Author name code: grossmann-doerth ADS astronomy entries on 2022-09-14 author:"Grossmann-Doerth, Ulrich" ------------------------------------------------------------------------ Title: Strong Stokes V asymmetries of photospheric spectral lines: What can they tell us about the magnetic field structure? Authors: Grossmann-Doerth, U.; Schüssler, M.; Sigwarth, M.; Steiner, O. Bibcode: 2000A&A...357..351G Altcode: In an attempt to identify the mechanism responsible for the extremely asymmetric Stokes V profiles which were recently observed we analyzed several simple atmospheric configurations with separated layers of mass flow and magnetic field. We found that under appropriate conditions the models are capable of producing the observed one-lobe profiles. Title: Understanding Small Solar Magnetic Elements: Comparing Models and Observations Authors: Leka, K. D.; Steiner, O.; Grossmann-Doerth, U. Bibcode: 1999AAS...194.5507L Altcode: 1999BAAS...31R.911L We perform direct comparisons of high-resolution spectropolarimetric observations with a full MHD model of the magnetized solar atmosphere. In this manner we investigate the evolution and dynamics of small magnetic elements by fully utilizing the diagnostics available with Stokes spectropolarimetry, both computed and observed. The model is a 2-D time-dependent numerical simulation of a small (~ 600 km diameter) magnetic feature embedded in a non-magnetized atmosphere (Steiner et al., 1998). At select time-steps, synthetic emergent Stokes I and V profiles are computed using a polarized radiation transfer code. The data consist of Stokes I and V spectra from the Advanced Stokes Polarimeter for seventeen small magnetic elements located near disk-center. For both the observed and computed Stokes spectra, diagnostics are computed including the emergent continuum intensity, V-crossing shift, and amplitude and area asymmetries of the V-profile. We find that it is possible to differentiate between salient processes occurring in the magnetic atmosphere (strong flows, gradients, etc.) by their spectropolarimetric signature; from this, we determine the dominant processes present in the observed magnetic structures. The results are extremely encouraging. We find good qualitative agreement between the amplitude and area asymmetries and their spatial variation. Quantitatively, the agreement is surprisingly good in many cases. While limitations exist for both the model and observations, this stringent test allows us to comment on the dynamics and possible evolutionary differences present in the observed magnetic features. This work is funded in part by NSF grant ATM-9710782. Reference: - Steiner, O., Grossmann-Doerth, U., Knolker, M., Schussler, M.: 1998, ApJ 495, 468 Title: The formation of extremely asymmetric Stokes V profiles Authors: Steiner, O.; Grossmann-Doerth, U.; Schüssler, M.; Sigwarth, M. Bibcode: 1999AGAb...15R..10S Altcode: 1999AGM....15..A11S Recent polarimetric observations at high spatial resolution (< 1 arcsec) and with high polarimetric accuracy (noise of Stokes V/I_c < 3 cdot 10^{-4}) have revealed that about 10% of all Stokes V profiles of a quiet Sun region have an abnormal shape that strongly deviates from the more common, nearly antisymmetric profiles. 35% of the abnormal Stokes V profiles are of one-wing type, where the profile shows essentially one lobe only. We show, that a strongly asymmetric Stokes V profile can be obtained when the atmosphere, in which the profile is formed, is divided into two or more layers of different magnetic, flow, and thermal properties. A simple configuration of that kind (sometimes referred to as magnetic canopy) consists of a layer of plasma at rest with a magnetic field component parallel to the line of sight, located on top of a field-free layer with downdraft. The Stokes V asymmetry in this case sensitively depends on the position of the layers interface and may assume values up to about 80%. An asymmetry of nearly 100% (corresponding to a one-wing profile) is obtained when the plasma below the magnetic canopy is relatively cool, giving rise to the formation of a strong, redshifted spectral line in the field-free plasma beneath the canopy. If the core of this line is formed at lower temperature than the temperature of the canopy layer, the red wing of the resulting Stokes V profile is driven into emission, leading to a ``pathological'' V profile. Title: Convective intensification of