Author name code: bunte ADS astronomy entries on 2022-09-14 =author:"Bunte, M." year:1990-2005 ------------------------------------------------------------------------ Title: Kelvin-Helmholtz and shear instability of a helical flow around a magnetic flux tube Authors: Kolesnikov, F.; Bünte, M.; Schmitt, D.; Schüssler, M. Bibcode: 2004A&A...420..737K Altcode: Magnetic flux concentrations in the solar (sub)photosphere are surrounded by strong downflows, which come into swirling motion owing to the conservation of angular momentum. While such a whirl flow can stabilize a magnetic flux tube against the MHD fluting instability, it potentially becomes subject to Kelvin-Helmholtz and shear instability near the edge of the flux tube, which may lead to twisting of the magnetic field and perhaps even to the disruption of the magnetic structure. As a first step towards studying the relevance of such instabilities, we investigate the stability of an incompressible flow with longitudinal and azimuthal (whirl) components surrounding a cylinder with a uniform longitudinal magnetic field. We find that a sharp jump of the azimuthal flow component at the cylinder boundary always leads to Kelvin-Helmholtz-type instability for sufficiently small wavelength of the perturbation. On the other hand, a smooth and wide enough transition of the azimuthal velocity towards the surface of the cylinder leads to stable configurations, even for a discontinuous profile of the longitudinal flow. Title: Magneto-atmospheric waves subject to Newtonian cooling Authors: Bunte, M.; Bogdan, T. J. Bibcode: 1994A&A...283..642B Altcode: We present a new class of analytic solutions for linear magneto-atmospheric waves in a stratified, isothermal atmosphere. This new class spans from the solutions found by Yu (1965) for a constant Alfven speed atmosphere to those of Nye & Thomas (1967 a,b) for an atmosphere with exponentially increasing Alfven speed. The inclusion of radiative dissipation in magneto-atmospheric wave problems - even in the over-simplified Newtonian cooling approximation - is of great importance in many applications. We show how the effects of Newtonian cooling can be incorporated in any isothermal magneto-atmospheric wave problem by letting the ratio of specific heats, gamma, be a complex, frequency dependent quantity. This technique is discussed in the context of the new solutions presented here. In particular, we discuss its application to the case studied by Souffrin (1966, 1972) and to the more general case of a constant Alfven speed atmosphere, where now three, rather than two regions of mainly propagating modes are found. In the case studied by Nye & Thomas, Newtonian cooling leads to a fast temporal decay of the eigenmodes. Title: Flux Tube Shredding Its Infrared Signature Authors: Bunte, M.; Steiner, O.; Solanki, S. K.; Pizzo, V. J. Bibcode: 1994IAUS..154..459B Altcode: No abstract at ADS Title: Surface waves: Origin of the Evershed phenomenon? Authors: Bünte, M.; Solanki, S. K. Bibcode: 1994smf..conf..179B Altcode: No abstract at ADS Title: On the interchange instability of solar magnetic flux tubes. Ill. The influence of the magnetic field geometry Authors: Bunte, M. Bibcode: 1993A&A...276..236B Altcode: The geometry of small magnetic flux concentrations in the solar photo sphere lies somewhere between the two extremes of a cylindrical tube and an elongated magnetic slab. A comparison of the two geometries shows that small slabs - like tubes - are unstable to fluting just below continuum optical depth unity (τc = 1), but - unlike tubes - remain unstable deeper down in the convection zone. Whereas tubes may be stabilized by the surrounding intergranular whirl motion, there is no natural mechanism to stabilize a flux sheet against fluting. This suggests that immediately after their formation slabs with diameters ≲ 1000 km are fragmented into tube-filaments below Tc = 1. Title: Surface waves as the origin of the Evershed phenomenon Authors: Bunte, M.; Darconza, G.; Solanki, S. K. Bibcode: 1993A&A...274..478B Altcode: We investigate the spectral signature of magnetoacoustic-gravity surface waves (MAGS-waves), which we expect to exist at the interface between the lower boundary of the magnetic field of the sunspot penumbra and the non-magnetic gas below. MAGS-waves have a number of properties that make them attractive candidates for explaining the photo spheric Evershed effect: 1. Since they transport only energy but no mass, they resolve the problem of mass conservation at the outer penumbral boundary. 2. Since they are restricted to magnetic interfaces, they are only (or dominantly) present in the penumbra and absent in the umbra. 3. The phase relation between the temperature, the horizontal, and vertical velocity perturbations leads to line shifts and asymmetries in observations at the limb, but none at disc centre. 4. The amplitude of the velocity perturbation drops approximately exponentially with height, in good agreement with observations. 5. Waves travelling in opposite directions with respect to the observer produce oppositely directed line shifts and asymmetries, making it straightforward to explain the opposite shifts and asymmetries observed in the limb-side and centre-side penumbra. Title: On the interchange instability of solar magnetic flux tubes. II. The influence of energy transport effects Authors: Bunte, M.; Hasan, S.; Kalkofen, W. Bibcode: 1993A&A...273..287B Altcode: We examine the interchange instability of thin photo spheric magnetic flux tube models which satisfy both force and energy balance with their surroundings. The stability of the tubes is independent of the efficiency of internal convective energy transport and shows only a weak dependence on the plasma beta. The structures are susceptible to the instability in a layer 200 - 300 km deep immediately below optical depth unity in the quiet photosphere. The presence of an internal atmosphere reduces the magnetic field strength in comparison with that of an evacuated tube. While this has a stabilizing effect on the tube surface, temperature differences between interior and exterior are usually destabilizing. We find that the two effects approximately cancel each other for tubes with radii R ≲ 200 km for which the stability properties are very similar to those of completely evacuated structures. For larger tubes, the temperature contrast with respect to the surroundings begins to dominate and destabilizes the tubes. Thus, despite the inclusion of energy transport effects on the tube structure, the stability problem of small tubes (with magnetic fluxes Φ < 1019-1020 Mx) remains. Consequences for photospheric magnetic fields are discussed. Title: The interchange instability of stellar magnetic flux tubes Authors: Bunte, M.; Saar, S. H. Bibcode: 1993A&A...271..167B Altcode: We investigate the stability of magnetic flux tubes in the surface layers of late-type stars, concentrating on the interchange (fluting) instability. Flux tubes on low gravity stars (log g ≲ 3.5) are generally unaffected by this instability. On stars with higher surface gravity, tubes are stable if their magnetic flux exceeds 1019 - 1021 Mx (the precise value depending on Teff and log g). Smaller structures on these stars may be stabilized by a sufficient external whirl flow. The magnitude of the required whirl flow shows only weak dependence on Teff but increases sharply with log g. In some stars, the whirl velocities needed for stabilization are larger than extant photo spheric velocities. In these cases, stable tubes are possibly separated into two regimes of permitted fluxes. Consequences for the field structure on the surfaces of late-type stars are discussed. Title: Center-to-limb variation of the Stokes V asymmetry in solar magnetic flux tubes. Authors: Bünte, M.; Steiner, O.; Solanki, S. K. Bibcode: 1991sopo.work..468B Altcode: The center-to-limb-variation of synthetic Stokes V line profiles of the spectral line Fe I 5250.22 Å is presented and compared with observations. These synthetic profiles are calculated using models that contain the main features of the current basic pictures of small scale magnetic fields on the Sun.