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Ulmschneider, P.

Publications and source records attributed to Ulmschneider, P..

13 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Chromospheric heating and metal deficiency in cool giants: Theoretical results versus observations

We compute acoustic shock wave-heated chromosphere models for moderately cool giant stars which differ greatly in metallicity. Subsequently, we simulate the emerging Mg II k lines assuming partial redistribution. The initial acoustic energy fluxes and the wave periods are taken from acoustic wave generation calculations based on traditional convection zone models. We find that the Mg II and Ca II core emissions are close to the observed basal flux limits which are common for giants and dwarfs. In addition, we find that the Mg II core emission is independent of the metallicity, in agreement with observations. We argue that these results should be considered as further evidence that the basal flux limits are indeed due to acoustic shock heating. The acoustic heating mechanism seems to be dominant in all nonmagnetic nonpulsating late-type stars.

Cuntz, M.↗
NASA NTRS1994

On sound generation by turbulent convection: A new look at old results

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 F(sub A) = 5 x 10(exp 7) 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 alpha(exp 3.8). 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.

Musielak, Z. E.↗
NASA NTRS1990

Wave pressure in stellar atmospheres due to shock wave trains

Analytic expressions for the wave pressure of propagating shock wave trains in stellar atmospheres or winds are derived. Applications to weak shocks and stronger shocks with sawtooth profiles are discussed in detail. The shocks are treated as discontinuities. The results provide insight in the momentum balance of time-dependent stellar wind flows. The analytic expressions can be used as an independent test of hydrodynamic codes.

Gail, H.-P.↗
NASA NTRS1989

On the generation of flux tube waves in stellar convection zones. I - Longitudinal tube waves driven by external turbulence

The source functions and the energy fluxes for wave generation in magnetic flux tubes embedded in an otherwise magnetic field-free, turbulent, and compressible fluid are derived. The calculations presented here assume that the tube interior is not itself turbulent, e.g., that motions within the flux tube are due simply to external excitation. Specific results for the generation of longitudinal tube waves are presented.

Musielak, Z. E.↗
NASA NTRS1987

Magnetic flux tubes as sources of wave generation

The structure of solar, and very likely stellar, surface magnetic fields is highly inhomogeneous: at the photospheric level, the fields are locally strong, and show concentration into a flux tube structure. In this case, the wave energy generated in stellar convection zones may be largely carried away by flux tube waves, which can then become important sources for the heating of the outer atmospheric layers. Such flux tube wave generation may help to explain the UV and X-ray fluxes observed by the IUE and Einstein observatories. The generation of longitudinal tube waves in magnetic flux tubes embedded in an otherwise magnetic field-free, turbulent, and stratified medium was considered. It is shown that compressible tube waves are generated by dipole emission and that the generation efficiency is a strong function of the magnetic field strength. Energy flux calculations are presented for different magnetic flux tubes, and show how the results depend on the magnetic field strength and the characteristics of the convective turbulence.

Musielak, Z. E.↗
NASA NTRS1987

A computational code for two-dimensional unsteady magnetohydrodynamics by the method of characteristics

A computational code for solving two-dimensional, time-dependent MHD equations by the method of characteristics is presented. Its capabilities are demonstrated by solving two very different problems for which analytical solutions exist: linearized, standing MHD wave motions in a magnetized cylindrical plasma, and nonlinear self-similar expansion of a magnetized plasma ball. The nonlinear development of standing MHD wave solutions in a cylindrical plasma is also studied. The method can be naturally embedded in the computational architecture of massively parallel processors.

