Detecting and Characterizing Geomagnetic Storm-Driven ULF Disturbances Beyond the Ionosphere with GNSS TEC

For decades, GNSS total electron content (TEC) has been widely used to study ionospheric disturbances. However, as TEC measurements are integrated electron densities along the receiver-satellite line-of-sight (LOS), they contain contributions from regions above the ionosphere. While the bulk of the integrated electron density is generally expected to be from the F region, perturbations at higher altitudes of sufficiently large amplitude, including shock-driven disturbances, can conceivably be embedded in the integrated measurement and may be sensed with a favorable LOS geometry. This creates an opportunity for investigating disturbances beyond the ionosphere, where conventional TEC processing steps, such as shell height assumption, the projection of measurements to ionospheric pierce points (IPPs), as well as transforming slant to vertical TEC (vTEC) via a mapping function, can obscure the true altitude and characterization of the electron density perturbations.

We present a new approach to study ultra-low frequency (ULF) disturbances occurring beyond the ionosphere with 3D TEC maps using global GNSS TEC observations, and apply this method to the storm sudden commencement (SSC) of the January 19, 2026 geomagnetic storm. The approach provides the opportunity to sample different regions of the plasmasphere and provide insight into the spatial location of its perturbations with the use of selected receiver-satellite LOS geometries. We also demonstrate that, by combining observations from specific LOSs, one can estimate the tailward azimuthal phase speed of ULF magnetosonic mode driven by the solar wind shock.

The results show that ground-based GNSS TEC, mainly associated with ionospheric structure and dynamics, can also reveal disturbances beyond the ionosphere that are not evident from standard TEC data processing and analysis approaches. This highlights GNSS TEC as a practical means for investigating such disturbances and shows how 3D TEC analysis can expand the use of existing GNSS networks for geospace coupling studies.