Radio Recombination Lines

A radio recombination line (RRL) in astrophysics is a spectral emission line produced by the transition of electrons between two energy levels in an atom, and with a rest frequency in the radio band of the electromagnetic spectrum. This effect is produced in ionized gas, driven by the recombination of ions with electrons, when the energy of the electron falls enough to be captured by the ion.

Source: Wikipedia — Radio Recombination Lines (CC BY-SA 4.0)