22 research outputs found
Towards a long-term record of solar total and spectral irradiance
The variation of total solar irradiance (TSI) has been measured since 1978
and that of the spectral irradiance for an even shorter amount of time.
Semi-empirical models are now available that reproduce over 80% of the measured
irradiance variations. An extension of these models into the more distant past
is needed in order to serve as input to climate simulations. Here we review our
most recent efforts to model solar total and spectral irradiance on time scales
from days to centuries and even longer. Solar spectral irradiance has been
reconstructed since 1947. Reconstruction of solar total irradiance goes back to
1610 and suggests a value of about 1-1.5 Wm for the increase in the
cycle-averaged TSI since the end of the Maunder minimum, which is significantly
lower than previously assumed but agrees with other modern models. First steps
have also been made towards reconstructions of solar total and spectral
irradiance on time scales of millennia
Detection of Solar Rotational Variability in the LYRA 190 - 222 nm Spectral Band
We analyze the variability of the spectral solar irradiance during the period
from 7 January, 2010 until 20 January, 2010 as measured by the Herzberg channel
(190-222 nm) of the Large Yield RAdiometer (LYRA) onboard PROBA2. In this
period of time observations by the LYRA nominal unit experienced degradation
and the signal produced by the Herzberg channel frequently jumped from one
level to another. Both these factors significantly complicates the analysis. We
present the algorithm which allowed us to extract the solar variability from
the LYRA data and compare the results with SORCE/SOLSTICE measurements and with
modeling based on the Code for the Solar Irradiance (COSI)
Eclipses observed by LYRA - a sensitive tool to test the models for the solar irradiance
We analyze the light curves of the recent solar eclipses measured by the
Herzberg channel (200-220 nm) of the Large Yield RAdiometer (LYRA) onboard
PROBA-2. The measurements allow us to accurately retrieve the center- to-limb
variations (CLV) of the solar brightness. The formation height of the radiation
depends on the observing angle so the examination of the CLV provide
information about a broad range of heights in the solar atmosphere. We employ
the 1D NLTE radiative transfer COde for Solar Irradiance (COSI) to model the
measured light curves and corresponding CLV dependencies. The modeling is used
to test and constrain the existing 1D models of the solar atmosphere, e.g. the
temperature structure of the photosphere and the treatment of the pseudo-
continuum opacities in the Herzberg continuum range. We show that COSI can
accurately reproduce not only the irradiance from the entire solar disk, but
also the measured CLV. It hence can be used as a reliable tool for modeling the
variability of the spectral solar irradiance.Comment: 19 pages, 9 figures, Solar Physic
Doppler imaging of late-type stars
SIGLEAvailable from British Library Document Supply Centre- DSC:DX185432 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Prominences on Rapidly-Rotating Solar-Type Stars
Original paper can be found at: http://www.astrosociety.org/pubs/cs/113-155.html--Copyright Astronomical Society of the PacificWe present Ha observations of two rapidly-rotating G2 dwarfs in the Alpha Persei cluster and of AB Dor, a young K0 dwarf
An IPHAS-based search for accreting very low-mass objects using VO tools
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO) DOI: 10.1051/0004-6361/200810256Context. The main goal of this paper is to prove that accreting very low-mass stars and brown dwarfs can be identified in IPHAS, an H emission survey of the northern Milky Way. Full exploitation of the IPHAS database and a future extension of it in the southern hemisphere will be useful in identifying very low-mass accreting objects near to and far from well-known star-forming regions. Aims. We have used Virtual Observatory tools to cross-match the IPHAS catalogue with the 2MASS catalogue. We defined photometric criteria to identify H emission sources with near-infrared colours similar to those of known young very low-mass stars and brown dwarfs. 4000 candidates were identified that met our criteria over an area of 1600 square degrees. We present low-resolution optical spectra of 113 candidates. Spectral types have been derived for the 33 candidates that have spectroscopically confirmed H emission, negligible reddening and spectral class M. We have also measured H emission and investigated the NaI doublet (818.3 nm, 819.5 nm) in these 33 objects. Methods. We confirm that 33 IPHAS candidates have strong H emission indicative of disc accretion for their spectral type. Twenty-three of them have spectral class M4 or later, of which ten have classes in the range M5.5–M7.0 and could thus be very young brown dwarfs. Many objects also have a weak NaI doublet, an indication of low surface gravity. Results. IPHAS provides a very valuable database for identifying accreting very low-mass stars and brown dwarfs. Virtual Observatory tools provide an efficient method for identifying these objects over large areas of the sky. Based on our success rate of 23 H emission objects with spectral type in the range M4–M7 out of 113 candidates with spectroscopic follow-up, we estimate that there could be hundreds of such objects in the full IPHAS survey.Peer reviewe
