Does a Non-magnetic Solar Chromosphere Exist?

Enhanced chromospheric emission which corresponds to an outwardly increasing semiempirical temperature structure can be produced by wave motion without any increase in the mean gas temperture. Hence, the sun may not have a classical chromosphere in magnetic eld free internetwork regions. Other signiicant diierences between the properties of dynamic and static atmospheres should be considered when analyzing chromospheric observations. Subject headings: hydrodynamics { radiative transfer { shock waves { Sun:chromosphere 1. INTRODUCTION Does the Sun have a chromosphere in magnetic eld free re-gions? We certainly observe enhanced emission over radiative equilibrium values, but does this mean the gas kinetic temperature increases outward above a temperature minimum? It must at some altitude since the existence of a million degree corona has long been known from the identiication of coronal emission lines as arising from highly ionized atoms. Initially, the existence of a chromosphere as the onset of this rise was inferred from the sharp increase in the emission scale height of lines and continua just above the limb of the Sun. Later on, starting with the analysis of the 1952 eclipse data by Thomas & Athay (1961) and others, the chromosphere was inferred from the increase in radiation temperature with increasing opacity in lines and continua. Since then, many one-dimensional hy-drostatic solar models have been constructed that reproduce the observed continuum and line intensities in an average sense (both temporally and spatially), with a chromospheric temperature rise starting at about 500 km above the photosphere. Familiar examples are the Vernazza, Avrett & Loeser (1981) models representing regions of diierent continuum intensity corresponding to dark cell interior to very bright network locations. However, spatially and temporally resolved observations of Ca ii (e.g. Lites et al. 1993), and recently of CO (Solanki et al. 1994, Uitenbroek et al. 1994) are consistent with a large fraction of the internetwork solar surface having no temperature rise in the rst 500 km above the location of the temperature minimum in the VAL models. We present results here that show the semi-empirical chro-mospheric temperature rise to be an artifact of temporal averaging of the highly non-linear UV Planck function. Also, di-agnosticians must be especially wary because dynamic eeects can drastically alter the heights at which the dominant contributions to emission occur. We brieey describe our calculations