solar surface magnetic fields: results of numerical experiments Authors: Grossmann-Doerth, U.; Schuessler, M.; Steiner, O. Bibcode: 1998A&A...337..928G Altcode: The concentration of magnetic flux by convective flows in the solar surface layers is studied by means of two-dimensional numerical simulations with radiative transfer. We follow the evolution of an initially homogeneous, vertical magnetic field, starting from an evolved state of simulated solar granulation. The results of three simulation runs with initial field strengths, B_0, of 100 G, 200 G, and 400 G, respectively, are shown. In all cases, horizontal convective flows rapidly sweep magnetic flux into the intergranular downflow channels. The field is further amplified up to kilogauss values by partial evacuation due to a strongly accelerated downflow within the magnetic structure. The value of the field strength reached at a given depth and the size of the flux concentrations grows with the initial field strength (i.e., the amount of magnetic flux within the computional box). In the case of B_0 = 400 G, the downflow within the flux concentration becomes so strong that it `bounces' off the high density plasma in the deeper layers; the resulting upflow leads to a strong, upward moving shock and to the dispersal of the flux sheet after a lifetime of about 200 s. In the cases with less magnetic flux (B_0 100 G, 200 G), the downflow is less vigorous and the flux concentrations persist to the end of the simulation (about 5 minutes). Radiation diagnostics in the continuum and in spectral lines predicts observable signatures of the intensification process. The accelerated downflow leads to a conspicuous Doppler shift and a negative area asymmetry of Stokes V-profiles of spectral lines, while the intensification of the magnetic field may be detectable through the `magnetic line ratio' method in the visible and by direct Zeeman splitting of magnetically sensitive lines in the infrared wavelength ranges. Title: Dynamical Interaction of Solar Magnetic Elements and Granular Convection: Results of a Numerical Simulation Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler, M. Bibcode: 1998ApJ...495..468S Altcode: Nonstationary convection in the solar photosphere and its interaction with photospheric magnetic structures (flux sheets in intergranular lanes) have been simulated using a numerical code for two-dimensional MHD with radiative energy transfer. Dynamical phenomena are identified in the simulations, which may contribute to chromospheric and coronal heating. Among these are the bending and horizontal displacement of a flux sheet by convective flows and the excitation and propagation of shock waves both within and outside the magnetic structure. Observational signatures of these phenomena are derived from calculated Stokes profiles of Zeeman-sensitive spectral lines. We suggest that the extended red wings of the observed Stokes V profiles are due to downward coacceleration of magnetized material in a turbulent boundary layer between the flux sheet and the strong external downflow. Upward-propagating shocks in magnetic structures should be detectable if a time resolution of about 10 s is achieved, together with a spatial resolution that allows one to isolate individual magnetic structures. Determination of the complicated internal dynamics of magnetic elements requires observations with a spatial resolution better than 100 km in the solar photosphere. Title: Observations of the quiet Sun's magnetic field. Authors: Grossmann-Doerth, U.; Keller, C. U.; Schuessler, M. Bibcode: 1996A&A...315..610G Altcode: The profiles of Stokes I and V of FeI 5247A, CrI 5247A, FeI 5250.2A and Fe I 5250.6A have been measured in the quiet photosphere with ZIMPOL, a new polarimeter, with very low noise level. The spatial resolution element of about 1Mm was essentially determined by seeing. In about 15% of the observed area, the signal-to-noise ratio was sufficiently large to determine the strength of the magnetic field and to derive various properties of the Stokes V profiles (asymmetries, zero-crossing shift and thermal line ratio). Our results show that, at least in this fraction of area, the magnetic field in the quiet Sun is dominated by strong flux concentrations which must be rather small. The data on the Stokes V properties provide observational constraints for theoretical models of flux concentrations. Title: Polarized Radiation