Showing papers on "Solar transition region published in 1990"

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

Abstract: 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.

Turbulent thermal conduction in the solar transition region

Abstract: Emission measures E(T) derived from EUV observations of the lower solar transition region indicate that there is far more material in that temperature range than can be explained within the steady single flux-tube scenario if the classical energy transfer and loss mechanisms are adopted. This suggests that some mechanism may be operating which reduces the temperature gradient. The inclusion of a turbulent thermal conductivity kappa(T) = phi rho c(p)ul in a one-dimensional model is investigated, where u is the rms microvelocity in one direction, l is the mixing length, and phi is a constant of order unity. If it is assumed that that l is proportional to T exp-alpha, it is shown that near-perfect agreement with the observed emission measures is obtained for alpha = 3/2, both as regards the slope d log E/d log T equal to about -3 in the LTR, and the upturn at the top of this range. 27 refs.

The effect of nonequilibrium ionization on ultraviolet line shifts in the solar transition region

Abstract: The line profiles and wavelength positions of all the important emission lines due to carbon were computed for a variety of steady state siphon flow loop models. For the lines from the lower ionization states (C II-C IV) a preponderance of blueshifts was found, contrary to the observations. The lines from the higher ionization states can show either a net red- or blueshift, depending on the position of the loop on the solar disk. Similar results are expected for oxygen. It is concluded that the observed redshifts cannot be explained by the models proposed here.

Force and energy balance in the transition region network

Abstract: Two-dimensional numerical models of the solar transition region are calculated using an inverse coordinates method which attains pressure equilibrium between the network magnetic field and the external comparatively field-free gas. If A(y, z) is the magnetic potential (a scalar in 2D), which is constant on field lines, the method involves interchanging dependent and independent variables to obtain a quasi-linear PDE for y(A, z), which is solved iteratively. The advantage of this approach is that magnetic field lines, including any magnetic interface, become coordinate lines, thereby simplifying the energy equation and free boundary problem. In order to examine the effects of self-consistent geometry on the thermal structure of the transition region network, we calculate four models. The energy balance includes the effects of radiation, conduction, and enthalpy flux. It is confirmed that the lower branch of the emission measure curve cannot be explained within the single fluxtube model if the classical Spitzer thermal conductivity is used. However, by including a turbulent thermal conductivity as proposed by Cally (1990a), transition region models are obtained for which the resulting emission measure curves exhibit the correct behaviour, including the observed turn-up below about 200 000 K. In summary, the broad conclusions of previous non-turbulent 2D models are confirmed, but most importantly, the turbulent conductivity hypothesis tested in 1D by Cally is shown to produce excellent agreement with observations in the more realistic geometry.

Absolute flows in the solar transition region

Abstract: It is the objective of the present study to establish an absolute scale for flows in the solar transition region in observations obtained with the UVSP/SMM. By use of the polar limbs as reference one finds that the downflows range between 3 and 10 km s−1. The brighter regions show the largest downward flows.

The Transition of Supergiant CS Matter fom Cool Winds to Coronae — New Insights with X Aur Binary Systems

Abstract: The age of astronomical satellites gave us access to X-rays and UV spectroscopy. New challenging questions for astronomy have been raised, one of which is the transition from hot, corona-like matter to cool winds and chromospheres. Such a transition occurs in a region of the HR diagram which extends as a band from G super giants to K giants.