869 research outputs found
Stokes diagnostics of simulated solar magneto-convection
No full textWe present results of synthetic spectro-polarimetric diagnostics of radiative MHD simulations of solar surface convection with magnetic fields. Stokes profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared spectral ranges emerging from the simulated atmosphere have been calculated in order to study their relation to the relevant physical quantities and compare with observational results. We have analyzed the dependence of the Stokes-I line strength and width as well as of the Stokes-V signal and asymmetries on the magnetic field strength. Furthermore, we have evaluated the correspondence between the actual velocities in the simulation with values determined from the Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles (zero-crossing shift). We confirm that the line weakening in strong magnetic fields results from a higher temperature (at equal optical depth) in the magnetic flux concentrations. We also confirm that considerable Stokes-V asymmetries originate in the peripheral parts of strong magnetic flux concentrations, where the line of sight cuts through the magnetopause of the expanding flux concentration into the surrounding convective donwflow
Opacity distribution functions for stellar spectra synthesis
No full textContext. Stellar spectra synthesis is essential for the characterization of potential planetary hosts. In addition, comprehensive stellar variability calculations with fast radiative transfer are needed to disentangle planetary transits from stellar magnetically driven variability. The planet-hunting space telescopes, such as CoRoT, Kepler, and TESS, bring vast quantities of data, rekindling the interest in fast calculations of the radiative transfer. Aims. We revisit the opacity distribution functions (ODF) approach routinely applied to speed up stellar spectral synthesis. To achieve a considerable speedup relative to the state of the art, we further optimize the approach and search for the best ODF configuration. Furthermore, we generalize the ODF approach for fast calculations of flux in various filters often used in stellar observations. Methods. In a parameter-sweep fashion, we generated ODF in the spectral range from UV to IR with different setups. The most accurate ODF configuration for each spectral interval was determined. We adapted the wavelength grid based on the transmission curve for calculations of the radiative fluxes through filters before performing the normal ODF procedure. Results. Our optimum ODF configuration allows for a three-fold speedup, compared to the previously used ODF configurations. The ODF generalization to calculate fluxes through filters results in a speedup of more than two orders of magnitude
Stokes diagnostics of simulations of magnetoconvection of mixed-polarity quiet-Sun regions
No full textRealistic solar magneto-convection simulations including the photospheric layers are used to study the polarization of the Fe I Zeeman-sensitive spectral lines at 6301.5, 6302.5, 15 648 and 15 652 Å. The Stokes spectra are synthesized in a series of snapshots with a mixed-polarity magnetic field whose average unsigned strength varies from = 10 to 140 G. The effects of spatial resolution and of the amount of magnetic flux in the simulation box on the profiles shapes, amplitudes and shifts are discussed. The synthetic spectra show many properties in common with those observed in quiet solar regions. In particular, the simulations reproduce the width and depth of spatially averaged Stokes I profiles, the basic classes of the Stokes V profiles and their amplitude and area asymmetries, as well as the abundance of the irregular-shaped Stokes V profiles. It is demonstrated that the amplitudes of the 1.56μm lines observed in the inter-network are consistent with a "true" average unsigned magnetic field strength of 20 G. We show that observations using these and visible lines, carried out under different seeing conditions (e.g., simultaneous observations at different telescopes), may result in different asymmetries and even opposite polarities of the profiles in the two spectral regions observed at the same spatial point
Power spectra of solar brightness variations at different inclinations
No full textMagnetic features on the surfaces of cool stars cause variations of their
brightness. Such variations have been extensively studied for the Sun. Recent
planet-hunting space telescopes allowed measuring brightness variations in
hundred thousands of other stars. The new data posed the question of how
typical is the Sun as a variable star. Putting solar variability into the
stellar context suffers, however, from the bias of solar observations being
made from its near-equatorial plane, whereas stars are observed at all possible
inclinations. We model solar brightness variations at timescales from days to
years as they would be observed at different inclinations. In particular, we
consider the effect of the inclination on the power spectrum of solar
brightness variations. The variations are calculated in several passbands
routinely used for stellar measurements. We employ the Surface Flux Transport
Model (SFTM) to simulate the time-dependent spatial distribution of magnetic
