|MadSci Network: Physics|
The physical processes in a gas discharge tube are quite complicated, but the phenomenon that you describe is explained by a rather simple mechanism which has to do with the so-called mean free path of electrons in the gas.
In the largest part of a gas discharge tube there is a small but constant electric field present which `pulls' electrons through the gas. These electrons interact with atoms, exciting their electron configurations and thus making them emit light. A single electron will lose most of its energy in such a collision and will be accelerated again by the field. The average distance over which the electron gets accelerated before it collides again is called its mean free path. Usually this mean free path is very short (sub-mm range) and there are no visual consequences because electrons get scattered in a random manner and on a small scale. But when the pressure is lowered further and further, the mean free path gets larger and larger, just because there are fewer and fewer atoms to collide with. At some point the mean free path is so large that you can actually see regions where no electron-atom interaction takes place - these are then the `dark' parts of the tube. Electrons travelling through such a region are accelerated by the field, but will not interact (on the average) with the atoms. But at some point - about one mean free path away from the last collision - they are just bound to interact, and they have enough energy to excite atoms, so this is where a `glowing region' will be present.
The important point is that the `breaking up' of the glow is a `collective phenomenon' because many electrons reach their `interaction point' at about the same distance from the electrodes. When the pressure is larger there are so many atoms around that electrons get accelerated and decelerated very randomly. At low pressure, scattering is very much reduced and electrons in a certain region of the tube get accelerated more or less `synchronously'. In a way, the presence of many atoms destroys the order that would take place with fewer atoms around.
Hope that helps,
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