Diagnostics of Magnetohydrodynamic Models of the Solar Atmosphere Authors: Steiner, O.; Grossmann-Doerth, U.; Schüssler, M.; Knölker, M. Bibcode: 1996SoPh..164..223S Altcode: Solar magnetic elements and their dynamical interaction with the convective surface layers of the Sun are numerically simulated. Radiation transfer in the photosphere is taken into account. A simulation run over 18.5 minutes real time shows that the granular flow is capable of moving and bending a magnetic flux sheet (the magnetic element). At times it becomes inclined by up to 30° with respect to the vertical around the level τ5000 = 1 and it moves horizontally with a maximal velocity of 4 km/s. Shock waves form outside and within the magnetic flux sheet. The latter cause a distinctive signature in a time series of synthetic Stokes V-profiles. Such shock events occur with a mean frequency of about 2.5 minutes. A time resolution of at least 10 seconds in Stokes V recordings is needed to reveal an individual shock event by observation. Title: Convective intensification of photospheric magnetic fields. Authors: Schüssler, M.; Grossmann-Doerth, U.; Steiner, O.; Knölker, M. Bibcode: 1996AGAb...12...89S Altcode: No abstract at ADS Title: Simulation of the Interaction of Convective Flow with Magnetic Elements in the Solar Atmosphere. Authors: Steiner, O.; Grossmann-Doerth, U.; Knoelker, M.; Schuessler, M. Bibcode: 1995RvMA....8...81S Altcode: No abstract at ADS Title: The deep layers of solar magnetic elements Authors: Grossmann-Doerth, U.; Knoelker, M.; Schuessler, M.; Solanki, S. K. Bibcode: 1994A&A...285..648G Altcode: We compare self-consistent theoretical models of solar magnetic flux sheets with spectropolarimetric observations of a solar plage and a network region. Our observational diagnostics mainly provide information on temperature and magnetic field of the deep photospheric layers. They are used to constrain the two free parameters of the models, viz. width and initial evacuation of the flux sheets. We find that the width of flux sheets in the network is approximately 200 km, while it is 300-350 km in an active plage. The flux sheets turn out to be less evacuated than previously thought, so they have continuum intensities close to unity. Since these are average values, however, our results do not exclude the presence of either smaller and brighter or larger and darker magnetic structures. Title: Height of formation of solar photospheric spectral lines Authors: Grossmann-Doerth, U. Bibcode: 1994A&A...285.1012G Altcode: The dependence of the line depression contribution functions of spectral lines on the relevant line parameters in the solar photosphere is investigated. It is shown how the formation height range varies as function of excitation and ionization potential, line strength, central wavelength and wavelength offset. Furthermore it is shown that the type of contribution function used provides reliable information on the depth where a given line - or part of the line - is sensitive to atmospheric field quantities like, for example, the bulk velocity. Title: Solar Magnetic Elements: Models Compared with Observations Authors: Grossmann-Doerth, U.; Knolker, M.; Schussler, M.; Solanki, S. K. Bibcode: 1994ASPC...68...96G Altcode: 1994sare.conf...96G No abstract at ADS Title: MHD simulations with adaptive mesh refinement Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler, M. Bibcode: 1994smf..conf..282S Altcode: No abstract at ADS Title: Simulation of magneto-convection with radiative transfer Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler, M. Bibcode: 1994smf..conf..286S Altcode: No abstract at ADS Title: Chromospheric fine structure revisited Authors: Grossmann-Doerth, U.; Schmidt, W. Bibcode: 1992A&A...264..236G Altcode: We obtained H-alpha spectra from both spicules at the limb and mottles at disk center. We compare our results with data obtained by other groups and derive two distributions of axial spicule velocity, one based on limb, the other on disk observations. The two distributions disagree. We conclude that our knowledge on the basic properties of the chromospheric fine structure has not much improved in the last two decades; it is still rather scanty. Title: Stokes Inversion in the Presence of Canopy-Like Structures and Unresolved Flux-Tubes Authors: Skumanich, A.; Grossmann-Doerth, U.; Lites, B. W. Bibcode: 1992AAS...180.1203S Altcode: 1992BAAS...24Q.748S The issue