features on both near- and far-sides of the Sun. This distribution is then used
to calculate solar brightness variations following the SATIRE (Spectral And
Total Irradiance REconstruction) approach. We have quantified the effect of the
inclination on solar brightness variability at timescales down to a day. Thus,
our results allow making solar brightness records directly comparable to those
obtained by the planet-hunting space telescopes. Furthermore, we decompose
solar brightness variations into the components originating from the solar
rotation and from the evolution of magnetic features
Observation of a bright coronal downflow by SOHO/EIT
No full textA distinct coronal downflow has been discovered in the course of a prominence
eruption associated coronal mass ejection (CME) imaged by EIT (Extreme ultraviolet
Imaging Telescope) and LASCO (Large Angle Spectrometric Coronagraph) on board SOHO (Solar
and Heliospheric Observatory) on 5-Mar.-2000. Evolution of the prominences seen by EIT
was tracked into the LASCO/C2 and C3 field-of-view where they developed as the core of a
typical three-part CME. In contrast to the inflow structures reported earlier in the
literatures, which were dark and were interpreted as plasma voids moving down, the
downflow reported here was bright. The downflow, which was only seen in EIT FOV had an onset
time that coincided with the deceleration phase of the core of the CME. The downflow
showed a rapid acceleration followed by a strong deceleration. The downflow followed a
curved path which may be explained by material following the apex of a contracting
magnetic loop sliding down along other field lines, although other explanations are
also possible. Irrespective of the detailed geometry, this observation provides support
for the pinching off of the field lines drawn-out by the erupting prominences and
the contraction of the arcade formed by the reconnection.
Stellar limb darkening. A new MPS-ATLAS library for Kepler, TESS, CHEOPS, and PLATO passbands
No full textThe detection of the first exoplanet paved the way into the era of transit
photometry space missions with a revolutionary photometric precision that aim
at discovering new exoplanetary systems around different types of stars. With
this high precision, it is possible to derive very accurately the radii of
exoplanets which is crucial for constraining their type and composition.
However, it requires an accurate description of host stars, especially their
center-to-limb variation of intensities (so called limb darkening) as it
affects the planet-to-star radius ratio determination. We aim at improving the
accuracy of limb darkening calculations for stars with a wide range of
fundamental parameters. We used the recently developed 1D MPS-ATLAS code to
compute model atmosphere structures and to synthesize stellar limb darkening on
a very fine grid of stellar parameters. For the computations we utilized the
most accurate information on chemical element abundances and mixing length
parameters including convective overshoot. The stellar limb darkening was
fitted using the two most accurate limb darkening laws: the power-2 and
4-parameters non-linear laws. We present a new extensive library of stellar
model atmospheric structures, the synthesized stellar limb darkening curves,
and the coefficients of parameterized limb-darkening laws on a very fine grid
of stellar parameters in the Kepler, TESS, CHEOPS, and PLATO passbands. The
fine grid allows overcoming the sizable errors introduced by the need to
interpolate. Our computations of solar limb darkening are in a good agreement
with available solar measurements at different view angles and wavelengths. Our
computations of stellar limb darkening agree well with available measurements
of Kepler stars. A new grid of stellar model structures, limb darkening and
their fitted coefficients in different broad filters is provided in CDS.Comment: 19 pages, 17 figures, accepted to Astronomy and Astrophysics. All
tables are available in CD
G-band spectral synthesis and diagnostics of simulated solar magneto-convection
No full textRealistic simulations of radiative magneto-convection in the solar (sub)photosphere are used for a spectral synthesis of Fraunhofer's G band, which is dominated by spectral lines from the CH molecule. It is found that the spatial pattern of integrated G-band brightness closely matches the spatial structure of magnetic flux concentrations in the convective downflow regions. The brightness contrast is mainly caused by the weakening of CH lines due to the reduced CH abundance and the resulting shift of the optical depth scale in the hot and tenuous magnetic flux concentrations. Various properties of the synthetic brightness images agree well with G-band observations. These results lends credit to the observational usage of G-band bright features as proxies for magnetic flux concentrations in the solar photosphere. However, the converse is only correct in a limited sense: only a fraction of the magnetic flux concentrations turn out to be bright in the G band
Synthesis and Characterization of some new perovskite type mixed oxide materials
No full textNot availabl
Synthesis and Physico Chemical Properties of some Bio Active Heterocyclic Entities
No full textNot availabl
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