explanation blue bibcodes open ADS page with paths to full text
Author name code: bunte
ADS astronomy entries on 2022-09-14
=author:"Bunte, M." year:1990-2005
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Title: Kelvin-Helmholtz and shear instability of a helical flow
around a magnetic flux tube
Authors: Kolesnikov, F.; Bünte, M.; Schmitt, D.; Schüssler, M.
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.
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Title: Magneto-atmospheric waves subject to Newtonian cooling
Authors: Bunte, M.; Bogdan, T. J.
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.
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Title: Flux Tube Shredding Its Infrared Signature
Authors: Bunte, M.; Steiner, O.; Solanki, S. K.; Pizzo, V. J.
1994IAUS..154..459B Altcode:
No abstract at ADS
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Title: Surface waves: Origin of the Evershed phenomenon?
Authors: Bünte, M.; Solanki, S. K.
1994smf..conf..179B Altcode:
No abstract at ADS
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Title: On the interchange instability of solar magnetic flux
tubes. Ill. The influence of the magnetic field geometry
Authors: Bunte, M.
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 (τ<SUB>c</SUB> = 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 T<SUB>c</SUB> = 1.
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Title: Surface waves as the origin of the Evershed phenomenon
Authors: Bunte, M.; Darconza, G.; Solanki, S. K.
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.
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Title: On the interchange instability of solar magnetic flux
tubes. II. The influence of energy transport effects
Authors: Bunte, M.; Hasan, S.; Kalkofen, W.
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 < 10<SUP>19</SUP>-10<SUP>20</SUP> Mx) remains. Consequences
for photospheric magnetic fields are discussed.
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Title: The interchange instability of stellar magnetic flux tubes
Authors: Bunte, M.; Saar, S. H.
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 10<SUP>19</SUP>
- 10<SUP>21</SUP> Mx (the precise value depending on T<SUB>eff</SUB>
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 T<SUB>eff</SUB> 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.
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Title: Center-to-limb variation of the Stokes V asymmetry in solar
magnetic flux tubes.
Authors: Bünte, M.; Steiner, O.; Solanki, S. K.
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.
