Magnetospheric control of ionospheric TEC perturbations via whistler-mode and ULF waves

Radio signals experience group delay and phase advance as they transit the ionosphere filled with free electrons. The total number of these electrons along the raypath from the satellite to a receiver defines the total electron content (TEC) of the ionosphere. The ionosphere TEC is vital for space weather applications, including monitoring Global Navigation Satellite Systems (GNSS) scintillations and detecting natural hazards, making comprehensive understanding, modeling, and forecasting of TEC perturbations critical for modern society. While modulations of TEC perturbations (dTEC) in the auroral region have been observed to be associated with ultra-low-frequency (ULF) waves, their driving mechanisms remain unclear. Using fortuitously timed and positioned conjugate observations from the THEMIS spacecraft and a GPS receiver at Fairbanks, Alaska, we provide direct evidence that dTEC modulations are driven by magnetospheric precipitation due to electron scattering into the loss cone by ULF-modulated whistler-mode waves. Peak-to-peak dTEC amplitudes reached ~0.5 TECU with modulations spanning scales of ~5--80 km. The cross-correlation between modeled and observed dTEC reached ~0.8 during the conjugacy period but decreased outside of it. The amplitude spectra of whistler-mode waves and dTEC also matched closely from 1 mHz to tens of mHz during the conjugacy period but diverged outside of it. Our findings offer crucial insights that could improve physics-based TEC modeling and ultimately enhance TEC forecast capabilities.