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The discovery of chemical elements in celestial bodies and the
first estimates of the chemical composition of the solar atmosphere
were early results of Astrophysics - the subdiscipline of Astronomy
that was originally concerned with the general laws of radiation
and with spectroscopy. Following the initial quantitative abundance
studies by Henry Norris Russell and by Cecilia Payne-Gaposchkin, a
tremendous amount of theoretical, observa tional, laboratory and
computational work led to a steadily improving body of knowledge of
photospheric abundances - a body of knowledge that served to guide
the theory of stellar evolution. Solar abundances determined from
photospheric spectra, together with the very similar abundances
determined from carbonaceous chondrites (where extensive
information on isotopic composition is available as well), are
nowadays the reference for all cosmic composition measures. Early
astrophysical studies of the solar photospheric composition made
use of atmosphere models and atomic data. Consistent abundances
derived from different atmospheric layers and from lines of
different strength helped to confirm and estab lish both models and
atomic data, and eventually led to the now accepted, so-called
"absolute" abundance values - which, for practical reasons,
however, are usually given relative to the number of hydrogen
nuclei.
Starting in 1995 numerical modeling of the Earth's dynamo has
ourished with remarkable success. Direct numerical simulation of
convection-driven MHD- ow in a rotating spherical shell show
magnetic elds that resemble the geomagnetic eld in many respects:
they are dominated by the axial dipole of approximately the right
strength, they show spatial power spectra similar to that of Earth,
and the magnetic eld morphology and the temporal var- tion of the
eld resembles that of the geomagnetic eld (Christensen and Wicht
2007). Some models show stochastic dipole reversals whose details
agree with what has been inferred from paleomagnetic data
(Glatzmaier and Roberts 1995; Kutzner and Christensen 2002; Wicht
2005). While these models represent direct numerical simulations of
the fundamental MHD equations without parameterized induction
effects, they do not match actual pla- tary conditions in a number
of respects. Speci cally, they rotate too slowly, are much less
turbulent, and use a viscosity and thermal diffusivity that is far
too large in comparison to magnetic diffusivity. Because of these
discrepancies, the success of geodynamo models may seem surprising.
In order to better understand the extent to which the models are
applicable to planetary dynamos, scaling laws that relate basic
properties of the dynamo to the fundamental control parameters play
an important role. In recent years rst attempts have been made to
derive such scaling laws from a set of numerical simulations that
span the accessible parameter space (Christensen and Tilgner 2004;
Christensen and Aubert 2006).
Starting in 1995 numerical modeling of the Earth's dynamo has
ourished with remarkable success. Direct numerical simulation of
convection-driven MHD- ow in a rotating spherical shell show
magnetic elds that resemble the geomagnetic eld in many respects:
they are dominated by the axial dipole of approximately the right
strength, they show spatial power spectra similar to that of Earth,
and the magnetic eld morphology and the temporal var- tion of the
eld resembles that of the geomagnetic eld (Christensen and Wicht
2007). Some models show stochastic dipole reversals whose details
agree with what has been inferred from paleomagnetic data
(Glatzmaier and Roberts 1995; Kutzner and Christensen 2002; Wicht
2005). While these models represent direct numerical simulations of
the fundamental MHD equations without parameterized induction
effects, they do not match actual pla- tary conditions in a number
of respects. Speci cally, they rotate too slowly, are much less
turbulent, and use a viscosity and thermal diffusivity that is far
too large in comparison to magnetic diffusivity. Because of these
discrepancies, the success of geodynamo models may seem surprising.
In order to better understand the extent to which the models are
applicable to planetary dynamos, scaling laws that relate basic
properties of the dynamo to the fundamental control parameters play
an important role. In recent years rst attempts have been made to
derive such scaling laws from a set of numerical simulations that
span the accessible parameter space (Christensen and Tilgner 2004;
Christensen and Aubert 2006).
The discovery of chemical elements in celestial bodies and the
first estimates of the chemical composition of the solar atmosphere
were early results of Astrophysics - the subdiscipline of Astronomy
that was originally concerned with the general laws of radiation
and with spectroscopy. Following the initial quantitative abundance
studies by Henry Norris Russell and by Cecilia Payne-Gaposchkin, a
tremendous amount of theoretical, observa tional, laboratory and
computational work led to a steadily improving body of knowledge of
photospheric abundances - a body of knowledge that served to guide
the theory of stellar evolution. Solar abundances determined from
photospheric spectra, together with the very similar abundances
determined from carbonaceous chondrites (where extensive
information on isotopic composition is available as well), are
nowadays the reference for all cosmic composition measures. Early
astrophysical studies of the solar photospheric composition made
use of atmosphere models and atomic data. Consistent abundances
derived from different atmospheric layers and from lines of
different strength helped to confirm and estab lish both models and
atomic data, and eventually led to the now accepted, so-called
"absolute" abundance values - which, for practical reasons,
however, are usually given relative to the number of hydrogen
nuclei.
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