Radiation transfer problems in the rocket ultra-violet lines

Abstract

WE PRESENT here a summary of a current investigation of the radiative transfer problem in the solar corona, in its effect upon the excitation state of coronal ions. [Complete details of the investigation will be published elsewhere; C. PECKER and THOMAS”).] The investigation is one phase of a systematic treatment of the influence of ionic configuration and physical environment upon the excitation state of an ion. We have called this treatment the New Spectroscopy, to contrast it with the Local Thermodynamic Equilibrium treatment of a gas, where the excitation state of an ion depends only upon the local electron temperature, and is independent of ionic configuration or the physical environment. The situation usually assumed for the solar corona falls, conceptually, intermediate to the LTE situation and that of the chromososphere and photosphere. The coronal optical thickness is taken to be so small that any radiation field produced by the corona itself has negligible effect upon excitation of the ions. Consequently, the excitation state depends upon ionic configuration, but only locally upon physical environment. That is, the excitation state at a point depends only upon the values of electron density, electron temperature, and photospheric radiation field at the point. It does not depend upon the atmosphere as a whole-upon values of electron density and temperature at other points in the atmosphere-as it would were the optical thickness of the atmosphere large enough to produce significant self-emission. In the present work we have asked whether the assumption of negligible optical thickness for the corona is valid and, if not, what is the influence of coronal self-emission as a function of coronal optical thickness. We show that an upper limit on the effect of coronal self-emission in any transition comes from isolating the transition to solve the radiative transfer problem in it. That is, the actual atomic configuration is replaced by that of the 2-level-atom. Thus this upper limit does not depend upon ionic configuration, but only upon physical environment. An estimate of the largest optical depths likely to be encountered in resonance transitions of the most abundant ions falls below 104. At coronal densities, the ratio of collisional to radiative de-excitation, c, does not exceed 10-r. In this situation, the excitation effect due to coronal self-emission in some transition can be expressed as a function only of the coronal optical thickness in this transition, for an isothermal, constant-ns corona. Numerical results are given in the complete investigation cited. Here, we note only that a detection of the presence of a significant, optical depth in some line, by observing a self-reversed emission profile, appears to demand an optical thickness exceeding a value which lies between 10 and 100.