Structure of the solar chromosphere. III. Models of the EUV brightness components of the quiet sun
TL;DR: In this paper, the solution of the non-LTE optically thick transfer equations for hydrogen, carbon, and other constituents to determine semi-empirical models for six components of the quiet solar chromosphere was investigated.
Abstract: The described investigation is concerned with the solution of the non-LTE optically thick transfer equations for hydrogen, carbon, and other constituents to determine semiempirical models for six components of the quiet solar chromosphere. For a given temperature-height distribution, the solution is obtained of the equations of statistical equilibrium, radiative transfer for lines and continua, and hydrostatic equilibrium to find the ionization and excitation conditions for each atomic constituent. The emergent spectrum is calculated, and a trial and error approach is used to adjust the temperature distribution so that the emergent spectrum is in best agreement with the observed one. The relationship between semiempirical models determined in this way and theoretical models based on radiative equilibrium is discussed by Avrett (1977). Harvard Skylab EUV observations are used to determine models for a number of quiet-sun regions.
Citations
More filters
TL;DR: In this paper, the authors study a class of metric-variation models that accelerates the expansion without a cosmological constant and satisfies both cosmologically and solar-system tests in the small-field limit of the parameter space.
Abstract: We study a class of metric-variation $f(R)$ models that accelerates the expansion without a cosmological constant and satisfies both cosmological and solar-system tests in the small-field limit of the parameter space. Solar-system tests alone place only weak bounds on these models, since the additional scalar degree of freedom is locked to the high-curvature general-relativistic prediction across more than 25 orders of magnitude in density, out through the solar corona. This agreement requires that the galactic halo be of sufficient extent to maintain the galaxy at high curvature in the presence of the low-curvature cosmological background. If the galactic halo and local environment in $f(R)$ models do not have substantially deeper potentials than expected in $\ensuremath{\Lambda}\mathrm{CDM}$, then cosmological field amplitudes $|{f}_{R}|\ensuremath{\gtrsim}{10}^{\ensuremath{-}6}$ will cause the galactic interior to evolve to low curvature during the acceleration epoch. Viability of large-deviation models therefore rests on the structure and evolution of the galactic halo, requiring cosmological simulations of $f(R)$ models, and not directly on solar-system tests. Even small deviations that conservatively satisfy both galactic and solar-system constraints can still be tested by future, percent-level measurements of the linear power spectrum, while they remain undetectable to cosmological-distance measures. Although we illustrate these effects in a specific class of models, the requirements on $f(R)$ are phrased in a nearly model-independent manner.
1,673 citations
TL;DR: An overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era, is presented in this paper, where the focus is on different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections.
Abstract: We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections We also discuss flare soft X-ray spectroscopy and the energetics of the process The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations
774 citations
Cites background from "Structure of the solar chromosphere..."
...…were around 1017 cm−2, then assuming a chromospheric slab of thickness 108 cm would require on average 3×1010 electrons cm−3 to be accelerated – on average about a tenth of the total electron population (bund or unbound) in the top 1000 km of the VAL-C chromospheric model (Vernazza et al. 1981)....
[...]
TL;DR: In this article, the authors review possible systematic errors in studies of F-, G- and K-type stars introduced by these questionable approximations and discuss the most vulnerable species and low-excitation transitions.
Abstract: ▪ Abstract The information on the chemical compositions of stars encoded in their spectra plays a central role in contemporary astrophysics. Stellar element abundances are, however, not observed: to decipher the spectral fingerprints in terms of abundances requires realistic models for the stellar atmospheres and the line-formation processes. Still today, the vast majority of abundance analyses of late-type stars rely on one-dimensional (1D), hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE). In this review possible systematic errors in studies of F-, G- and K-type stars introduced by these questionable approximations are discussed. Departures from LTE are commonplace and often quite severe, in particular for low surface gravities or metallicities, with minority species and low-excitation transitions being the most vulnerable. Recently, time-dependent, 3D, hydrodynamical model atmospheres have started to be employed for stellar abundance purposes, with large differe...
755 citations
TL;DR: In this article, a model of the plasma beta above an active region and its consequences in terms of coronal magnetic field modeling is presented. But the model is based on a collection of sources and the resulting β variation with height in the solar atmosphere is used to emphasize that the assumption that the magnetic pressure dominates over the plasma pressure must be carefully employed when extrapolating the magnetic field.
Abstract: In this paper, we present a model of the plasma beta above an active region and discuss its consequences in terms of coronal magnetic field modeling. The β-plasma model is representative and derived from a collection of sources. The resulting β variation with height in the solar atmosphere is used to emphasize that the assumption that the magnetic pressure dominates over the plasma pressure must be carefully employed when extrapolating the magnetic field. This paper points out (1) that the paradigm that the coronal magnetic field can be constructed from a force-free magnetic field must be used in the correct context, since the force-free region is sandwiched between two regions which have β>1, (2) that the chromospheric Mg ii–C iv magnetic measurements occur near the β-minimum, and (3) that, moving from the photosphere upwards, β can return to ∼1 at relatively low coronal heights, e.g., R∼1.2 R
s.
546 citations
TL;DR: In this article, a detailed examination of spectra of the upper solar atmosphere has confirmed the above results and proceeded to show that elemental abundances change from region to region by as much as an order of magnitude.
Abstract: Elemental abundances in the solar wind (SW) and in solar energetic particles (SEP) are different from abundances in the photosphere. A reassessment of spectroscopic abundance measurements from high temperature solar plasmas [1] showed that, indeed, on the average, the solar upper atmosphere possesses a composition which is similar in nature to the composition of the SW and SEPs. In the last few years a detailed examination of spectra of the upper solar atmosphere has confirmed the above results and proceeded to show that elemental abundances change from region to region by as much as an order of magnitude. An account of these changes, a description of the regions in which the changes occur, and a possible elemental enrichment model is given.
510 citations