Theoretical Models of Magnetic Flux Tubes: Structure and Dynamics

To explain the heating of stellar chromospheres and transition regions, two classes of heating mechanisms have been considered: dissipation of acoustic and magnetic waves generated in stellar convection zones; and dissipation of currents generated by photospheric motions of surface magnetic fields. The focus of this paper is on the wave heating mechanisms and on recent results which demonstrate that theoretical models of stellar chromospheres based on the wave heating can explain the “basal flux” and the observed Ca II emission in most stars but cannot account for the observed Mg II emission in active stars. The obtained results clearly show that the base of stellar chromospheres is heated by acoustic waves, the heating of the middle and upper chromospheric layers is dominated by magnetic waves associated with magnetic flux tubes, and that other non-wave heating mechanisms are required to explain the structure of the highest layers of stellar chromospheres and transition regions.

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Magnetic structure of sunspot under the photosphere

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310009737 ◽

2009 ◽

Vol 5 (H15) ◽

pp. 351-351

Author(s):

Elena A. Kirichek ◽

Alexandr A. Solov'ev

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Large Scale ◽

Propagation Speed ◽

Theoretical Models ◽

Flux Tubes ◽

Magnetoacoustic Waves ◽

Deep Layers ◽

Magnetic Flux Tubes ◽

Wave Propagation Speed

In recent years, the local helioseismology has become a highly effective tool for investigating subphotospheric layers of the Sun, which can yield fairly detailed distributions of the subphotospheric temperatures and large-scale plasma flows based on the spectra of the oscillations observed at the photospheric layers and the observed peculiarities of propagation of magnetoacoustic waves in this medium (Zhao et al. (2001), Kosovichev (2006)). Unfortunately, the effects of temperature and the magnetic field on the wave propagation speed have not yet been separated Kosovichev (2006), so that the structure of the sunspot magnetic field in deep layers, beneath the photosphere, remains a subject of purely theoretical analysis. In his analysis of some theoretical models of the subphotospheric layers of sunspots based on recent helioseismological data, Kosovichev (2006) concluded that Parker's (“spaghetti”) cluster model Parker (1979) is most appropriate. In this model, the magnetic flux in the sunspot umbra is concentrated into separate, strongly compressed, vertical magnetic flux tubes that are interspaced with plasma that is almost free of magnetic field; the plasma can move between these tubes.

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Theoretical Models of Magnetic Flux Tubes: Structure and Dynamics

Symposium - International Astronomical Union ◽

10.1017/s0074180900124672 ◽

1994 ◽

Vol 154 ◽

pp. 407-421

Author(s):

O Steiner

Keyword(s):

Magnetic Flux ◽

Line Emission ◽

Theoretical Models ◽

Small Scale ◽

Solar Photosphere ◽

Model Calculations ◽

Flux Tubes ◽

Wide Range ◽

Magnetic Flux Tubes ◽

Lower Chromosphere

Two types of model calculations for small scale magnetic flux tubes in the solar atmosphere are reviewed. In the first kind, one follows the temporal evolution governed by the complete set of the MHD and radiative transfer equations to a (quasi) stationary solution. From such a solution the continuum contrasts of a photospheric flux tube in the visible and in the infrared continuum at 1.6 μm have been computed and are briefly discussed. The second, more empirical type of method assumes the flux tubes to be in magnetohydrostatic equilibrium. It is computationally faster and more flexible and allows us to explore a wide range of parameters. Models and insights obtained from such parameter studies are discussed in some detail. These include an explanation for the peculiar variation of the area asymmetry of Stokes V profiles across the solar disk in terms of mass motions in the surroundings of magnetic flux tubes.Furthermore, a two-dimensional model of the lower chromosphere that has been developed is presented. Emphasis is laid on the effect of thermal bifurcation of the lower chromosphere on the structure of the chromospheric magnetic field. If the cool carbon monoxide clouds, observed in the infrared, occupy the non-magnetic regions, the flux tubes expand very strongly and form a magnetic canopy with an almost horizontal base. This has consequences for the spatial distribution of the Ca II K spectral line emission.Finally, some consideration is given to the formation and destruction of intense magnetic flux tubes in the solar photosphere. The formation is described as a consequence of the flux expulsion process that leads to a convective instability. A possible observational signature of this mechanism is proposed.

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Heating of Intense Magnetic Flux Tubes by Magnetohydrodynamic Waves

Symposium - International Astronomical Union ◽

10.1017/s0074180900030084 ◽

1983 ◽

Vol 102 ◽

pp. 413-416

Author(s):

Z. Musielak ◽

E. Bielicz

Keyword(s):

Magnetic Flux ◽

Flux Tube ◽

Theoretical Models ◽

Convective Zone ◽

Temperature Minimum ◽

Late Type ◽

Flux Tubes ◽

Magnetohydrodynamic Waves ◽

Chromospheric Activity ◽

Magnetic Flux Tubes

Theoretical models of intense magnetic flux tubes embedded in the photospheres of late-type stars have been calculated. Magnetohydrodynamic waves generated in the convective zone are radiatively damped in the lower part of the flux tube and then dissipated. We discuss how the presence of flux tubes in stellar photospheres influences the temperature minimum and the chromospheric activity. We suggest a possible interpretation of the Wilson-Bappu effect in stars with strong chromospheric activity.

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