Nonlinear Interaction of the Lunar Tide M₂ and the Diurnal Variation of Electron Density in the Ionosphere

Ground-based observations of time series of ionospheric electron density indicated the existence of spectral components at the periods of the lunar tidal constituents (Formula presented.) (25.82 hr) and (Formula presented.) (8.18 hr). It was unclear whether these variations are excited by (Formula presented.) and (Formula presented.) tides propagating from the surface into the ionosphere or if the variations are due to a nonlinear interaction of the semidiurnal lunar tide (Formula presented.) with the diurnal variation of ionization ((Formula presented.)). A simulation was performed with the NSF National Center for Atmospheric Research thermosphere-ionosphere-mesosphere electrodynamics general circulation model (TIME-GCM). In the stratosphere at the lower boundary of TIME-GCM, the signal of the atmospheric semidiurnal lunar tide (Formula presented.) is introduced. The TIME-GCM simulation of January–February 2009 shows that the electron density variation in the equatorial (Formula presented.) region contains not only a spectral component of the (Formula presented.) period but also spectral components of the (Formula presented.) and (Formula presented.) periods fulfilling the nonlinear interaction resonance condition for the frequencies of the wave triads ((Formula presented.) + (Formula presented.) = (Formula presented.)) and ((Formula presented.) − (Formula presented.) = (Formula presented.)). The nonlinear interaction resonance condition of the zonal wavenumbers of these wave triads is also fulfilled. Moreover, the relevance of nonlinear interaction of (Formula presented.) and (Formula presented.) is supported by observations of total electron content (TEC). The amplitude spectrum of the long-term time series of TEC shows that the quasi-diurnal and terdiurnal lunar spectral lines precisely occur at the frequencies of (Formula presented.) − (Formula presented.) and (Formula presented.) + (Formula presented.).