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Author name code: steffens
ADS astronomy entries on 2022-09-14
author:"Steffens, Sebastian"
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Title: Regional Geologic Mapping of the Oxia Planum Landing Site
for the Exomars 2022 Mission
Authors: Hauber, E.; Tirsch, D.; Adeli, S.; Acktories, S.; Steffens,
S.; Nass, A.; Exomars Rsowg
2021LPI....52.2057H Altcode:
A rover will land / In a small landing ellipse / But more is around.
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Title: Regional Geologic Mapping of the Oxia Planum Landing Site
for ExoMars
Authors: Adeli, S.; Nass, A.; Tirsch, D.; Acktories, S.; Steffens,
S.; Hauber, E.
2020LPICo2357.7035A Altcode:
Here we present a preliminary regional map of the ExoMars landing site
in Oxia Planum, at CTX-scale. This map will provide a contextualization
of the geological units within the stratigraphy of western Arabia
Terra and Chryse Planitia.
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Title: Regional Geologic Mapping of the Oxia Planum Landing Site
for the Exomars Mission
Authors: Hauber, E.; Acktories, S.; Steffens, S.; Naß, A.; Tirsch,
D.; Adeli, S.; Schmitz, N.; Trauthan, F.; Stephan, K.; Jaumann, R.;
Exomars Rsowg
2020LPI....51.2001H Altcode:
Regional geologic mapping of the ExoMars landing site in Oxia Planum
provides stratigraphic and morphologic context and constraints on the
geologic evolution.
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Title: Linear adiabatic dynamics of a polytropic convection zone
with an isothermal atmosphere. II. Quasi-stationary solutions
Authors: Schmitz, F.; Steffens, S.
2000A&A...356..319S Altcode:
For a plane model of the exterior parts of the sun, the behavior
of adiabatic waves with complex frequencies is investigated. The
equilibrium configuration is a one-layer model with isentropic
stratification at great depth and an asymptotically isothermal
atmosphere. The wave equation reduces to Whittaker's equation with
complex parameters. By the assumption that only outgoing progressive
waves are present in the atmosphere, we obtain a discrete spectrum
of complex frequencies. The dispersion relation F(omega ,k) = 0
is a third-order algebraic equation in omega <SUP>2</SUP> with real
coefficients. There are no connections of the ridges of the eigenmodes
with the ridges of the quasi-stationary waves. Instead, there are
striking gaps, and the ridges of quasi-stationary waves extend into
the region below the acoustic cut-off frequency. The findings indicate
that the ridges of the quasi-stationary solutions cannot explain the
ridges of the observed pseudo-modes. As the solutions are not quadratic
integrable and form no basis, they do not represent eigenmodes. The
behavior of the quasi-stationary solutions is related to the behavior
of quasi-stationary states of certain quantum mechanical systems. To
answer the question whether quasi-stationary waves are limiting cases
of instationary waves, we consider a simple one-dimensional two-layer
model. For this case, instationary solutions are compared with the
corresponding quasi-stationary solutions.
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Title: The influence of the solar atmospheric stratification on the
form of p-mode ridges
Authors: Steffens, S.; Schmitz, F.
2000A&A...354..280S Altcode:
We investigate properties of non-radial solar p-modes of high angular
degree. We consider linear adiabatic oscillations with the transition
layer as an ideal reflector. Ionization of hydrogen and helium and
dissociation of hydrogen are included in the equation of state and
consequently in the adiabatic sound speed. Because of the restriction
to high-degree modes we use the plane layer approximation with constant
gravity. Our standard atmospheric model is the VAL-C atmosphere. This
atmosphere is joined to the upper part of a convection zone. A model
corona is matched to the transition region. Boundary conditions are
applied at the temperature maximum of the corona and at a depth in the
convection zone far below the lower turning point of the non-radial
p-modes determined by the Lamb-frequency. We vary the temperature
stratification of the atmosphere and shift the position of the
transition region to obtain a family of eight different equilibrium
models. By this strategy we can study the formation of structures
in the diagnostic diagram and we can take into account uncertainties
of the VAL-chromosphere. It is shown how the classical p-modes of a
convection zone with zero pressure boundary condition are deformed when
the thickness of the overlying atmosphere is enlarged. In no case, the
atmosphere generates additional modes. By strong bending, horizontally
passing parts of the ridges are formed. These parts produce more or
less pronounced chromospheric ridges or features. These chromospheric
ridges appear at frequencies where observations show enhanced power
in the diagnostic diagram. Their locations sensitively depend on the
atmospheric model. A simple two layer model shows that the occurence
of bending of the ridges in the diagnostic diagram is quite natural
and independent of atmospheric details.
