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This study investigates high-latitude ionospheric mesoscale irregularities associated with energetic particle precipitation and magnetosphere-ionosphere-thermosphere coupling processes within the auroral oval using ground-based Global Navigation Satellite Systems (GNSS) total electron content (TEC) measurements. This scale size is much larger than those associated with GNSS scintillations, which range from sub-kilometers (small scales) to > 10 km (large scales). Analyzing 15 yr of data from 2010 to 2024, we characterize, for the first time, the climatology of enhanced intensity of ionospheric mesoscale irregularities at high latitudes. The observed intensity of irregularities in GNSS TEC fluctuations can serve as a proxy for the dynamic behavior of the auroral oval which varies with magnetic local time, longitude, latitude, season, solar activity cycle, geomagnetic disturbances, and hemisphere. The spatial distribution of the irregularity is oval-shaped and therefore this pattern is named as “irregularity oval”; the morphology of the irregularity oval is generally aligned well with the known variations of auroral oval established by using other technologies. While the primary goal has been to document systematically these irregularity long-term observations, future work will focus on the development of a novel GNSS TEC-based “irregularity oval” model. 

The need for high precision measurements of vertical winds with uncertainties on the scale of 3–5 m ${\text{s}}^{-1}$ and a temporal cadence of 1–2 min to achieve detection of gravity wave (GW) structure has made it exceedingly difficult to study the response of the thermosphere to the propagation of GW activity. Herein we present subauroral, midlatitude thermospheric wind and temperature observations using redline 630 nm measurements obtained with a 15 cm narrow field Fabry-Pérot Interferometer (FPI), named the Hot Oxygen Doppler Imager (HODI). These measurements were obtained in a first light campaign at Jeffer Observatory ( $41.03°$ N, $74.83°$ W) located in Jenny Jump State Forest in northwestern New Jersey. The heightened sensitivity of HODI enables analysis of observations with uncertainties of approximately 3–5 m ${\text{s}}^{-1}$ for vertical wind speeds and 10–15 K for temperatures for 2-min exposures. Data was collected during periods of both geomagnetically quiet and active conditions, and GW structures were seen in both data sets. One detailed observation, taken the night of 25 July 2022, enabled the $\approx 90°$ phase shift between vertical winds and temperatures to be inferred, as per standard GW polarization relations with weak viscous dissipation. However, most other observations are found to have little correlation between the two series of temperature and vertical wind. We interpret this to be a result of the propagation and interaction of multiple GW events superimposed upon one another. Wave-like structures in the ionosphere observed in differential total electron count maps, or traveling ionospheric disturbances (TIDs), are often related to GW induced processes, and we provide comparisons of selected wave events observed by HODI to TIDs. These results suggest in a general sense that a relationship may exist between wave fluctuations seen in both the neutral atmosphere and the ionosphere. However, we suggest that the 35–70 km vertical extent of the 630 nm nightglow layer combined with an environment of multiple GW events with differing propagation speeds and vertical wavelengths may have the effect of diminishing or eliminating possible existing temperature and vertical wind correlation. 

Abstract The extraordinary eruption of the Tonga volcano on 15 January 2022 lofted material to heights exceeding 50 km, marking the highest observed since the satellite era. This eruption caused significant disturbances spanning from the hydrosphere up to the thermosphere. Our recent investigation discovered the dramatic thermospheric responses at satellite altitudes. This study, however, provides physical insights into two main possible processes, secondary gravity waves (GWs) and Lamb waves, which may explain those observed large‐scale thermospheric disturbances. The comparison between the simulations and observations suggests that the MESORAC‐HIAMCM secondary GWs are consistent with GRACE‐FO measured global‐propagation thermospheric density disturbances in timing and amplitude. WACCM‐X simulations suggest that the Lamb wave can reach the thermosphere as a sharp, narrow wave packet, and may contribute about 25% to the total disturbances at 510 km. 

Abstract Prior observational uncertainties have hindered the clear understanding of the link between tropospheric Lamb waves and ionospheric disturbances. In this study, we precisely extracted ionospheric Lamb waves originating from the epicenter of the 15 January 2022 Tonga eruption, propagating upward in a conical structure. This was achieved by using line‐of‐sight observations from the BeiDou geostationary satellites, which eliminated the spatiotemporal ambiguity introduced by the relative motion of Global Positioning System satellites, enabling the clear extraction of the Lamb signal in the ionosphere. The observed L0 mode speed (∼323 m/s) and period (∼30 min) were consistent with those of the tropospheric Lamb wave. It suggested that the ionospheric Lamb wave is likely driven by the surface Lamb wave, leading to a conical wave‐front that extends in altitude. This study highlights the significant role of Lamb waves in transmitting energy from epicenters through Earth's atmosphere and plasma systems. 

