in its operational period between 2001 and 2003. Several highly
ionized silicon lines were observed within the range of the long-wavelength
channel (5.00 -- 6.05 \AA). The fluxes of the \sixiv Ly- line (5.217
\AA) and the \sixiii line (5.688 \AA) during 21 flares with
optimized pulse-height analyzer settings on RESIK have been analyzed to obtain
the silicon abundance relative to hydrogen in flare plasmas. As in previous
work, the emitting plasma for each spectrum is assumed to be characterized by a
single temperature and emission measure given by the ratio of emission in the
two channels of GOES. The silicon abundance is determined to be (\sixiv) and (\sixiii) on a logarithmic scale with
H = 12. These values, which vary by only very small amounts from flare to flare
and times within flares, are and times the
photospheric abundance, and are about a factor of three higher than RESIK
measurements during a period of very low activity. There is a suggestion that
the Si/S abundance ratio increases from active regions to flares.Comment: To be published, Solar Physic
A solar spectroscopic absolute abundance of argon from RESIK
Observations of He-like and H-like Ar (Ar XVII and Ar XVIII) lines at 3.949
Angstroms and 3.733 Angstroms respectively with the RESIK X-ray spectrometer on
the CORONAS-F spacecraft, together with temperatures and emission measures from
the two channels of GOES, have been analyzed to obtain the abundance of Ar in
flare plasmas in the solar corona. The line fluxes per unit emission measure
show a temperature dependence like that predicted from theory, and lead to
spectroscopically determined values for the absolute Ar abundance, A(Ar) = 6.44
pm 0.07 (Ar XVII) and 6.49 pm 0.16 (Ar XVIII) which are in agreement to within
uncertainties. The weighted mean is 6.45 pm 0.06, which is between two recent
compilations of the solar Ar abundance and suggest that the photospheric and
coronal abundances of Ar are very similar.Comment: 4 figure
Review of industry-proposed in-pile thermionic space reactors. Volume I - General
Diode and reactor design and nuclear fuels including uranium carbide alloys, uranium dioxide and uranium dioxide cermets for industry proposed in-pile thermionic space reactor
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