of the Stokes polarization signature of canopy-like structures and the inversion of the resulting Stokes profiles is considered for a self-consistent flux tube (sheet) model. It is found that when the `canopy' and its attendant current sheet have an optical thickness at line center tau_0 <= 5, the intensity and net polarization profiles become decoupled. An effective inversion strategy is to reduce or eliminate the role of the intensity parameter. The Milne-Eddington (ME) analytic inversion in current use is easily adapted for this purpose. It is found that the use of a magnetic fill factor allows for an approximate compensation for the presence of a field discontinuity along the line of sight but a more accurate procedure is to restrict the inversion to the net polarization profiles. In the case of sufficiently thin canopies the ME inversion, when reduced to the Sears form, yields fields within ~ 100 G of the true field. When the Stokes profiles are averaged over the flux sheet to simulate the effects of limited angular resolution, the fill factor strategy yields an accurate measure of both the axial field and the width of the flux sheet at tau_0 =~ 1 for observing angles <= 35(deg) to the normal. For larger angles, opacity shielding occurs and reduced fields are derived. Title: MHD simulations with adaptive mesh refinement. Authors: Steiner, O.; Grossmann-Doerth, U.; Knölker, M.; Schüssler, M. Bibcode: 1992AGAb....7..213S Altcode: No abstract at ADS Title: The effect of non-linear oscillations in magnetic flux tubes on Stokes V asymmetry Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K. Bibcode: 1991A&A...249..239G Altcode: The present knowledge on, and interpretation of, the asymmetry of Stokes V of spectral lines thought to be formed in and around magnetic flux tubes is reviewed. Crude models of nonlinear oscillations in the flux tube were investigated in order to explain the observed values of Stokes V amplitude and area asymmetry. It was found that flux tube models with quasi-oscillatory motions, consisting of a slow upflow and a rapid downflow in the magnetic region, as well as a downflow in the nonmagnetic surroundings, may reproduce the observations, both asymmetries and zero-crossing shift of Stokes V. Title: Solar magnetic elements: results of MHD simulations. Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.; Weisshaar, E. Bibcode: 1991AGAb....6...31G Altcode: No abstract at ADS Title: Stokes profile formation heights in solar magnetic flux tubes. Authors: Larson, B.; Solanki, S. K.; Grossmann-Doerth, U. Bibcode: 1991sopo.work..479L Altcode: The formation heights of magnetically split lines in small solar magnetic flux tubes are investigated. In particular, the authors are interested in how the heights of formation depend on different flux tube parameters, like magnetic field strength, temperature and temperature gradient, as well as on line parameters, such as line strength, excitation potential and ionization stage. Title: Some developments in the theory of magnetic flux concentrations in the solar atmosphere Authors: Knoelker, M.; Grossmann-Doerth, U.; Schuessler, M.; Weisshaar, E. Bibcode: 1991AdSpR..11e.285K Altcode: 1991AdSpR..11..285K Most of the magnetic flux in the solar photosphere is concentrated in small-scale structures of large field strength, called magnetic elements. We discuss briefly the observationally determined properties of magnetic elements and the theoretical concepts for the origin of magnetic flux filamentation and concentration. New results of model calculations for 2D magnetic flux sheets on the basis of numerical simulation of the compressible MHD equations including a full (grey) radiative transfer are presented. Synthetic Stokes profiles of spectral lines and continuum intensity distributions serve to compare the theoretical models with observational data. Among the key results are: (1) The upper layers of the magnetic structure become hotter than the environment due to radiative illumination effects; (2) a strong convective flow evolves with horizontal velocity of 2 km/s towards the flux sheet and a narrow ``downflow jet'' with velocity up to 6 km/s adjacent to the magnetic structure; (3) both flux sheet and non-magnetic environment oscillate with a period around 5 minutes. Comparison with observed properties of solar magnetic elements reveals: (4) Calculated and semi-empirical temperature profiles as function of height