Author name code: steffens ADS astronomy entries on 2022-09-14 author:"Steffens, Sebastian" ------------------------------------------------------------------------ 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 Bibcode: 2021LPI....52.2057H Altcode: A rover will land / In a small landing ellipse / But more is around. 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. Bibcode: 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. 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 Bibcode: 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. Title: Linear adiabatic dynamics of a polytropic convection zone with an isothermal atmosphere. II. Quasi-stationary solutions Authors: Schmitz, F.; Steffens, S. Bibcode: 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 2 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. Title: The influence of the solar atmospheric stratification on the form of p-mode ridges Authors: Steffens, S.; Schmitz, F. Bibcode: 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. Title: Observers' Views of the Bustling Atmosphere of the Sun Authors: Deubner, F. -L.; Steffens, S. Bibcode: 1999ESASP.448..149D Altcode: 1999ESPM....9..149D; 1999mfsp.conf..149D No abstract at ADS Title: Linear adiabatic dynamics of a polytropic convection zone with an isothermal atmosphere. I. General features and real modes Authors: Schmitz, F.; Steffens, S. Bibcode: 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. Title: υ - υ Phase Relations in the Vicinity of the Transition Region Derived from SUMER Observations Authors: Steffens, S.; Deubner, F. Bibcode: 1999ASPC..183..426S Altcode: 1999hrsp.conf..426S No abstract at ADS Title: Probing 5-minute oscillations in the solar wind with comet Hale-Bopp (C/1995 O1) Authors: Steffens, Sebastian; Nuernberger, Dieter Bibcode: 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.