Solar transition region
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The solar transition region is a region of the
It is important because it is the site of several unrelated but important transitions in the physics of the solar atmosphere:- Below, gravity tends to dominate the shape of most features, so that the Sun may often be described in terms of layers and horizontal features (like sunspots); above, dynamic forces dominate the shape of most features, so that the transition region itself is not a well-defined layer at a particular altitude.
- Below, most of the helium is not fully ionized, so that it radiates energy very effectively; above, it becomes fully ionized. This has a profound effect on the equilibrium temperature (see below).
- Below, the material is opaque to the particular colors associated with of energy within the transition region very complicated.
- Below, electrodynamics.
Helium
corona: when solar material is cool enough that the helium within it is only partially ionized (i.e. retains one of its two electrons), the material cools by radiation very effectively via both black-body radiation and direct coupling to the helium Lyman continuum. This condition holds at the top of the chromosphere, where the equilibrium temperature is a few tens of thousands of kelvins
.
Applying slightly more heat causes the helium to ionize fully, at which point it ceases to couple well to the Lyman continuum and does not radiate nearly as effectively. The temperature jumps up rapidly to nearly one million kelvin, the temperature of the solar corona. This phenomenon is called the temperature catastrophe and is a
solar physicists
refer to the process as evaporation by analogy to the more familiar process with water. Likewise, if the amount of heat being applied to coronal material is slightly reduced, the material very rapidly cools down past the temperature catastrophe to around one hundred thousand kelvin, and is said to have condensed. The transition region consists of material at or around this temperature catastrophe.
See also
References
- ^ "The Transition Region". Solar Physics, NASA Marshall Space Flight Center. NASA.
- ISBN 978-0521382618.
External links
- Animated explanation of the Transition Region (and Chromosphere) (University of South Wales).
- Animated explanation of the temperature of the Transition Region (and Chromosphere) (University of South Wales).
