333,114 research outputs found
Inclinations of small quiet-Sun magnetic features based on a new geometric approach
High levels of horizontal magnetic flux have been reported in the quiet-Sun
internetwork, often based on Stokes profile inversions. Here we introduce a new
method for deducing the inclination of magnetic elements and use it to test
magnetic field inclinations from inversions. We determine accurate positions of
a set of small, bright magnetic elements in high spatial resolution images
sampling different photospheric heights obtained by the Sunrise balloon-borne
solar observatory. Together with estimates of the formation heights of the
employed spectral bands, these provide us with the inclinations of the magnetic
features. We also compute the magnetic inclination angle of the same magnetic
features from the inversion of simultaneously recorded Stokes parameters. Our
new, geometric method returns nearly vertical fields (average inclination of
around 14 deg with a relatively narrow distribution having a standard deviation
of 6 deg). In strong contrast to this, the traditionally used inversions give
almost horizontal fields (average inclination of 75+-8 deg) for the same small
magnetic features, whose linearly polarised Stokes profiles are adversely
affected by noise. The almost vertical field of bright magnetic features from
our geometric method is clearly incompatible with the nearly horizontal
magnetic fields obtained from the inversions. This indicates that the amount of
magnetic flux in horizontal fields deduced from inversions is overestimated in
the presence of weak Stokes signals, in particular if Stokes Q and U are close
to or under the noise level. By combining the proposed method with inversions
we are not just improving the inclination, but also the field strength. This
technique allows us to analyse features that are not reliably treated by
inversions, thus greatly extending our capability to study the complete
magnetic field of the quiet Sun.Comment: 12 pages, 9 figures, 1 table; Accepted for publication in Astronomy &
Astrophysic
A comparison of spectroscopic methods for detecting starlight scattered by transiting hot Jupiters, with application to Subaru data for HD 209458b and HD 189733b
The measurement of the light scattered from extrasolar planets informs
atmospheric and formation models. With the discovery of many hot Jupiter
planets orbiting nearby stars, this motivates the development of robust methods
of characterisation from follow up observations. In this paper we discuss two
methods for determining the planetary albedo in transiting systems. First, the
most widely used method for measuring the light scattered by hot Jupiters
(Collier Cameron et al.) is investigated for application for typical echelle
spectra of a transiting planet system, showing that detection requires high
signal-to-noise ratio data of bright planets. Secondly a new Fourier analysis
method is also presented, which is model-independent and utilises the benefits
of the reduced number of unknown parameters in transiting systems. This
approach involves solving for the planet and stellar spectra in Fourier space
by least-squares. The sensitivities of the methods are determined via Monte
Carlo simulations for a range of planet-to-star fluxes. We find the Fourier
analysis method to be better suited to the ideal case of typical observations
of a well constrained transiting system than the Collier Cameron et al. method.
