The Case Against Cold, Dark Chromospheres

Abstract

Is the solar chromosphere always hot, with relatively small temperature variations ($δT/T\sim0.1$); or is it cold most of the time, with temperature fluctuations that reach $δT/T\sim 10$ at the top of the chromosphere? Or, equivalently: Is the chromosphere heated continually, or only for a few seconds once every three minutes? Two types of empirical model, one essentially time independent and always hot, the other highly time dependent and mostly cold, come to fundamentally different conclusions. This paper analyzes the time-dependent model of the quiet, nonmagnetic chromosphere by Carlsson & Stein (1994: CS94) and shows that it predicts deep absorption lines, none of which is observed; intensity fluctuations in the Lyman continuum that are much larger than observed; and time-averaged emission that falls far short of the observed emission. The paper concludes that the solar chromosphere, while time dependent, is never cold and dark. The same conclusion applies for stellar chromospheres. A complete, time-dependent model of the nonmagnetic chromosphere must describe two phenomena: (1) dynamics, like that modeled by CS94 for chromospheric bright points but corrected for the geometrical properties of shocks propagating in an upward-expanding channel; and (2) the energetically more important general, sustained heating of the chromosphere, as described by current time-independent empirical models, but modified in the upper photosphere for the formation of molecular absorption lines of CO in a dynamical medium. This model is always hot and, except for absorption features caused by departures from local thermodynamic equilibrium, shows chromospheric lines only in emission.