in the photosphere are in reasonable agreement; (5) the calculated velocity field around flux concentrations explains the area asymmetry of the observed Stokes V-profiles including their center-limb variation; (6) the calculated continuum intensity of a flux sheet model is compatible with the values inferred from high spatial resolution observations of bright points at solar disk center; (7) the observed center-limb variation of facular contrast at low or medium spatial resolution is reproduced by arranging calculated flux sheets in arrays. We stress the importance of MHD simulation models for the analysis and interpretation of data from future facilities for high spatial resolution observations like OSL and LEST. Title: The influence of vertical magnetic field gradients on the measured field strength and filling factor in late-type stars. Authors: Grossmann-Doerth, U.; Solanki, S. K. Bibcode: 1990A&A...238..279G Altcode: The influence of a vertical gradient of the magnetic field in late type stars on the measurement of magnetic field strengths and filling factors is studied. Line profiles and contribution functions of spectral lines with large Lande factors are calculated in model stellar atmospheres in the presence of a magnetic field with a vertical gradient. It is found that the four lines, which have often been used to measure solar and stellar magnetic fields in the past, are formed at heights sufficiently different to account for differences in apparent field strength of up to 1000 G if the fields are assumed to be similarly structured as in the sun. Thus it is concluded that the vertical gradient of the field may contribute to a good part of the discrepancy between different published measurements of field strength and filling factor in the K2 dwarf Epsilon-Eri. Title: Model calculations of magnetic flux concentrations in the solar photosphere. Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.; Weisshaar, E. Bibcode: 1990AGAb....5...44G Altcode: No abstract at ADS Title: At what heights are spectral lines formed in solar magnetic flux tubes? Authors: Larsson, B.; Solanki, S.; Grossmann-Doerth, U. Bibcode: 1990nba..meet..169L Altcode: 1990taco.conf..169L Illustrative examples, taken from a study of the formation of magnetically split lines in solar magnetic flux tubes, are presented. A particular interest is: in how the heights of formation depend on different flux tube parameters, like the magnetic field strength, the temperature and temperature gradient, as well as on line parameters, such as line strength, excitation potential and ionization stage. The result should help to improve the construction of empirical models of magnetic features and provide a new understanding of the spectral diagnostics used in the study of the solar magnetic field. Title: Stokes V asymmetry and shift of spectral lines Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K. Bibcode: 1989A&A...221..338G Altcode: Further evidence is given for the interpretation of the observed unshifted and asymmetric Stokes V profiles in the solar atmosphere in terms of a magnetic flux concentration expanding with height and surrounded by a downflow. A general proof is given that a V profile originating in an atmosphere in which magnetic field and flow are spatially separated along the line of sight has a zero-crossing wavelength which is unshifted with respect to the rest wavelength of the line center. Heuristic considerations for the dependence of the asymmetry of Stokes V on Zeeman shift, Doppler shift, line width, and strength are described and confirmed by exploratory calculations. Title: Observational aspects of magnetic flux sheet models Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.; Weisshaar, E. Bibcode: 1989hsrs.conf..427G Altcode: No abstract at ADS Title: Models of Magnetic Flux Sheets Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M.; Weisshaar, E. Bibcode: 1989ASIC..263..481G Altcode: 1989ssg..conf..481G No abstract at ADS Title: Unshifted, asymmetric Stokes V-profiles - Possible solution of a riddle Authors: Grossmann-Doerth, U.; Schuessler, M.; Solanki, S. K. Bibcode: 1988A&A...206L..37G Altcode: We show that Stokes V-profiles originating in an atmosphere in which a magnetic field and a systematic velocity field are spatially separated along the line of sight are asymmetric with unshifted zero-crossings. Such a configuration is probably realized in the peripheral