14 research outputs found
Solar irradiance models and measurements: a comparison in the 220 nm to 240 nm wavelength band
Solar irradiance models that assume solar irradiance variations to be due to
changes in the solar surface magnetic flux have been successfully used to
reconstruct total solar irradiance on rotational as well as cyclical and
secular time scales. Modelling spectral solar irradiance is not yet as
advanced, and also suffers from a lack of comparison data, in particular on
solar-cycle time scales. Here we compare solar irradiance in the 220 nm to 240
nm band as modelled with SATIRE-S and measured by different instruments on the
UARS and SORCE satellites.
We find good agreement between the model and measurements on rotational time
scales. The long-term trends, however, show significant differences. Both SORCE
instruments, in particular, show a much steeper gradient over the decaying part
of cycle 23 than the modelled irradiance or that measured by UARS/SUSIM.Comment: 8 pages, 2 figures, conference proceedings to appear in Surveys in
Geophysic
A new SATIRE-S spectral solar irradiance reconstruction for solar cycles 21--23 and its implications for stratospheric ozone
We present a revised and extended total and spectral solar irradiance (SSI)
reconstruction, which includes a wavelength-dependent uncertainty estimate,
spanning the last three solar cycles using the SATIRE-S model. The SSI
reconstruction covers wavelengths between 115 and 160,000 nm and all dates
between August 1974 and October 2009. This represents the first full-wavelength
SATIRE-S reconstruction to cover the last three solar cycles without data gaps
and with an uncertainty estimate. SATIRE-S is compared with the NRLSSI model
and SORCE/SOLSTICE ultraviolet (UV) observations. SATIRE-S displays similar
cycle behaviour to NRLSSI for wavelengths below 242 nm and almost twice the
variability between 242 and 310 nm. During the decline of last solar cycle,
between 2003 and 2008, SSI from SORCE/SOLSTICE version 12 and 10 typically
displays more than three times the variability of SATIRE-S between 200 and 300
nm. All three datasets are used to model changes in stratospheric ozone within
a 2D atmospheric model for a decline from high solar activity to solar minimum.
The different flux changes result in different modelled ozone trends. Using
NRLSSI leads to a decline in mesospheric ozone, while SATIRE-S and
SORCE/SOLSTICE result in an increase. Recent publications have highlighted
increases in mesospheric ozone when considering version 10 SORCE/SOLSTICE
irradiances. The recalibrated SORCE/SOLSTICE version 12 irradiances result in a
much smaller mesospheric ozone response than when using version 10 and now
similar in magnitude to SATIRE-S. This shows that current knowledge of
variations in spectral irradiance is not sufficient to warrant robust
conclusions concerning the impact of solar variability on the atmosphere and
climate.Comment: 25 pages (18 pages in main article with 6 figures; 7 pages in
supplementary materials with 6 figures) in draft mode using the American
Meteorological Society package. Submitted to Journal of Atmospheric Sciences
for publicatio
Spectral irradiance variations: Comparison between observations and the SATIRE model on solar rotation time scales
Aims: We test the reliability of the observed and calculated spectral
irradiance variations between 200 and 1600 nm over a time span of three solar
rotations in 2004.
Methods: We compare our model calculations to spectral irradiance
observations taken with SORCE/SIM, SoHO/VIRGO and UARS/SUSIM. The calculations
assume LTE and are based on the SATIRE (Spectral And Total Irradiance
REconstruction) model. We analyse the variability as a function of wavelength
and present time series in a number of selected wavelength regions covering the
UV to the NIR. We also show the facular and spot contributions to the total
calculated variability.
Results: In most wavelength regions, the variability agrees well between all
sets of observations and the model calculations. The model does particularly
well between 400 and 1300 nm, but fails below 220 nm as well as for some of the
strong NUV lines. Our calculations clearly show the shift from
faculae-dominated variability in the NUV to spot-dominated variability above
approximately 400 nm. We also discuss some of the remaining problems, such as
the low sensitivity of SUSIM and SORCE for wavelengths between approximately
310 and 350 nm, where currently the model calculations still provide the best
estimates of solar variability.Comment: 15 pages, 11 figures, accepted by A&
Can 1D radiative equilibrium models of faculae be used for calculating contamination of transmission spectra?
The reliable characterization of planetary atmospheres with transmission
spectroscopy requires realistic modeling of stellar magnetic features, since
features that are attributable to an exoplanet atmosphere could instead stem
from the host star's magnetic activity. Current retrieval algorithms for
analysing transmission spectra rely on intensity contrasts of magnetic features
from 1D radiative-convective models. However, magnetic features, especially
faculae, are not fully captured by such simplified models. Here we investigate
how well such 1D models can reproduce 3D facular contrasts, taking a G2V star
as an example. We employ the well established radiative magnetohydrodynamic
code MURaM to obtain three-dimensional simulations of the magneto-convection
and photosphere harboring a local small-scale-dynamo. Simulations without
additional vertical magnetic fields are taken to describe the quiet solar
regions, while simulations with initially 100 G, 200 G and 300 G vertical
magnetic fields are used to represent different magnetic activity levels.
Subsequently, the spectra emergent from the MURaM cubes are calculated with the
MPS-ATLAS radiative transfer code. We find that the wavelength dependence of
facular contrast from 1D radiative-convective models cannot reproduce facular
contrasts obtained from 3D modeling. This has far reaching consequences for
exoplanet characterization using transmission spectroscopy, where accurate
knowledge of the host star is essential for unbiased inferences of the
planetary atmospheric properties.Comment: 7 pages, 2 figures, submitted to APJ
Recommended from our members
The effect of stellar contamination on low-resolution transmission spectroscopy: needs identified by NASA’s Exoplanet Exploration Program Study Analysis Group 21
Study Analysis Group 21 (SAG21) of NASA’s Exoplanet Exploration Program Analysis Group was organized to study the effect of stellar contamination on space-based transmission spectroscopy, a method for studying exoplanetary atmospheres by measuring the wavelength-dependent radius of a planet as it transits its star. Transmission spectroscopy relies on a precise understanding of the spectrum of the star being occulted. However, stars are not homogeneous, constant light sources but have temporally evolving photospheres and chromospheres with inhomogeneities like spots, faculae, plages, granules, and flares. This SAG brought together an interdisciplinary team of more than 100 scientists, with observers and theorists from the heliophysics, stellar astrophysics, planetary science, and exoplanetary atmosphere research communities, to study the current research needs that can be addressed in this context to make the most of transit studies from current NASA facilities like Hubble Space Telescope and JWST. The analysis produced 14 findings, which fall into three science themes encompassing (i) how the Sun is used as our best laboratory to calibrate our understanding of stellar heterogeneities (‘The Sun as the Stellar Benchmark’), (ii) how stars other than the Sun extend our knowledge of heterogeneities (‘Surface Heterogeneities of Other Stars’), and (iii) how to incorporate information gathered for the Sun and other stars into transit studies (‘Mapping Stellar Knowledge to Transit Studies’). In this invited review, we largely reproduce the final report of SAG21 as a contribution to the peer-reviewed literature