On Inferring the Properties of Dynamic Plasmas from Their Emitted Spectra: The Case of the Solar Transition Region

Abstract

We reexamine the issue of inferring physical properties of solar plasmas using EUV and UV observations. We focus on the question of whether one can determine if typical structures seen as bright in typical "transition-region" lines are formed in the thermal interface between the coronal and chromospheric plasmas. Since 1983, Feldman and colleagues have proposed, based upon Skylab and other data, that much of the transition-region emission is formed in so-called unresolved fine structures (UFS) that are magnetically and thermally disconnected from the corona. This has led others to consider theoretical models of the transition region that differ from classical models. We examine the evidence cited in support of the UFS picture, specifically by relaxing the implicit assumption of a static atmospheric structure. Noting that observational data alone do not contain the information necessary to infer essential properties of the emitting plasmas, we argue that additional information must be added through forward calculations using physical models. MHD models of coronal flux tubes are then examined with explicit assumptions and boundary conditions, not as an attempt to "fit" observed data, but in order to study the formation of emission lines in dynamically evolving plasmas that are unresolved in space and time. We show that incorrect conclusions can be drawn by applying reasonable and traditional diagnostic methods to spectral data when unresolved dynamic evolution of the emitting plasma is important but not accounted for. In the particular case of the transition region, we show that the UFS interpretation is not unique, and is likely to be incorrect in the presence of unresolved dynamics. Most or all of the evidence for UFS is amenable to a different, equally reasonable interpretation, in which the transition-region emission is at all times formed in the time-varying thermal interface between the corona and the chromosphere. This work is likely to be important for a wider range of astrophysical plasmas than simply in the solar transition region. At stake is our basic ability to correctly diagnose physical conditions of plasmas for which heating mechanisms are not yet understood, but which are likely to be time dependent.