Lou, Y. Q.↗
NASA NTRS1985

Acoustic waves in the solar atmosphere. VII - Non-grey, non-LTE H(-) models

The propagation and shock formation of radiatively damped acoustic waves in the solar chromosphere are studied under the assumption that H(-) is the only absorber; the opacity is non-grey. Deviations from local thermodynamic equilibrium (LTE) are permitted. The results of numerical simulations show the depth dependence of the heating by the acoustic waves to be insensitive to the mean state of the atmosphere. After the waves have developed into shocks, their energy flux decays exponentially with a constant damping length of about 1.4 times the pressure scale height, independent of initial flux and wave period. Departures from LTE have a strong influence on the mean temperature structure in dynamical chromosphere models; this is even more pronounced in models with reduced particle density - simulating conditions in magnetic flux tubes - which show significantly increased temperatures in response to mechanical heating. When the energy dissipation of the waves is sufficiently large to dissociate most of the H(-) ions, a strong temperature rise is found that is reminiscent of the temperature structure in the transition zone between chromosphere and corona; the energy flux remaining in the waves then drives mass motions.

Schmitz, F.↗
NASA NTRS1985

Propagation of nonlinear, radiatively damped longitudinal waves along magnetic flux tubes in the solar atmosphere

For solar magnetic flux tubes three types of waves are compared: longitudinal MHD tube waves, acoustic tube waves propagating in the same tube geometry but with rigid walls and ordinary acoustic waves in plane geometry. It is found that the effect of the distensibility of the tube is small and that longitudinal waves are essentially acoustic tube waves. Due to the tube geometry there is considerable difference between longitudinal waves or acoustic tube waves and ordinary acoustic waves. Longitudinal waves as well as acoustic tube waves show a smaller amplitude growth, larger shock formation heights, smaller mean chromospheric temperature but a steeper dependence of the temperature gradient on wave period.

Herbold, G.↗
NASA NTRS1984

Apparent solar temperature enhancement due to large-amplitude waves

The effect of slow-mode acoustic-type MHD waves propagating outward in the solar atmosphere on the temperature structures predicted by empirical models is investigated analytically. A model is constructed, and numerical results are presented for wing intensities, line profiles, temperature enhancements, waves with higher energy flux, temperature depression, and the Si continuum. The flux in the MgII and CaII UV resonance lines is found to be increased relative to that in the IR continuum, leading to model temperatures which depend systematically on which observations are used in the computation. It is suggested that mechanical heating may take place in smaller regions such as flux tubes rather than uniformly over the surface.

Kalkofen, W.↗
NASA NTRS1984

Core saturation in a moving medium

A numerical technique for solving the line transfer equation of a two-level atom in static equilibrium is presented. Complete redistribution of emitted photons is assumed, as is saturation at the line core. Emission intensity is calculated either by a generalized Eddington-Barber relation, a first-order differential equation for the specific intensity, or by a formal transfer integral. Sample calculations are performed of the line transfer equation in a semi-infinite atmosphere with a constant Planck function of the collision parameter and for the Mg II resonance line in a model solar atmosphere experiencing shocks. Attention is focused on the line wings in the latter problem. The first order differential equation approach yields the best intensity values and temperature structure.

Kalkofen, W.↗
NASA NTRS1983

Stellar coronae - What can be predicted with minimum flux models?

In order to determine the possible errors of various minimum flux corona (MFC) predictions, MFC models are compared with a grid of detailed coronal models covering a range of two orders of magnitude in coronal heating and damping length values. The MFC concept is totally unreliable in the prediction of mass loss and the relative importance of various kinds of energy losses, and MFC predictions for the mass loss rate and energy losses due to stellar wind can be wrong by many orders of magnitude. It is suggested that for future applications, the unreliable MFC formulas should be replaced by a grid of related models accounting for the coronal dependence on damping length, such as the models underlying the present study.

Hammer, R.↗
NASA NTRS1977

Acoustic waves in the solar atmosphere. I - The hydrodynamic code

This paper studies large-amplitude radiatively damped acoustic waves in the solar atmosphere. A modified method of characteristics is described for the solution of the time-dependent hydrodynamic equations in a gravitational atmosphere. A procedure for the detection of shocks is outlined. Several tests of the accuracy of the method are described. The evolution of the wave and the height of shock formation are computed for several values of the period and the initial acoustic flux in isothermal atmospheres with temperatures of 4000 and 5000 K as well as in a model solar atmosphere.

Ulmschneider, P.↗