The Case against Cold, Dark Chromospheres

Is the solar chromosphere always hot, with relatively small temperaturevariations ($\delta T/T\sim0.1$); or is it cold most of the time, withtemperature fluctuations that reach $\delta T/T\sim 10$ at the top of thechromosphere? Or, equivalently: Is the chromosphere heated continually, or onlyfor a few seconds once every three minutes? Two types of empirical model, oneessentially time independent and always hot, the other highly time dependentand mostly cold, come to fundamentally different conclusions. This paperanalyzes the time-dependent model of the quiet, nonmagnetic chromosphere byCarlsson & Stein (1994: CS94) and shows that it predicts deep absorption lines,none of which is observed; intensity fluctuations in the Lyman continuum thatare much larger than observed; and time-averaged emission that falls far shortof the observed emission. The paper concludes that the solar chromosphere,while time dependent, is never cold and dark. The same conclusion applies forstellar chromospheres. A complete, time-dependent model of the nonmagnetic chromosphere mustdescribe two phenomena: (1) dynamics, like that modeled by CS94 forchromospheric bright points but corrected for the geometrical properties ofshocks propagating in an upward-expanding channel; and (2) the energeticallymore important general, sustained heating of the chromosphere, as described bycurrent time-independent empirical models, but modified in the upperphotosphere for the formation of molecular absorption lines of CO in adynamical medium. This model is always hot and, except for absorption featurescaused by departures from local thermodynamic equilibrium, shows chromosphericlines only in emission.Comment: 22 pages, 5 figures (in 6 files