Depending on these wave interactions, coherent radio emission may be produced at the fundamental electron plasma frequency ( f p ; Equation 1) or its harmonic (2 f p ). Emission at f p is often referred to as fundamental plasma emission, while emission at 2 f p is called harmonic plasma emission. This distinction is important because the two types have different observed properties and imply different plasma conditions. For example, fundamental plasma emission exhibits a much larger circular polarization fraction and originates from plasma that is four times denser than harmonic plasma emission.