parts of magnetic flux concentrations in the solar photosphere: Since the magnetic field flares out with height, the line of sight traverses both a static magnetic region and a downdraft in the non-magnetic surroundings. V-profiles formed in these parts of the flux concentation will be strongly asymmetric but unshifted while the V-profiles from the central parts are symmetric and unshifted. The resulting mean V-profiles show the characteristics of the observed profiles, i.e. they are unshifted and asymmetric. Title: Contribution and response functions for Stokes line profiles formed in a magnetic field Authors: Grossmann-Doerth, U.; Larsson, B.; Solanki, S. K. Bibcode: 1988A&A...204..266G Altcode: Expressions defining the contribution functions of the 'line depression' Stokes profiles formed in a general magnetic field are derived. Such contribution functions are better suited to determining the heights at which the bound-bound transitions responsible for spectral lines are important than the contribution functions to the 'intensity' Stokes parameters defined by Van Ballegooijen (1985). Expressions defining response functions for both 'intensity' and 'line depression' Stokes parameters are also derived for an arbitrary atmosphere and magnetic field. A code for calculating the various Stokes contribution functions is described and some example calculations are presented. These clearly demonstrate the superior diagnostic value of the contribution functions to the 'line depression' Stokes profiles. Title: The solar granulation in the vicinity of sunspots Authors: Schmidt, W.; Grossmann-Doerth, U.; Schroeter, E. H. Bibcode: 1988A&A...197..306S Altcode: The mean size of the granulation close to sunspots is about 10 percent smaller than the corresponding value in the normal photosphere. The percentage of small granules increases steadily over a distance of about 30 arcsedc, as one approaches the sunspots. The observed rms intensity-contrast of the granulation at distances of less than 10 arcse to the penumbra is lower than in the quiet photosphere. This is the result of a power spectrum analysis of 11 white light pictures of different sunspots taken in 1980 with the 40 cm Newtonian Telescope of the Kiepenheuer-Institut operated at the Observatorio del Teide, Tenerife. Title: Models of small magnetic flux concentrations in the solar photosphere. Authors: Grossmann-Doerth, U.; Knölker, M.; Schüssler, M. Bibcode: 1988AGAb....1...11G Altcode: No abstract at ADS Title: Spurious variation of photospheric magnetic flux Authors: Grossmann-Doerth, U.; Pahlke, K. -D.; Schuessler, M. Bibcode: 1987A&A...176..139G Altcode: The response of a Babcock-type magnetograph to emission from different stages in the evolution of a solar active region is simulated by computing the Stokes I and V profiles of the 525.02-nm and 630.25-nm lines of Fe I in model atmospheres representing sunspot umbrae, magnetic knots, and magnetic elements. The results are presented in tables and graphs and characterized. Breakup of a single sunspot into small hot elements with the same total magnetic flux is shown to produce a significant (as much as factor of 10) spurious loss of signal in the magnetograph. It is suggested that recent observations of flux-loss events (in unipolar regions of the solar photosphere without magnetic neutral lines) be carefully reexamined and analyzed. Title: Size and temperature of umbral dots Authors: Grossmann-Doerth, U.; Schmidt, W.; Schroeter, E. H. Bibcode: 1986A&A...156..347G Altcode: In 12 white light broad band pictures of 6 different sunspots, taken in 1980 and 1981 with the 40 cm Newtonian Vacuum Telescope operated at the Observatorio del Teide (Iazaña), the authors identified nearly 100 umbral dots and attempted to derive their temperature and diameter. They found strong evidence that the diameter of the dots is in the range 0arcsec.4 to 0arcsec.9 and the temperature is several 100K up to more than 1000K cooler than the photosphere. Title: Bericht und Programm Authors: Grossmann-Doerth, U. Bibcode: 1984MitAG..62..184G Altcode: No abstract at ADS Title: The brightness distribution in sunspot penumbrae Authors: Grossmann-Doerth, U.; Schmidt, W. Bibcode: 1981A&A....95..366G Altcode: From a set of high quality sunspot photographs the intensity distribution in the penumbrae as well as mean intensities and rms contrast were derived. For image restoration a Fourier technique was employed. The