25,250 research outputs found
Zeeman-Tomography of the Solar Photosphere -- 3-Dimensional Surface Structures Retrieved from Hinode Observations
AIMS :The thermodynamic and magnetic field structure of the solar photosphere
is analyzed by means of a novel 3-dimensional spectropolarimetric inversion and
reconstruction technique. METHODS : On the basis of high-resolution,
mixed-polarity magnetoconvection simulations, we used an artificial neural
network (ANN) model to approximate the nonlinear inverse mapping between
synthesized Stokes spectra and the underlying stratification of atmospheric
parameters like temperature, line-of-sight (LOS) velocity and LOS magnetic
field. This approach not only allows us to incorporate more reliable physics
into the inversion process, it also enables the inversion on an absolute
geometrical height scale, which allows the subsequent combination of individual
line-of-sight stratifications to obtain a complete 3-dimensional reconstruction
(tomography) of the observed area. RESULTS : The magnetoconvection simulation
data, as well as the ANN inversion, have been properly processed to be
applicable to spectropolarimetric observations from the Hinode satellite. For
the first time, we show 3-dimensional tomographic reconstructions (temperature,
LOS velocity, and LOS magnetic field) of a quiet sun region observed by Hinode.
The reconstructed area covers a field of approximately 12000 by 12000 km and a
height range of 510 km in the photosphere. An enormous variety of small and
large scale structures can be identified in the 3-D reconstructions. The
low-flux region (B_{mag} = 20G) we analyzed exhibits a number of "tube-like"
magnetic structures with field strengths of several hundred Gauss. Most of
these structures rapidly loose their strength with height and only a few larger
structures can retain a higher field strength to the upper layers of the
photosphere.Comment: accepted for A&A Letter
Detecting and quantifying stellar magnetic fields -- Sparse Stokes profile approximation using orthogonal matching pursuit
In the recent years, we have seen a rapidly growing number of stellar
magnetic field detections for various types of stars. Many of these magnetic
fields are estimated from spectropolarimetric observations (Stokes V) by using
the so-called center-of-gravity (COG) method. Unfortunately, the accuracy of
this method rapidly deteriorates with increasing noise and thus calls for a
more robust procedure that combines signal detection and field estimation. We
introduce an estimation method that provides not only the effective or mean
longitudinal magnetic field from an observed Stokes V profile but also uses the
net absolute polarization of the profile to obtain an estimate of the apparent
(i.e., velocity resolved) absolute longitudinal magnetic field. By combining
the COG method with an orthogonal-matching-pursuit (OMP) approach, we were able
to decompose observed Stokes profiles with an overcomplete dictionary of
wavelet-basis functions to reliably reconstruct the observed Stokes profiles in
the presence of noise. The elementary wave functions of the sparse
reconstruction process were utilized to estimate the effective longitudinal
magnetic field and the apparent absolute longitudinal magnetic field. A
multiresolution analysis complements the OMP algorithm to provide a robust
detection and estimation method. An extensive Monte-Carlo simulation confirms
the reliability and accuracy of the magnetic OMP approach.Comment: A&A, in press, 15 pages, 14 figure
New continua for absorption spectroscopy from 40 to 2000 Å
The spectra of plasmas produced by focusing the output of a Q-switched ruby laser (output 1 J) on the rare-earth metals have been studied. From samarium (Z = 70), strong quasi-uniform continua are emitted in the wavelength range 40–2000 Å. Line emission from the target elements is absent over most of this wavelength region, particularly below about 600 Å. The use of these continua as simple, reliable background sources for absorption spectroscopy in the vacuum-ultraviolet and soft x-ray region down to 40 Å is demonstrated
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