On Spectroscopic Filling Factors and the Solar Transition Region

Abstract

The concept of spectroscopic Ðlling factors is examined by assuming that observable astrophysical plasmas are inhomogeneous and must therefore be described using functions f(n), the emission measure di †erential in electron density n. Using a formal deÐnition for the spectroscopic Ðlling factor and (f s) simple distribution functions, it is shown that (1) di †ers from the geometric Ðlling factor f unless the f s plasma is truly homogeneous (2) depends on the choice of line pairs for the same [f(n) \ f 0 d(n [ n 0)], f s f(n), (3) systematically underestimates f, and (4) departs more from f the broader the distribution f(n). f s f s Implications for the particular case of the unresolved solar transition region are discussed. A dynamic "" classical ÏÏ transition region model appears to satisfy observed properties, including tiny Ðlling factors (10~5È10~2) as well as Ðlamentary models. This work highlights the nonunique interpretation of spectral data in terms of unresolved plasma structure.