Rudolf Loeser

Energy balance in the solar transition region. III - Helium emission in hydrostatic, constant-abundance models with diffusion

In our previous papers we described the mathematical formalism and the computed results for energy-balance hydrostatic models of the solar transition region. In this paper we discuss in some detail the limitations of the hydrostatic and one-dimensional assumptions used. Then we analyze the determination of helium emission when diffusion is included. We use transport coefficients estimated from kinetic theory to determine the helium departures from local ionization balance. We calculate the helium spectra for each of our models and evaluate the role of helium in the energy transport. Also, we investigate the effects of coronal illumination on the structure of the transition region and upper chromosphere, and show how coronal illumination affects various EUV lines and the He I 10830 A line. Comparing with both absolute intensities and detailed line profiles, we show that our models are consistent not only with the observed hydrogen spectra but also with the available helium spectra.

Models of the Solar Chromosphere and Transition Region from SUMER and HRTS Observations: Formation of the Extreme-Ultraviolet Spectrum of Hydrogen, Carbon, and Oxygen

We present the results of optically thick non-LTE radiative transfer calculations of lines and continua of H, C I-IV, and O I-VI and other elements using a new one-dimensional, time-independent model corresponding to the average quiet-Sun chromosphere and transition region. The model is based principally on the Curdt et al. SUMER atlas of the extreme ultraviolet spectrum. Our model of the chromosphere is a semiempirical one, with the temperature distribution adjusted to obtain optimum agreement between calculated and observed continuum intensities, line intensities, and line profiles. Our model of the transition region is determined theoretically from a balance between (a) radiative losses and (b) the downward energy flow from the corona due to thermal conduction and particle diffusion, and using boundary conditions at the base of the transition region established at the top of the chromosphere from the semiempirical model. The quiet-Sun model presented here should be considered as a replacement of the earlier model C of Vernazza et al., since our new model is based on an energy-balance transition region, a better underlying photospheric model, a more extensive set of chromospheric observations, and improved calculations. The photospheric structure of the model given here is the same as in Table 3 of Fontenla, Avrett, Thuiller, & Harder. We show comparisons between calculated and observed continua, and between the calculated and observed profiles of all significant lines of H, C I-IV, and O I-VI in the wavelength range 67-173 nm. While some of the calculated lines are not in emission as observed, we find reasonable general agreement, given the uncertainties in atomic rates and cross sections, and we document the sources of the rates and cross sections used in the calculation. We anticipate that future improvements in the atomic data will give improved agreement with the observations.

Energy balance in the solar transition region. I - Hydrostatic thermal models with ambipolar diffusion

The energy balance in the lower transition region is analyzed by constructing theoretical models which satisfy the energy balance constraint. The energy balance is achieved by balancing the radiative losses and the energy flowing downward from the corona. This energy flow is mainly in two forms: conductive heat flow and hydrogen ionization energy flow due to ambipolar diffusion. Hydrostatic equilibrium is assumed, and, in a first calculation, local mechanical heating and Joule heating are ignored. In a second model, some mechanical heating compatible with chromospheric energy-balance calculations is introduced. The models are computed for a partial non-LTE approach in which radiation departs strongly from LTE but particles depart from Maxwellian distributions only to first order. The results, which apply to cases where the magnetic field is either absent, or uniform and vertical, are compared with the observed Lyman lines and continuum from the average quiet sun. The approximate agreement suggests that this type of model can roughly explain the observed intensities in a physically meaningful way, assuming only a few free parameters specified as chromospheric boundary conditions. 44 refs.

Energy balance in the solar transition region. II - Effects of pressure and energy input on hydrostatic models

The radiation of energy by hydrogen lines and continua in hydrostatic energy-balance models of the transition region between the solar chromosphere and corona is studied using models which assume that mechanical or magnetic energy is dissipated in the hot corona and is then transported toward the chromosphere down the steep temperature gradient of the transition region. These models explain the average quiet sun and also the entire range of variability of the Ly-alpha lines. The relations between the downward energy flux, the pressure of the transition region, and the different hydrogen emission are described.