intensity distribution was found to be almost symmetrical and singly peaked. Title: Observations of Chromospheric Lines from OSO-8 Authors: Grossmann-Doerth, U.; Kneer, F.; Uexkuell, M.; Artzner, G. E.; Vial, J. C. Bibcode: 1980SoPh...66....3G Altcode: The line profiles of Lα, Ca II K and Mg IIk were measured with the spectrometer of the `Laboratoire de Physique Stellaire et Planétaire' on board of OSO-8. The results of these measurements are presented. Title: Some comments on Suemoto's paper "An empirical, statistical model for the formation of the cores of chromospheric Fraunhofer lines". Authors: Cram, L. E.; Durrant, C. J.; Grossmann-Doerth, U. Bibcode: 1978SoPh...58..279C Altcode: We discuss a recent model for the solar chromosphere proposed by Suemoto (Solar Phys.54, 3). We conclude that the model is incompatible with the basic constraints imposed by high resolution observations and by line formation theory, and that consequently the model does not adequately describe the solar chromosphere. Title: Spectral investigation of the chromosphere. VI: Observations of Halpha close to the limb. Authors: Grossmann-Doerth, U.; von Uexkuell, M. Bibcode: 1977SoPh...55..321G Altcode: Several hundred Hα spectrograms from areas close to the solar limb were taken with the 35 cm Coudé refractor at Anacapri. The 41 spectra with the greatest spatial resolution were selected and analysed. At the supergranular boundaries a considerable fraction of the line profiles were found to correspond to Beckers' Cloud Model (BCM). Moreover, the BCM parameters of the dark mottles at the limb appear to be approximately equal to those from the center of the disk. On the other hand, we also obtained evidence in disagreement with the general applicability of the BCM to all features of the chromospheric fine structure. Title: The sun's chromospheric velocity field as inferred from the Ca II K line. Authors: Durrant, C. J.; Grossmann-Doerth, U.; Kneer, F. J. Bibcode: 1976A&A....51...95D Altcode: Theory and observation of the solar Ca II K line under high spatial resolution are briefly reviewed. It is shown that contrary to popular belief, present theory is not capable of explaining the observed features of the K line. An attempt is made to develop a schematic model of the temporal behavior of the spatially resolved K line, based on Leibacher's (1971) investigation of waves in the solar atmosphere Title: Solar activity (Activité solaire). Authors: Newkirk, G.; Dunn, R. B.; Mehltretter, P.; MacQueen, R.; Bonnet, R. M.; White, O. R.; Fokker, A. D.; Zwaan, C.; Bruzek, A.; Durrant, C.; Grossmann-Doerth, U.; Mehltretter, J. P.; Svestka, Z.; de Feiter, L. D.; Tandberg-Hanssen, E.; Howard, R.; Stix, M.; Pneuman, G. W.; Hundhausen, A. J.; Sawyer, C.; Simon, P. Bibcode: 1976IAUTA..16b..13N Altcode: No abstract at ADS Title: Spectral Investigation of the Chromosphere. V: Observation and Analysis of Hβ Authors: Grossmann-Doerth, U.; Uexkuell, M. V. Bibcode: 1975SoPh...42..303G Altcode: The results of the analysis of 16 Hβ spectra taken with high spatial resolution are presented. The properties of the line profile in the boundary regions of supergranular cells (SGB) are described by the statistical distribution functions and correlation coefficients of four parameters which determine the shape of a suitable fitting curve. Although some of the line profile properties of both Hβ and Hα in the SGB can be interpreted in terms of a model which was recently proposed by Durrant (1975) the agreement between that theory and our observations is not entirely satisfactory. Title: Properties of the solar Ca ii K-Line at high spatial resolution Authors: Grossmann-Doerth, U.; Kneer, F.; Uexküll, M. v. Bibcode: 1974SoPh...37...85G Altcode: The analysis of three Ca II K-spectra with spatial resolution of ∼1″ is described and its results presented. The comparison of the observed single peak line profiles with model computation leads to some conclusions regarding the non-uniform large-scale velocity fields in the chromosphere. Title: On the Design of Chromospheric Models Authors: Grossmann-Doerth, U. Bibcode: 1974IAUS...56...93G Altcode: No abstract at ADS Title: Spectroscopic Investigation of the Chromosphere. III: Hα Line Profile from the Interior Supergranular Cells Authors: Grossmann-Doerth, Ulrich; Von Uexküll, Marina Bibcode: 1973SoPh...30...71G Altcode: The line profile of H⇓ as emanating from the interior of supergranular