We apply the Fourier analysis method for extracting the light scattered by
transiting hot Jupiters from high resolution spectra to echelle spectra of HD
209458 and HD 189733. Unfortunately we are unable to improve on the previous
upper limit of the planet-to-star flux for HD 209458b set by space-based
observations. A 1{\sigma}upper limit on the planet-to-star flux of HD 189733b
is measured in the wavelength range of 558.83-599.56 nm yielding {\epsilon} <
4.5 \times 10-4. Improvement in the measurement of the upper limit of the
planet-to-star flux of this system, with ground-based capabilities, requires
data with a higher signal-to-noise ratio, and increased stability of the
telescope.Comment: 15 pages, 8 figures, 2 tables. Monthly Notices of the Royal
Astronomical Society, in press. Accepted 2011 March 17. Received 2011 March
17; in original form 2010 June 2
The Lyman-alpha Forest Power Spectrum from the Sloan Digital Sky Survey
We measure the power spectrum, P_F(k,z), of the transmitted flux in the
Ly-alpha forest using 3035 high redshift quasar spectra from the Sloan Digital
Sky Survey. This sample is almost two orders of magnitude larger than any
previously available data set, yielding statistical errors of ~0.6% and ~0.005
on, respectively, the overall amplitude and logarithmic slope of P_F(k,z). This
unprecedented statistical power requires a correspondingly careful analysis of
the data and of possible systematic contaminations in it. For this purpose we
reanalyze the raw spectra to make use of information not preserved by the
standard pipeline. We investigate the details of the noise in the data,
resolution of the spectrograph, sky subtraction, quasar continuum, and metal
absorption. We find that background sources such as metals contribute
significantly to the total power and have to be subtracted properly. We also
find clear evidence for SiIII correlations with the Ly-alpha forest and suggest
a simple model to account for this contribution to the power. While it is
likely that our newly developed analysis technique does not eliminate all
systematic errors in the P_F(k,z) measurement below the level of the
statistical errors, our tests indicate that any residual systematics in the
analysis are unlikely to affect the inference of cosmological parameters from
P_F(k,z). These results should provide an essential ingredient for all future
attempts to constrain modeling of structure formation, cosmological parameters,
and theories for the origin of primordial fluctuations.Comment: 92 pages, 45 of them figures, submitted to ApJ, data available at
http://feynman.princeton.edu/~pmcdonal/LyaF/sdss.htm
LOFAR Sparse Image Reconstruction
Context. The LOw Frequency ARray (LOFAR) radio telescope is a giant digital
phased array interferometer with multiple antennas distributed in Europe. It
provides discrete sets of Fourier components of the sky brightness. Recovering
the original brightness distribution with aperture synthesis forms an inverse
problem that can be solved by various deconvolution and minimization methods
Aims. Recent papers have established a clear link between the discrete nature
of radio interferometry measurement and the "compressed sensing" (CS) theory,
which supports sparse reconstruction methods to form an image from the measured
visibilities. Empowered by proximal theory, CS offers a sound framework for
efficient global minimization and sparse data representation using fast
algorithms. Combined with instrumental direction-dependent effects (DDE) in the
scope of a real instrument, we developed and validated a new method based on
this framework Methods. We implemented a sparse reconstruction method in the
standard LOFAR imaging tool and compared the photometric and resolution
performance of this new imager with that of CLEAN-based methods (CLEAN and
MS-CLEAN) with simulated and real LOFAR data Results. We show that i) sparse
reconstruction performs as well as CLEAN in recovering the flux of point
sources; ii) performs much better on extended objects (the root mean square
error is reduced by a factor of up to 10); and iii) provides a solution with an
effective angular resolution 2-3 times better than the CLEAN images.
Conclusions. Sparse recovery gives a correct photometry on high dynamic and
wide-field images and improved realistic structures of extended sources (of
simulated and real LOFAR datasets). This sparse reconstruction method is
compatible with modern interferometric imagers that handle DDE corrections (A-
and W-projections) required for current and future instruments such as LOFAR
and SKAComment: Published in A&A, 19 pages, 9 figure
The VLA Low-frequency Sky Survey
The Very Large Array (VLA) Low-frequency Sky Survey (VLSS) has imaged 95% of
the 3*pi sr of sky north of declination = -30 degrees at a frequency of 74 MHz
(4 meter wavelength). The resolution is 80" (FWHM) throughout, and the typical
RMS noise level is ~0.1 Jy/beam. The typical point-source detection limit is
0.7 Jy/beam and so far nearly 70,000 sources have been catalogued. This survey
used the 74 MHz system added to the VLA in 1998. It required new imaging
algorithms to remove the large ionospheric distortions at this very low
frequency throughout the entire ~11.9 degree field of view. This paper
describes the observation and data reduction methods used for the VLSS and
presents the survey images and source catalog. All of the calibrated images and
the source catalog are available online (http://lwa.nrl.navy.mil/VLSS) for use
by the astronomical community.Comment: 53 pages, including 3 tables and 15 figures. Has been accepted for
publication in the Astronomical Journa
Multi-scale initial conditions for cosmological simulations
We discuss a new algorithm to generate multi-scale initial conditions with
multiple levels of refinements for cosmological "zoom-in" simulations. The
method uses an adaptive convolution of Gaussian white noise with a real space
transfer function kernel together with an adaptive multi-grid Poisson solver to
generate displacements and velocities following first (1LPT) or second order
Lagrangian perturbation theory (2LPT). The new algorithm achieves RMS relative
errors of order 10^(-4) for displacements and velocities in the refinement
region and thus improves in terms of errors by about two orders of magnitude
over previous approaches. In addition, errors are localized at coarse-fine
boundaries and do not suffer from Fourier-space induced interference ringing.
