This image is a schematic of a comet. The center part of the comet, or nucleus, is represented by the flame. The solar wind particles are shown as green dots with arrows. And the ionized particles are shown as green/red dots with arrows. Neutral particles are shown by the other dots (without arrows).
Click on image for full size
JPL
The Comet Coma
As the ices of the comet nucleus
evaporate, they expand rapidly into a large cloud around the central part of the comet.
This cloud, called the coma, is the atmosphere of the comet and can extend for millions of miles. The cloud is very thin, however, with only a 100 particles in a cubic centimeter. By comparison, a cloud in the Earth's atmosphere might contain millions of particles per cubic centimeter.
Just as in any atmosphere, there are electrically charged particles present, called ions, as well as electrically neutral particles. Ions are formed when the cloud of neutral particles collides with the solar wind. A continual stream of neutral particles is produced as long as the nucleus is evaporating, and these neutral particles are continually converted to ions. These ions are what help form the comet tail.
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