Is the solar chromosphere always hot , with relatively small temperature variations ( \delta T / T \sim 0.1 ) ; or is it cold most of the time , with temperature fluctuations that reach \delta T / 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 .