ON INFERRING THE PROPERTIES OF DYNAMIC PLASMAS FROM THEIR EMITTED SPECTRA - THE CASE OF THE SOLAR TRANSITION REGION

We reexamine the issue of inferring physical properties of solar plasm as using EUV and UV observations. We focus on the question of whether one can determine if typical structures seen as bright in typical ''tr ansition-region'' lines are formed in the thermal interface between th e coronal and chromospheric plasmas. Since 1983, Feldman and colleague s have proposed, based upon Skylab and other data, that much of the tr ansition-region emission is formed in so-called unresolved fine struct ures (UFS) that are magnetically and thermally disconnected from the c orona. This has led others to consider theoretical models of the trans ition region that differ from classical models. We examine the evidenc e cited in support of the UFS picture, specifically by relaxing the im plicit assumption of a static atmospheric structure. Noting that obser vational 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 expli cit assumptions and boundary conditions, not as an attempt to ''fit'' observed data, but in order to study the formation of emission lines i n 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 d ynamic evolution of the emitting plasma is important but not accounted for. In the particular case of the transition region, we show that th e UFS interpretation is not unique, and is likely to be incorrect in t he presence of unresolved dynamics. Most or all of the evidence for UF S is amenable to a different, equally reasonable interpretation, in wh ich the transition-region emission is at all times formed in the time- varying thermal interface between the corona and the chromosphere. Thi s work is likely to be important for a wider range of astrophysical pl asmas than simply in the solar transition region. At stake is our basi c ability to correctly diagnose physical conditions of plasmas for whi ch heating mechanisms are not yet understood, but which are likely to be time dependent.