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Title: Observers' Views of the Bustling Atmosphere of the Sun
Authors: Deubner, F. -L.; Steffens, S.
1999ESASP.448..149D Altcode: 1999ESPM....9..149D; 1999mfsp.conf..149D
No abstract at ADS
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Title: Linear adiabatic dynamics of a polytropic convection zone
with an isothermal atmosphere. I. General features and real modes
Authors: Schmitz, F.; Steffens, S.
1999A&A...344..973S Altcode:
To investigate and understand basic properties of non-radial solar
p-modes with high wave numbers l, it is sufficient to consider only
the outer layers of the sun. As an atmosphere, the upper part of the
convection zone may be approximated by a plane layer with constant
gravity. A simple standard model is a polytropic convection zone
with an overlying isothermal atmosphere. In this case, the adiabatic
wave equation of each layer can be solved analytically. However, the
dispersion relation F(omega ,k) = 0 of the acoustic and gravity modes of
the whole layer is complicated and cannot be solved in closed form. In
this paper, we present a model with a smooth transition between the
poytropic convection zone and the isothermal atmosphere. For this
model, using the column mass instead of the geometrical height, the
adiabatic wave equation can be reduced to Whittakers differential
equation. The geometrical height is a simple elementary function of
the column mass. The dispersion relation F(omega ,k) = 0 is a fourth
order algebraic equation in omega (2) . In the important case of an
isentropically stratified polytropic convection zone, it reduces to
a cubic equation in omega (2) . In any case, the dispersion curves
omega (k) can be given in closed form. As in the case of a purely
polytropic convection zone, the z-dependence of the waves and the
modes is represented by Whittaker functions. We analyze the behavior
of the dispersion curves of modes with an adiabatic exponent gamma =
5/3 for layers with polytropic indices n = 3 and n = 3/2. Further,
we investigate the appearance of resonances in the region of the
continuous spectrum of acoustic waves. We find that these resonances
are present only at frequencies slightly above the acoustic cutoff
frequency of the isothermal atmosphere. The case of purely vertical
wave propagation is considered separately. In the present paper,
we deal only with real frequencies.
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Title: υ - υ Phase Relations in the Vicinity of the Transition
Region Derived from SUMER Observations
Authors: Steffens, S.; Deubner, F.
1999ASPC..183..426S Altcode: 1999hrsp.conf..426S
No abstract at ADS
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Title: Probing 5-minute oscillations in the solar wind with comet
Hale-Bopp (C/1995 O1)
Authors: Steffens, Sebastian; Nuernberger, Dieter
1998A&A...336..769S Altcode:
This study aims at short-period fluctuations of the solar wind by
using comet Hale-Bopp (C/1995 O1) as a probe. The comet's intensity
fluctuations due to changes in the reflectivity and emission of
molecular lines are investigated both in the coma and the tails
at about 1 AU heliocentric distance. The existence of oscillations
with periods of a few minutes as an already suggested by Isserstedt
& Schlosser (1975) on the basis of observations of comet 1973f
(Kohoutek). We focus here on the time scale of the solar p-modes (5
min.); Thomson et al. (1995) have recently claimed to observe these
periods in the flux density of the solar wind. Both in the comet's
coma and in the ion tail we found oscillations on the time scale of
minutes if mainly the stimulated emission of the C_3 line was observed
through a `narrow-band' filter. No comparable oscillations were seen
through a `broad band' filter where reflected sunlight dominates the
intensity. We show that the observed fluctuations are neither induced
by the earth's atmosphere nor due to genuine comet activity.
