Abstract Almost all nondegenerate stars have chromospheres and coronae. These hot outer layers are produced by mechanical heating. The heating mechanisms of chromospheres and coronae, classified as hydrodynamic and magnetic mechanisms, are reviewed here. Both types of mechanisms can be further subdivided on basis of the fluctuation frequency into acoustic and pulsational waves for hydrodynamic and into AC- and DC-mechanisms for magnetic heating. Intense heating is usually associated with the formation of very small spatial scales, which are difficult to observe. Yet, global stellar observations, because of the dependence of the mechanical energy generation on the basic stellar parameters ( T eff , gravity, rotation, metallicity) can be extremely important to identify the dominant heating mechanisms.
view Abstract Citations (145) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On Sound Generation by Turbulent Convection: A New Look at Old Results Musielak, Z. E. ; Rosner, R. ; Stein, R. F. ; Ulmschneider, P. Abstract We have revisited the problem of acoustic wave generation by turbulent convection in stellar atmospheres. The theory of aerodynamically generated sound, originally developed by Lighthill and later modified by Stein to include the effects of stratification, has been used to estimate the acoustic wave energy flux generated in solar and stellar convection zones. We correct the earlier computations by incorporating an improved description of the spatial and temporal spectrum of the turbulent convection. We show the dependence of the resulting wave fluxes on the nature of the turbulence, and compute the wave energy spectra and wave energy fluxes generated in the Sun on the basis of a mixing-length model of the solar convection zone. In contrast to the previous results, we show that the acoustic energy generation does not depend very sensitively on the turbulent energy spectrum. However, typical total acoustic fluxes of order FA = 5 x 107 ergs/sq cm/s with a peak of the acoustic frequency spectrum near omega = 100 mHz are found to be comparable to those previously calculated. The acoustic flux turns out to be strongly dependent on the solar model, scaling with the mixing-length parameter alpha as alpha3.8. The computed fluxes most likely constitute a lower limit on the acoustic energy produced in the solar convection zone if recent convection simulations suggesting the presence of shocks near the upper layers of the convection zone apply to the Sun. Publication: The Astrophysical Journal Pub Date: March 1994 DOI: 10.1086/173825 Bibcode: 1994ApJ...423..474M Keywords: Sound Waves; Stellar Atmospheres; Stellar Convection; Turbulent Flow; Wave Generation; Energy Spectra; Fourier Transformation; Reynolds Stress; Wave Equations; Astrophysics; CONVECTION; STARS: ATMOSPHERES; SUN: ATMOSPHERE; TURBULENCE full text sources ADS |
The nonlinear time-dependent response to external pressure fluctuations acting on a thin vertical magnetic flux tube embedded in the solar atmosphere is investigated numerically. The continuous and impulsive fluctuations are imposed on the tube at different atmospheric heights and the resulting longitu- dinal tube wave energy fluxes are calculated for an observation- ally established range of velocity amplitudes and tube magnetic fields. The obtained results show that typical wave energy fluxes carried by nonlinear longitudinal tube waves are of the order of 2 10 8 erg=cm 2 s, which is roughly a factor of 30 less than the flux for transverse waves. In contrast to our linear analytical re- sults the generated nonlinear longitudinal tube wave fluxes can be up to an order of magnitude higher. gitudinal and transverse tube waves. The obtained results show that the fluxes can be important for the heating observed in the solar chromospheric network and that they should be regarded only as lower bounds for realistic energy fluxes carried by these waves. Choudhuri et al. have investigated the generation of mag- netic kink waves by rapid foot point motions of the magnetic flux tube. They argue that occasional rapid motions can account for the entire energy flux needed to heat the quiet corona. They find that pulses are much more efficient than continuous exci- tation for the transfer of wave energy to the solar corona and that the energy flux from pulses actually increases if there is a transition layer temperature jump in the atmosphere. The approach presented by us in Paper I and in this paper
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