The Case against Cold, Dark Chromospheres
Abstract
Is the solar chromosphere always hot, with relatively small temperature variations (δT/T~0.1), or is it cold most of the time, with temperature fluctuations that reach δT/T~10 at the top of the chromosphere? Or, equivalently, is the chromosphere heated continually or only for a few seconds once every 3 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 and shows that it predicts deep absorption lines, none of which are 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 Carlsson & Stein 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2001
- DOI:
- 10.1086/321638
- arXiv:
- arXiv:astro-ph/0106072
- Bibcode:
- 2001ApJ...557..376K
- Keywords:
-
- Hydrodynamics;
- Shock Waves;
- Sun: Atmospheric Motions;
- Sun: Chromosphere;
- Sun: Infrared;
- Sun: UV Radiation;
- Astrophysics
- E-Print:
- 22 pages, 5 figures (in 6 files)