Rapid Conversion of Electromagnetic Waves to Electrostatic Waves in the Ionosphere

Thomson radar returns from the ionosphere subjected to a step-function excitation of electromagnetic waves show the immediate rise of electrostatic waves near the resonant height ${\omega }_p\simeq {\omega }_0$. The observed secular temporal variation $E\propto t$ agrees with a theory based on direct conversion of electromagnetic waves to electrostatic waves by preexisting density inhomogeneities. The rise time is at least an order of magnitude smaller than previously reported. The frequency spectrum shows this conversion is characterized by frequencies at the pump frequency ${\omega }_0$.