Ionospheric Day‐to‐Day Variability Driven by Nonlinear Interactions Between the Terdiurnal Tide and Planetary Waves Under Magnetospheric Modulation

Abstract Day‐to‐day variability is a persistent feature of the ionosphere, but distinguishing contributions from the atmosphere below or geospace above remains unclear. Here, simultaneous observations of thermospheric meridional winds and ionospheric electron density from the Sanya Incoherent Scatter Radar (SYISR) reveal that the terdiurnal tide (TDT), a crucial agent in atmosphere‐ionosphere coupling, is strongly correlated with ionospheric F‐layer oscillations on a day‐to‐day basis. During a major sudden stratospheric warming in 2024, day‐to‐day variability of both the TDT amplitudes and F‐layer oscillations exhibited pronounced modulation by enhanced planetary waves (PWs). Bispectral analysis revealed nonlinear interactions between PWs, the TDT, and secondary waves at lower and upper sidebands. The ionospheric bispectrum showed peaks shifted toward shorter periods than the PWs themselves. Numerical simulations indicate that variations in magnetospheric forcing modify the ionospheric response to PW‐modulated tides. Our results clarify how the ionosphere responds to coupled lower atmospheric and geospace disturbances on daily timescales.