Eugene H. Avrett

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.

A new sunspot umbral model and its variation with the solar cycle

Semiempirical model atmospheres are presented for the darkest parts of large sunspot umbrae, regions have called umbral cores. The approach is based on general-purpose computational procedures that are applicable to different types of stellar atmospheres. It is shown that recent umbral intensity measurements of the spectral energy distribution may be accounted for by an umbral core atmospheric model that varies with time during the solar cycle; the observed center-limb variation can be accounted for by the properties of the model. Three umbral core models are presented, corresponding to the early, middle, and late phases of the solar cycle. These three models also may be regarded as having the properties of dark, average, and bright umbral cores respectively. The effects of atomic, opacity, and abundance data uncertainties on the model calculations are briefly discussed. For comparison, a new reference model for the average quiet solar photosphere is given. 94 references.

Calculation of Solar Irradiances. I. Synthesis of the Solar Spectrum

Variations in the total radiative output of the Sun as well as the detailed spectral irradiance are of interest to terrestrial and solar-stellar atmosphere studies. Recent observations provide measurements of spectral irradiance variations at wavelengths in the range 1100-8650 ? with improved accuracy, and correlative studies give procedures for estimating the spectral irradiance changes from solar activity records using indicators such as those derived from Ca II K and Mg II indices. Here we describe our approach to physical modeling of irradiance variations using seven semiempirical models to represent sunspots, plage, network, and quiet atmosphere. This paper gives methods and details, and some preliminary results of our synthesis of the variations of the entire irradiance spectrum. Our calculation uses object-oriented programming techniques that are very efficient and flexible. We compute at high spectral resolution the intensity as a function of wavelength and position on the disk for each of the structure types corresponding to our models. These calculations include three different approximations for the line source function: one suited for the very strong resonance lines where partial redistribution (PRD) is important, another for the most important nonresonance lines, and another approximation for the many narrow lines that are provided in Kuruczs listings. The image analysis and calculations of the irradiance variation as a function of time will be described in a later paper. This work provides an understanding of the sources of variability arising from solar-activity surface structures. We compute the Ly? irradiance to within 3% of the observed values. The difference between our computations and the Neckel & Labs data is 3% or less in the near-IR wavelengths at 8650 ?, and less than 1% in the red at 6080 ?. Near 4100 ? we overestimate the irradiance by 9%-19% because of opacity sources missing in our calculations. We also compute a solar cycle variability of 49% in the Ly? irradiance, which is very close to observed values. At wavelengths between 4100 ? and 1.6 ?m, we obtain spectral irradiance variations ranging from -0.06% to 0.46% in the visible?the higher values correspond to the presence of strong lines. The variability in the IR between 1.3 and 2.2 ?m is ~-0.15%.

Semiempirical models of chromospheric flare regions

Homogeneous plane-parallel semiempirical flare model atmospheres which reproduce observations in lines and continua of H I, Si I, C I, Ca II, and Mg II have a thin transition zone at the top of the enhanced chromosphere, indicating a significant amount of heating from the zone to the temperature minimum level. The minimum temperature is located deeper and is higher than in the quiet-sun and active-region models. The results do not agree with the particle-heated theoretical models, and it is suggested that the models of Brown (1973) and Henoux and Nakagawa (1977, 1978) do not include an essential term for heat conduction in their energy balance equations. It is concluded that substantial Ly-alpha radiative heating occurs in the upper chromosphere resulting from the conductive energy flux in the transition zone where the Ly-alpha line cools the gas.

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.

Semiempirical Models of the Solar Atmosphere. I. The Quiet- and Active Sun Photosphere at Moderate Resolution

Inthispaperwestudyandmodifyprevioussemiempiricalmodelsofthesolarphotosphereasobservedatmoderate spatial and temporal resolution (� 3 00 and � 30 minutes, respectively) in the main quiet- and active Sun component features. Our present models are constructed to match the relevant available observations at this resolution for which a one-dimensional and time-independent stratification is reasonable. The models do not describe the fine structure and temporal variability observed in high-resolution images but correspond to a ‘‘radiation averaging’’ over the finestructure andp-modevariations. Weuse theobservedlimbdarkeningin therange0.3‐2.4 � m,as wellasthe absolute intensities and details of the spectral continua and lines in this range, to validate and adjust the models. Using the methoddescribedinapreviouspaper,wecomputetheemergentradiationfromourmodelsinfulldetailforthevisible and IR continuum and the lines in the interval 0.3‐5 � mf or which we have atomic data from NIST (� 13,000 lines used) and molecular data from HITRAN and Gray & Corbally (� 480,000 molecular lines used). The observations, abundances, and atomic/molecular data are improved over previous work and yield models that better fit the observations. In addition, we construct a new penumbra model. The visible and IR detailed spectra computed from these models provide insight for understanding the effects of magnetic fields on the solar irradiance and are useful tools for computing synthetic spectral irradiances in different solar activity configurations.

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.

Radiative backwarming in white-light flares

We examine empirical atmospheric structures that are consistent with enhanced white-light continuum emission in solar flares. This continuum can be produced either by hydrogen bound-free emission in an enhanced region in the upper chromosphere, or by H- emission in an enhanced region around the temperature minimum. In the former case, weak Paschen jumps in the spectrum will be present, with the spectrum being dominated by a strong Balmer continuum, while in the latter case the spectrum exhibits a weaker, flat enhancement over the entire visible spectrum.We find that when proper account is taken of radiative backwarming processes, the two enhanced atmospheric regions above are not independent, in that irradiation by Balmer continuum photons from the upper chromosphere creates sufficient heating around the temperature minimum to account for the temperature enhancements there. Thus the problem of main phase white-light flare production reduces to one of creating temperature enhancements of order 104 K in the upper chromosphere; radiative backwarming then naturally accounts for the enhancements of order 100 K around the temperature minimum.Heating by electron and proton bombardment, and by XUV irradiation from above, are then considered as candidates for creating the necessary enhancements in the upper chromosphere. We find that electron bombardment can be ruled out, whereas bombardment by protons in the few-MeV energy range is a viable candidate, but one without strong observational support. The XUV irradiation hypothesis is examined by incorporating it self-consistently into the PANDORA radiative transfer algorithm used to construct the empirical model atmospheres; we find that the introduction of XUV radiation, with flux and spectrum appropriate to white-light flare events, does indeed produce sufficient radiative heating in the upper chromosphere to balance the radiative losses associated with the required temperature enhancements.In summary, we find that the radiative coupling of (i) the upper chromosphere and temperature minimum regions (through Balmer continuum photons) and (ii) the transition region and upper chromosphere (through XUV photons) can account for white-light emission in solar flares.

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.