Intensity contrast from MHD simulations and HINODE observations
Context. Changes in the solar surface area, which is covered by small-scale magnetic elements, are thought to cause long-term changes in the solar spectral irradiance, which are important for determining the impact on Earth’s climate. Aims. To study the effect of small-scale magnetic elements on the total and spectral irradiance, we derive their contrasts from 3-D MHD simulations of the solar atmosphere. These calculations are necessary because measurements of small-scale flux tube contrasts are confined to a few wavelengths and affected by scattered light and instrument defocus, even for space observations. Methods. To test the contrast calculations, we compare rms contrasts from simulations with those obtained with the broad-band filter imager mounted on the Solar Optical Telescope (SOT) onboard the Hinode satellite and also analyse centre-to-limb variations (CLV). The 3-D MHD simulations include the interaction between convection and magnetic flux tubes. They are performed by assuming non-grey radiative transfer and using the MURaM code. The simulations have an average vertical magnetic field of 0 G, 50 G, and 200 G. Emergent intensities are calculated with the spectral synthesis code ATLAS9 and are convolved with a theoretical point-spread function to account for the properties of the observations’ optical system. Results. We find reasonable agreement between simulated and observed intensity distributions in the visible continuum bands. Agreement is poorer for the CN and G-bands. The analysis of the simulations uncovers a potentially more realistic centre-to-limb behaviour than calculations based on 1-D model atmospheres. Conclusions. We conclude that starting from 3-D MHD simulations represents a powerful approach to obtaining intensity contrasts for a wide wavelength coverage and different positions across on the solar disk. This also paves the way for future calculations of facular and network contrast as a function of magnetic fluxes
Intensity contrast from MHD simulations and HINODE observations
Context. Changes in the solar surface area, which is covered by small-scale magnetic elements, are thought to cause long-term changes in the solar spectral irradiance, which are important for determining the impact on Earth’s climate. Aims. To study the effect of small-scale magnetic elements on the total and spectral irradiance, we derive their contrasts from 3-D MHD simulations of the solar atmosphere. These calculations are necessary because measurements of small-scale flux tube contrasts are confined to a few wavelengths and affected by scattered light and instrument defocus, even for space observations. Methods. To test the contrast calculations, we compare rms contrasts from simulations with those obtained with the broad-band filter imager mounted on the Solar Optical Telescope (SOT) onboard the Hinode satellite and also analyse centre-to-limb variations (CLV). The 3-D MHD simulations include the interaction between convection and magnetic flux tubes. They are performed by assuming non-grey radiative transfer and using the MURaM code. The simulations have an average vertical magnetic field of 0 G, 50 G, and 200 G. Emergent intensities are calculated with the spectral synthesis code ATLAS9 and are convolved with a theoretical point-spread function to account for the properties of the observations’ optical system. Results. We find reasonable agreement between simulated and observed intensity distributions in the visible continuum bands. Agreement is poorer for the CN and G-bands. The analysis of the simulations uncovers a potentially more realistic centre-to-limb behaviour than calculations based on 1-D model atmospheres. Conclusions. We conclude that starting from 3-D MHD simulations represents a powerful approach to obtaining intensity contrasts for a wide wavelength coverage and different positions across on the solar disk. This also paves the way for future calculations of facular and network contrast as a function of magnetic fluxes