cells was measured at sin θ = 0, 0.6, 0.8 and 0.9. The measurements are described and the results presented. Title: Spectral Investigation of the Chromosphere. II. The Nature of the Mottles and a Model of the Overall Structure Authors: Grossmann-Doerth, Ulrich; Von Uexküll, Marina Bibcode: 1973SoPh...28..319G Altcode: Highly resolved Hα spectra and filtergrams obtained at the Fraunhofer Observatory on Capri were analysed by a method whose principles have been described before. As a result the tentative conclusion of our previous work has been confirmed: The mottles of the chromospheric fine structure are clouds superimposed on the low chromosphere. Furthermore, it is proposed that the latter is identical with the interior of the supergranular cells whose spatial averages lend themselves to an interpretation in terms of a spherically symmetric model. On the other hand, we present evidence that the boundary regions which are formed by the mottles of the fine structure pattern and which constitute the upper chromosphere are not adequately described by spatial averages. Instead, the properties of the individual structural elements should be taken into account by an appropriate theory, if only in a statistical fashion. Some of these properties, viz. source function, optical thickness, Doppler width and velocity, were measured for a large number of elements. On the basis of indirect evidence we suggest that the spicules seen on limb photographs of chromospheric lines are identical with the elements of the supergranule boundary structure. Title: Die Struktur der solaren Chromosphäre Authors: Grossmann-Doerth, U.; von Uexküll, M. Bibcode: 1973MitAG..32..187G Altcode: No abstract at ADS Title: Spectral Investigation of Chromospheric Fine Structure Authors: Grossmann-Doerth, Ulrich; Von Uexküll, Marina Bibcode: 1971SoPh...20...31G Altcode: Hα spectra and effectively simultaneous filtergrams were taken at the Fraunhofer Observatory on Capri with the 35 cm domeless Coudé. The spatial resolution of the 19 best spectra selected for analysis was estimated to be 1-2 arc sec. The comparison of several hundred Hα line profiles emitted by typical chromospheric structure elements with theoretical prediction yielded strong evidence to suggest that the chromosphere consists of two parts: A lower, rather uniform layer at rest superposed by `clouds' (condensations of great spatial variability) which constitute the well-known structure pattern of Hα filtergrams. For most image points the line-of-sight velocity, optical thickness, source function and Doppler broadening of these clouds could be determined. While the values of the latter three quantities were found to be similar to what Beckers (1968) has found for limb spicules the velocity of the bright and of the dark mottles is considerably smaller than to be expected if these features were the spicules as seen on the disk. However, our results do not rule out the possibility that the spicules rise at the centers of rosettes where they are difficult to detect. Title: On Temperature in Line-Gap Regions Authors: Grossmann-Doerth, U. Bibcode: 1970SoPh...13..287G Altcode: The small regions in the photosphere where many absorption lines weaken have been investigated by several authors. Sheeley attempted to explain the weakening of neutral lines in terms of a temperature increase on the basis of a simple theoretical model. It is shown that this model is not applicable but that a modified theory is able to interpret the observed behaviour of both neutral and ionized lines in terms of a temperature increase of a few percent. Title: On Astronomical Seeing: The Single Schlieren Model Authors: Grossmann-Doerth, Ulrich Bibcode: 1969SoPh....9..210G Altcode: A model of astronomical seeing with particular view to solar observations is developed which assumes the atmospheric disturbances to consist of individual turbulence elements called Schlieren. A quantitative account is given of each image motion, image blurring and scintillation as function of Schlieren properties and telescope parameters. The theory permits to explain the observational results under conditions of good seeing; furthermore, it provides a basis for the discussion of the physical phenomena in the atmosphere that cause image deterioration. As an illustration it is shown how the theory could be applied to a problem of interest to solar astronomers searching for a suitable observatory site.