  Is the solar chromosphere always hot, with relatively small temperature
variations ($\delta T/T\sim0.1$); 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.