An optional hybrid multi-grid and Fast Fourier Transform (FFT) based scheme is
introduced which has identical Fourier space behaviour as traditional
approaches. Using a suite of re-simulations of a galaxy cluster halo our real
space based approach is found to reproduce correlation functions, density
profiles, key halo properties and subhalo abundances with per cent level
accuracy. Finally, we generalize our approach for two-component baryon and
dark-matter simulations and demonstrate that the power spectrum evolution is in
excellent agreement with linear perturbation theory. For initial baryon density
fields, it is suggested to use the local Lagrangian approximation in order to
generate a density field for mesh based codes that is consistent with
Lagrangian perturbation theory instead of the current practice of using the
Eulerian linearly scaled densities.Comment: 22 pages, 24 figures. MNRAS in press. Updated affiliation
Coupled atmosphere-wildland fire modeling with WRF-Fire
We describe the physical model, numerical algorithms, and software structure
of WRF-Fire. WRF-Fire consists of a fire-spread model, implemented by the
level-set method, coupled with the Weather Research and Forecasting model. In
every time step, the fire model inputs the surface wind, which drives the fire,
and outputs the heat flux from the fire into the atmosphere, which in turn
influences the atmosphere. The level-set method allows submesh representation
of the burning region and flexible implementation of various ignition modes.
WRF-Fire is distributed as a part of WRF and it uses the WRF parallel
infrastructure for parallel computing.Comment: Version 3.3, 41 pages, 2 tables, 12 figures. As published in
Discussions, under review for Geoscientific Model Developmen
Bias-free Measurement of Giant Molecular Cloud Properties
(abridged) We review methods for measuring the sizes, line widths, and
luminosities of giant molecular clouds (GMCs) in molecular-line data cubes with
low resolution and sensitivity. We find that moment methods are robust and
sensitive -- making full use of both position and intensity information -- and
we recommend a standard method to measure the position angle, major and minor
axis sizes, line width, and luminosity using moment methods. Without
corrections for the effects of beam convolution and sensitivity to GMC
properties, the resulting properties may be severely biased. This is
particularly true for extragalactic observations, where resolution and
sensitivity effects often bias measured values by 40% or more. We correct for
finite spatial and spectral resolutions with a simple deconvolution and we
correct for sensitivity biases by extrapolating properties of a GMC to those we
would expect to measure with perfect sensitivity. The resulting method recovers
the properties of a GMC to within 10% over a large range of resolutions and
sensitivities, provided the clouds are marginally resolved with a peak
signal-to-noise ratio greater than 10. We note that interferometers
systematically underestimate cloud properties, particularly the flux from a
cloud. The degree of bias depends on the sensitivity of the observations and
the (u,v) coverage of the observations. In the Appendix to the paper we present
a conservative, new decomposition algorithm for identifying GMCs in
molecular-line observations. This algorithm treats the data in physical rather
than observational units, does not produce spurious clouds in the presence of
noise, and is sensitive to a range of morphologies. As a result, the output of
this decomposition should be directly comparable among disparate data sets.Comment: Accepted to PASP (19 pgs., 12 figures). The submission describes an
IDL software package available from
http://cfa-www.harvard.edu/~erosolow/cprops
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