Continuum Emissions from Ionized Gas
Thermal blackbody radiation is also emitted by gases. Plasmas are ionized gases and are considered to be a fourth state of matter, after the solid, liquid, and gaseous states. As a matter of fact, plasmas are the most common form of matter in the known universe (constituting up to 99% of it!) since they occur inside stars and in the interstellar gas. However, naturally occurring plasmas are relatively rare on Earth primarily because temperatures are seldom high enough to produce the necessary degree of ionization. The flash of a lightning bolt and the glow of the aurora borealis are examples of plasmas. But immediately beyond Earth’s atmosphere is the plasma comprising the Van Allen radiation belts and the solar wind.
An atom in a gas becomes ionized when another atom bombards it with sufficient energy to knock out an electron, thus leaving a positively charged ion and a negatively charged electron. Once separated, the charged particles tend to recombine with their opposites at a rate dependent on the density of the electrons. As the electron and ion accelerate toward one another, the electron emits electromagnetic energy. Again, the kinetic energy of the colliding atoms tends to separate them into electron and positive ion, making the process continue indefinitely. The gas will always have some proportion of neutral to ionized atoms.
As the charged particles move around, they can generate local concentrations of positive or negative charge, which gives rise to electric and magnetic fields. These fields affect the motion of other charged particles far away. Thus, elements of the ionized gas exert a force on one another even at large distances. An ionized gas becomes a plasma when enough of the atoms are ionized so that the gas exhibits collective behavior.
Whenever a vast quantity of free and oppositely charged ions coexist in a relatively small space, the combination of their reactions can add up to intense, continuous, wideband radio frequency radiation. Such conditions prevail around stars, nebulae, clusters of stars, and even planets Jupiter being at least one we know of.