Nonthermal Electrostatic Plasma Modes in the Ionosphere

Nonthermal ion-acoustic and Langmuir modes are important to a variety of topics within the fields of ionospheric, magnetospheric, and space physics. Several mechanisms have been proposed to explain the origins of these electrostatic modes. Although more than one mechanism may be effective in enhancing one or both modes, only Langmuir turbulence driven by low-energy electron precipitation is well-supported by both observations and modeling as a mechanism for enhancing ion-acoustic and Langmuir modes simultaneously. We discuss the current understanding of naturally-enhanced electrostatic wave modes, Langmuir turbulence in the ionosphere, and the expected signatures of natural cavitating Langmuir turbulence; present observations of the radar spectra of up- and down-going Langmuir and ion-acoustic waves; and show related modeling results.