Lawrence E. Cram

Multi-component models for the formation of the chromospheric Caii K line

Recent high resolution observations of the Ca ii fine structure are discussed. An analytic method is applied to examine the effects of velocity fields on multi-component model atmospheres in which the central reversal in the bright components is due to self-absorption. It is shown that the inclusion of quite reasonable velocity fields permits the reproduction of not only the high resolution profiles of the small scale emission features but also the qualitative centre-limb behaviour of the spatially averaged profiles. The method is also used to examine models suggested by Pasachoff and others in which the double reversal is a statistical effect of singly peaked velocity shifted profiles. These models are shown to encounter severe difficulties near the limb.Recent high resolution observations of the Ca ii fine structure are discussed. An analytic method is applied to examine the effects of velocity fields on multi-component model atmospheres in which the central reversal in the bright components is due to self-absorption. It is shown that the inclusion of quite reasonable velocity fields permits the reproduction of not only the high resolution profiles of the small scale emission features but also the qualitative centre-limb behaviour of the spatially averaged profiles. The method is also used to examine models suggested by Pasachoff and others in which the double reversal is a statistical effect of singly peaked velocity shifted profiles. These models are shown to encounter severe difficulties near the limb.

Hydromagnetic waves in structured magnetic fields

Although the inhomogeneous nature of solar magnetic fields is now well established, most theoretical analyses of hydromagnetic wave propagation assume infinite homogeneous fields. Here we reformulate the hydromagnetic wave problem for magnetic fields which vary in one direction perpendicular to the field. The permitted modes of small amplitude hydromagnetic oscillations are considered, first in the case of a single interface between semi-infinite magnetic and non-magnetic compressible regions, and secondly for a magnetic flux sheath of given thickness imbedded in a nonmagnetic region. It is shown that, for small values of R (the ratio of the Alfvén to the sound speed), an acoustic or p-mode wave front passes through the flux sheath with only minor deformation. However, for large R, the transmitted acoustic wave is attenuated and, depending upon the thickness of the flux sheath and the angle of incidence, a hydromagnetic wave may be effectively trapped and guided along the flux sheath.It is also shown that, for the symmetric vibration of the flux sheath in the absence of incident acoustic waves, only slow mode type waves are permitted. Thus, in compressible regions for which R > 1 the Alfvénic-type fast mode is not a permitted mode of free vibration of a flux sheath.

Electrical conductivity in sunspots and the quiet photosphere

The electrical conductivity, thermal conductivity and viscosity of models of the quiet photosphere and the umbra of a sunspot have been calculated using LTE ionization equilibria and the Chapman-Enskog theory of transport coefficients. The results are presented in tabular form, and compared with previous calculations.

Determination of the temperature of the solar corona from the spectrum of the electron-scattering continuum

When the K-corona is formed by the scattering of photospheric radiation from free electrons, the Fraunhofer lines are greatly broadened by the thermal motions of the hot electrons. This paper discusses the possibility of measuring the coronal electron temperature from the residual depressions in the K-coronal spectrum. If the ratio of the intensities at 4100 Å and 3900 Å can be measured to an accuracy of ±1%, the coronal temperature can be inferred to an accuracy of ±0.2 MK. The temperature of a coronal inhomogeneity may also be measured by this method, provided the position angle is known.

Partially coherent scattering models for the formation of the chromospheric Ca ii K line

We discuss a model for the formation of the chromospheric Ca ii K line which does not make the usual assumption of complete redistribution. Using a physically reasonable scattering model, we find significant departures due to the frequency dependence of the line source function, particularly in the relative intensity and centre-to-limb behaviour of the K1 parts of the line and in the asymmetry produced by differential velocity fields. We conclude that the frequency dependence of the K line source function must be considered in quantitative models for the formation of the K line.

White-light radiation from semi-empirical flare models

We show that some recently published semi-empirical models for solar flares predict a significant flux of visible continuum radiation, due to bound-free radiation from hydrogen atoms and H− ions in the chromospheric flare. The ratio of the emergent intensity in one flare model to that in the quiet Sun is more than 100% at the head of the Balmer continuum for a flare close to the limb, and 8% at disk centre. The predicted flare spectrum has a relatively strong Balmer jump. We compare the theoretical flare continuum with observations and find disagreement in several important respects. The main disagreements are: (1) the fact that few flares are observed to emit a white-light continuum, while the models suggest that they should do so; (2) the prediction of a strong Balmer jump, which is not observed in most white-light flares; and (3) the absence of a ‘blue continuum’ in the theoretical prediction. We conclude that observations of flare continua provide useful constraints on semi-empirical models, and that at present the models do not satisfy these constraints.

Sacramento Peak Observatory

This article provides a brief summary of instrumental developments at Sacramento Peak Observatory over the past decade, and describes highlights of past and present staff research programs.

Inversion problems in radiative transfer theory: The Backus-Gilbert formalism

Abstract The method introduced by Backus and Gilbert to study geophysical inversion problems may be readily applied to the inversion problems of radiation transfer theory. Using the inversion of limb-darkening data as an example, we show how the method provides a quantitative estimate of the amount and reliability of the information that can be extracted from a given set of noisy data.

High resolution spectroscopy of the disk chromosphere

We compare temporal power spectra of solar atmospheric oscillations in plage and quiet Sun regions occurring on different parts of a time series of high-quality spectrograms. For periods shorter than ∼300 s, the oscillation amplitude in the photospheric and low chromospheric parts of the plage is reduced. There is a significant increase in long period power in the chromospheric plage. Our results provide no clear evidence that plages are heated by the dissipation of short-period waves.

Synthesis of emission line profiles from free-burning arcs

Abstract Theoretical emission line profiles have been evaluated by solving the radiative transfer equation and using physical parameters predicted by applying a two-dimensional model of a free-burning arc in argon at atmospheric pressure. Quantities describing the Stark shift and broadening of an ArI Line (λ763.5 nm) have been obtained by adjusting the theoretical profile to agree with a measured profile. A prominent absorption feature observed at the line center in the experimental profile is ascribed to absorption in a relatively cool layer surrounding the luminous arc column. There is strong evidence that this layer is not in LTE.