Physical Understanding of Solar Irradiance in Ultraviolet and Radio Wavelengths
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Date
2017
Publication Type
Doctoral Thesis
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Abstract
Understanding of solar and stellar brightness variability plays a crucial role in
the studies of solar-stellar and solar-terrestrial connection. In particular, the
modeling of solar brightness variations is important for understanding the role
of the Sun in the climate variability, while studying the stellar variability allows
to better constrain the physics of the stellar activity.
Since the launch of the NIMBUS-7 mission in 1978 the solar brightness has been
continuously monitored and has been found to vary on all time scales on which
it has ever been measured. Although the number of solar brightness datasets
has been increasing over the last years, the observations alone do not provide a
sufficient means to understand the influence of solar radiation on climate due to
large uncertainties and gaps in the available datasets. This calls for the devel-
opment of solar brightness modeling in order to complement the observational
data.
The physics-based modeling of solar and stellar brightness variations relies
on the spectra of magnetic features and surrounding quiet regions in the solar
and stellar atmospheres. These spectra are provided by radiative transfer codes.
Currently the radiative transfer codes oriented towards such modeling represent
the stellar atmospheres as a one-dimensional structure.
Development of 1D radiative transfer codes is a challenging task due to the
Non-Local Thermodynamic Equillibrium (NLTE) coupling of matter and radia-
tion. As a first approximation one can assume full coupling, i.e. Local Thermody-
namic Equillibrium (LTE), but by now a substantial evidence has been acquired
pointing to the inadequacy of this approximation. Hence, today one of the focal
points of solar and stellar physics is the development of NLTE radiative transfer
codes which is the main goal of this thesis.
In Chapter 2 we present the NLTE Spectral SYnthesis (NESSY) code. The
code was originally designed for modeling the spectra of hot stars with expand-
ing atmospheres. This purpose predisposed the numerical scheme of the code to
work in a way that is not efficient for spectrum synthesis under solar-like con-
ditions in which the expansion is much less pronounced. The aim of our work
was to adjust the code so that it can handle both expanding and non-expanding
cases. Such an adjustment was a significant step on the way to make NESSY efficiently applicable to the synthesis of spectra emerging from all kinds of stars.
It required a complete change of the algorithms of the code responsible for the
NLTE calculations. The new version of the code is very well suited for spec-
tral synthesis over broad spectral ranges which is required for modeling of solar
and stellar brightness variations. In the following chapters we demonstrate the
capabilities of the code in the exemplary case of the Sun.
In Chapter 3 we apply the code to modeling the Center-to-Limb Variation(s) of
solar brightness (CLV) which are a powerful diagnostic tool for constraining the
models of solar atmosphere. They are also important for modeling of the solar
brightness variations especially on the time scale of solar rotation. We compare
the CLVs modeled with NESSY to those derived from the measurements of solar
brightness variations during eclipses observed with the PREMOS instrument on-
board the PICARD mission. We use the light curves of the three solar eclipses
measured by the radiometers of PREMOS to derive CLVs in the UV, visible and
IR parts of the solar spectrum. We show that in the visible and IR the modeled
CLVs agree well with those derived from the eclipse observations which proves
that NESSY can not only reproduce the full-disk solar spectrum, but also the
distribution of brightness across the solar disk. In the UV the derived CLVs
allow us to constrain the source of the so-called “missing opacities” and make a
step toward the resolution of this well-known problem arising from the lack of
laboratory measurements of lines constituting the UV solar spectrum within 160
nm - 320 nm range.
Chapter 4 is devoted to the first ever physics-based reconstruction of the solar
brightness variability in the radio. This reconstruction became possible thanks to
the changes of the code described in Chapter 2. The NLTE effects are important
for the formation of radio wavelengths and therefore with NESSY we could for
the first time consistently model the brightness variability of the Sun in the
radio. We can model both the variability from UV (where the LTE assumption
also fails) to IR and in the radio without any empirical corrections. In Chapter 4
we take advantage of this capability and show how radio wavelengths can be used
for reconstruction of of variability of the entire solar spectrum from UV to IR.
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Examiner : Carollo, C. Marcella
Examiner : Schmutz, Werner
Examiner : Unruh, Yvonne C.
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ETH Zurich
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Subject
Radiative transfer, numerical methods, lambda-iteration, approximate lambda-operators, line formation, opacity, solar atmosphere, center-to-limb variation, eclipses, solar variability modelling, solar radio emission
Organisational unit
03612 - Carollo, Marcella (ehemalig) / Carollo, Marcella (former)