Energy Balance in the Solar Transition Region. IV. Hydrogen and Helium Mass Flows with Diffusion

In this paper we extend our previous modeling of energy balance in the chromosphere-corona transition region to cases with particle and mass flows. The cases considered here are quasi-steady and satisfy the momentum and energy balance equations in the transition region. We assume one-dimensional geometry and include the flow velocity terms in all equations, but we neglect the partial derivatives with respect to time. We present a complete and physically consistent formulation and method for solving the non-LTE and energy balance equations in these situations, including both particle diffusion and flows of H and He. Our calculations include partial frequency redistribution in the Lyα and Lyβ lines. Our results show quantitatively how mass flows affect the ionization and radiative losses of H and He, thereby affecting the structure and extent of the transition region. Furthermore, our computations show that the H and He line profiles are greatly affected by flows. We find that line shifts are much less important than the changes in line intensity and central reversal as a result of the influence of flows on the excitation and ionization. In this paper we use fixed conditions at the base of the transition region and in the underlying chromosphere. Our intent is to show the physical effects of flows on the transition region, not to match any particular observations. However, our computed Lyα profiles can account for the range of observed high spectral and spatial resolution from the quiet Sun. We suggest that dedicated modeling of specific sequences of observations based on physically consistent methods like those presented here will substantially improve our understanding of the energy balance in the chromosphere and corona.

Winds from T Tauri stars. I. Spherically symmetric models

Line fluxes and profiles are computed for a sequence of spherically symmetric T Tauri wind models. The calculations indicate that the H-alpha emission of T Tauri stars arises in an extended and probably turbulent circumstellar envelope at temperatures above about 8000 K. The models predict that Mg II resonance line emission should be strongly correlated with H-alpha fluxes; observed Mg II/H-alpha ratios are inconsistent with the models unless extinction corrections have been underestimated. The models predict that most of the Ca II resonance line and IR triplet emission arises in dense layers close to the star rather than in the wind. H-alpha emission levels suggest mass loss rates of about 10 to the -8th solar mass/yr for most T Tauri stars, in reasonable agreement with independent analysis of forbidden emission lines. These results should be useful for interpreting observed line profiles in terms of wind densities, temperatures, and velocity fields. 67 refs.

Mg I as a probe of the solar chromosphere - The atomic model

This paper presents a complete atomic model for Mg I line synthesis, where all the atomic parameters are based on recent experimental and theoretical data. It is shown how the computed profiles at 4571 A and 5173 A are influenced by the choice of these parameters and the number of levels included in the model atom. In addition, observed profiles of the 5173 A b2 line and theoretical profiles for comparison (based on a recent atmospheric model for the average quiet sun) are presented.

Formation of the Solar 10830 Å Line

One-dimensional hydrostatic-equilibrium models are shown here for faint, average, and bright components of the quiet Sun, and for a plage region, describing in each case how the atmosphere is stratified through the photosphere, chromosphere, and transition region up to a temperature of 105 K. The observed coronal line radiation is assumed to be the inward incident radiation at the 105 K boundary. This coronal radiation penetrates into the upper chromosphere causing sufficient helium ionization to populate the lower level of the He I 10830 A line, producing optically-thin absorption of the photospheric continuum at 10830A. The amount of absorption, which is proportional to the optical thickness of the upper chromosphere in the 10830 line, depends on 1) the strength of the coronal lines at wavelengths in the He I 504 A ionizing continuum, and 2) the density and geometrical thickness of the upper chromosphere. The computed 10830 A line is shown for the four atmospheric models and for three values of the coronal illumination. The calculated off-limb 10830 intensity distribution shows a minimum in the low chromosphere and a maximum at roughly 2000 km above the photosphere, in general agreement with observations, indicating that this is the predominant height of the transition region over most of the solar surface.

Formation of the infrared emission lines of Mg I in the solar atmosphere

A non-LTE radiative transfer investigation of the emission lines is conducted at 7 and 12 microns using a realistic atomic model for neutral magnesium. An average quiet sun atmospheric model is used to calculate emission-line profiles that resemble the observed ones, i.e., broad absorption troughs with narrow central emission, and significant limb brightening. The charge exchange rates are found to be significant, but the effects of high-n coupling between Mg and Mg(+) together with radiative low-n transitions are of greater importance. It is confirmed that the emission cores are formed no higher than the temperature minimum region, and that the emission is caused by non-LTE effects rather than by the chromospheric temperature rise. It is inferred from the model calculations that the line core is sensitive to magnetic fields located almost 400 km above those measured in ordinary magnetograms; the gas pressure decreases 20-fold between these two heights.

Chromospheric structure of cool carbon stars

A semiempirical chromospheric model is proposed for TX Psc which is a prototype for the N-type carbon stars. Observational data imply that the chromospheric temperature rise must begin at a low density, that the temperature gradient in the lower chromosphere must be steep, that partial redistribution must be employed in the Mg II calculation, and that the lower chromosphere is expanding away from the photosphere with a velocity of close to 50 km/s. The present model also shows that the microturbulent velocity is about 7 km/s at the temperature minimum region, dropping to 5 km/s in the chromosphere, and that the Lyman lines are optically thick in the chromosphere. 55 refs.