Ionospheric Disturbances and Scintillations Observations through CubeSats Coherent GNSS-Reflectometry Measurements

We present two case studies of traveling ionospheric disturbances (TIDs) triggered by the 2022 Tonga volcanic eruption and a case study of ionospheric scintillation observed by low-cost CubeSat-based global navigation satellite system reflectometry (GNSS-R) measurements. The GNSS-R data used in this work are from Spire Global CubeSats. Our analysis shows that coherent GNSS signals reflected over the ocean can be used to derive precise ionospheric total electron content (TEC) measurements. The first case shows clear TID structures with a TEC disturbance magnitude of ~1 TEC unit (TECu) and horizontal wavelength of ~330 km over Northwest Australia on the day of Tonga volcanic eruption. The second case shows the TIDs with ~0.05 TECu disturbance magnitude and horizontal wavelength of ~240 km over East Russia the day after the eruption. The second case is likely a TIDs propagated outward from Tonga for the second time after it traversed around the Earth. The GNSS-R observed TID may be associated with the incident or reflection signal ray path. In this paper, the appropriate ray path was identified using simultaneous observations from ground receiver networks in the areas of both ray paths.
For the scintillation case, a GNSS-R event with its specular point track over the sea south of Singapore contains sufficient coherent energy that enable the ionospheric scintillation observations. Clear phase fluctuations are shown at the ionospheric piercing points (IPP) located over the sea south of Singapore. The phase fluctuations are quantified through the phase scintillation index (σ_ϕ), and scintillation index (S4). Our analysis indicates that the magnitudes of the filtered phase fluctuation are ~18 cm on L2-band and ~11 cm on L1-band, respectively. The maximum values of σ_ϕ are ~6 cm and ~4 cm on L2-band and L1-band, respectively. The S4 values reach ~0.65 in this case. Validation through nearby Radio Occultation (RO) measurements from FORMOSAT-7/COSMIC-2 indicates that the GNSS-R observed high S4 value is likely a sporadic E layer event.