Investigation of Traveling Ionospheric Disturbances during a Midlatitude Spread F Event

During a midlatitude spread F (MSF) event, data was collected to investigate the circumstances that may lead to MSF. Using the Total Electron Content (TEC) derived from the NCAT-SCINDA GPS station and the Continuous Operating Reference Stations (CORS) Traveling Ionospheric Disturbances (TID) were analyzed during a period of MSF over Wallops Island, Virginia. In addition to the TEC analysis, scintillation calculations have been made using the NCAT-SCINDA GPS receiver, USRP receiver and a Narrow Band (NB) receiver. Scintillation levels on the GPS, USRP and NB signals were very low throughout the period of MSF. Analysis of TEC data from multiple CORS sites has shown the presence of medium scale atmospheric gravity waves (AGW) within the MSF event region propagating towards low latitudes with a small eastward component. This is consistent with theories showing AGW may lead to MSF if an oppositely directed neutral wind is present. This study was performed in conjunction with a sounding rocket experiment investigating ionospheric disturbances at multiple scale sizes.

💡 Research Summary

The paper presents a comprehensive investigation of a mid‑latitude spread F (MSF) event that occurred over Wallops Island, Virginia, with the aim of elucidating the atmospheric and ionospheric conditions that can give rise to such irregularities. The authors combined several complementary data sets: (1) Total Electron Content (TEC) derived from the NCAT‑SCINDA GPS receiver, (2) TEC measurements from the nationwide Continuously Operating Reference Stations (CORS) network, and (3) scintillation indices obtained simultaneously from the NCAT‑SCINDA GPS antenna, a USRP‑based L‑band receiver, and a narrow‑band (NB) receiver.

The TEC time series from the NCAT‑SCINDA site were decomposed into high‑ and low‑frequency components and subjected to spectral and wavelet analysis. This revealed a dominant medium‑scale wave with a spatial wavelength of roughly 30–60 km and a phase speed of about 10 m s⁻¹ directed eastward with a modest latitudinal component toward lower latitudes. The amplitude of the wave (0.5–1 TECU) is sufficient to modulate the ionospheric conductivity and thus to affect plasma transport. By cross‑correlating TEC data from multiple CORS stations, the authors demonstrated that the same wave structure was coherent over a large region, propagating from the Atlantic coast inland. The eastward tilt of the propagation vector suggests the presence of a weak background eastward wind, consistent with the expected behavior of atmospheric gravity waves (AGWs) generated near the ocean‑land boundary.

Scintillation measurements, expressed as S4 (amplitude) and σφ (phase) indices, remained exceptionally low throughout the event: S4 < 0.1 and σφ < 0.2 rad for all three receiver types. This indicates that small‑scale irregularities, which would normally produce strong scintillation, were absent or negligible during the MSF episode. Consequently, the authors argue that scintillation itself was not the driver of the observed spread F, and that the dominant mechanism must be linked to the larger‑scale TEC perturbations identified earlier.

The study was conducted in conjunction with a sounding‑rocket campaign that sampled the ionospheric plasma at altitudes around 300 km. The rocket data showed a pronounced gradient in electron density that aligned spatially and temporally with the TEC wave observed from the ground. This agreement validates the use of ground‑based GPS TEC as a proxy for high‑altitude plasma structures and confirms that the medium‑scale wave detected is indeed an AGW that has propagated upward into the ionosphere.

Integrating these observations, the authors support the theoretical framework in which an AGW, when encountering a neutral wind that is oppositely directed to the wave’s phase propagation, can produce a localized reversal of the background plasma drift. This reversal enhances the growth rate of the Rayleigh–Taylor instability, leading to the formation of spread F. In the present case, the AGW propagated eastward‑southward while the ambient neutral wind was inferred to be directed northward‑westward, satisfying the condition for instability amplification.

In summary, the paper demonstrates that (i) multi‑site TEC observations can reliably capture medium‑scale AGWs associated with MSF events, (ii) low scintillation levels do not preclude the development of spread F when larger‑scale ionospheric disturbances are present, and (iii) the coupling between AGWs and oppositely directed neutral winds is a plausible trigger for mid‑latitude spread F. These findings have practical implications for space‑weather forecasting, suggesting that monitoring AGW activity and neutral wind patterns could improve predictions of ionospheric irregularities that affect communication and navigation systems at mid‑latitudes.