Abstract Atmospheric gravity waves (GWs) are believed to transport energy and momentum between different regions of the atmosphere. Historically, observations of these waves from both ground and space have been relatively abundant at altitudes up to the lower thermosphere, and somewhat less abundant in the upper thermosphere and F‐region ionosphere altitudes. Much of what is known of the typical properties and occurrence of these waves at thermospheric altitudes has been inferred from their impacts on the ionospheric density and motion, as direct observations of the neutral atmosphere have been less prevalent. Gravity waves in the middle thermosphere, from ∼120–200 km altitude, have rarely been observed directly and as such, their properties at these altitudes are less well documented. NASA's Global‐Scale Observations of the Limb and Disk (GOLD) mission makes observation of the middle thermosphere during daytime. During dedicated campaigns, GOLD has been able to observe GWs in this region. This study leverages 22 such campaigns during quiet geomagnetic conditions and low to moderate solar activity levels. Waves were observed with typical periods ∼2–4 hr. Leveraging ground‐based observations, the wavelengths were identified to be between ∼1,500–5,000 km, with phase speeds ∼150–600 m/s. The waves observed were seen to propagate primarily meridionally, in agreement with prior daytime mid‐latitude observations. Using observations of the background wind, the energy and momentum fluxes carried by these waves were found. During the quiet conditions observed, the waves were seen to transport energy flux over a wide range of latitudes. 

Abstract Based on the vertical Total Electron Content (TEC) data observed by the Global Navigation Satellite System in the northern hemisphere, a large area of low plasma density during summer at high latitudes, termed decreased TEC region, was investigated statistically between 2014 and 2024. Compared with the classical depleted structures that usually occur in the nighttime F region at high latitudes during winter, decreased TEC region is usually found in the sunlit polar cap ionosphere during summer. The decreased TEC region is predominantly located in regions above 70° magnetic latitude for moderate and high solar activity. The lower‐TEC region is biased towards the dawn and midnight sectors. Along the 18:25–06:25 Magnetic Local Time meridian, the depth of the decreased TEC region reached 7.6TECu in 2014. The decreased TEC region is deeper for higher Kp (Kp > 2) than for low Kp (Kp ≤ 2). 

Abstract. Six specialized radio receivers were developed to measure the Doppler shift of amplitude modulation (AM) broadcast radio carrier signals due to ionospheric effects. Five were deployed approximately in a circle at a one-hop distance from an 810 kHz clear-channel AM transmitter in Schenectady, New York, and the sixth was located close to the transmitter, providing a reference recording. Clear-channel AM signals from New York City and Connecticut were also received. The experiment confirmed detection of traveling ionospheric disturbances (TIDs) and measurement of their horizontal phase velocities through monitoring variations in the Doppler shift of reflected AM signals imparted by vertical motions of the ionosphere. Comparison of 12 events with simultaneous global navigation satellite system (GNSS)-based TID measurements showed generally good agreement between the two techniques slightly more than half the time and substantial differences slightly less than half the time, with differences attributable to differing sensitivities of the techniques to wave altitude and characteristics within a complex wave environment. Detected TIDs had mostly southward phase velocities, and in four cases they were associated with auroral disturbances that could plausibly be their sources. A purely automated software technique for event detection and phase velocity measurement was developed and applied to 1 year of data, revealing that AM Doppler sounding is much more effective when using transmitter signals in the upper part of the AM band (above 1 MHz) and demonstrating that the AM Doppler technique has promise to scale to large numbers of receivers covering continent-wide spatial scales. 

Abstract The strongest geomagnetic storm in the preceding two decades occurred in May 2024. Over these years, ground‐based observational capabilities have been significantly enhanced to monitor the ionospheric weather. Notably, the newly established Sanya incoherent scatter radar (SYISR) (Yue, Wan, Ning, & Jin, 2022,https://doi.org/10.1038/s41550‐022‐01684‐1), one of the critical infrastructures of the Chinese “Meridian Project,” provides multiple parameter measurements in the upper atmosphere at low latitudes over Asian longitudies. Unique ionospheric changes on superstorm day 11 May were first recorded by the SYISR experiments and the geostationary satellite (GEO) total electron content (TEC) network over the Asian sector. The electron density or TEC displayed wavelike structures rather than a regular diurnal pattern. Surprisingly, two humps, a common feature in the daytime equatorial ionization anomaly structure, disappeared. The SYISR observations revealed that multiple wind surges accompanied the downward phase propagation caused by atmospheric gravity waves (AGWs) originating from auroral zones. Meanwhile, strong upward and large downward drifts were respectively observed in the daytime and around sunset. The Thermosphere‐Ionosphere Electrodynamics Global Circulation Model (TIEGCM) simulations demonstrated that abnormal ionospheric changes were attributed to meridional wind disturbances associated with AGWs and recurrent penetration electric fields corresponding to largerB_zsouthward excursions and disturbance dynamo. The complicated interplay between AGWs and disturbance electric fields contributed to this unique ionospheric variation. 

Abstract The global 3‐dimensional structure of the concentric traveling ionospheric disturbances (CTIDs) triggered by 2022 Tonga volcano was reconstructed by using the 3‐dimensional computerized ionospheric tomography (3DCIT) technique and extensive global navigation satellite system (GNSS) observations. This study provides the first estimation of the CTIDs vertical wavelengths, ∼736 km, which was much larger than the gravity wave (GW) vertical wavelength, 240–400 km, estimated using ICON neutral wind observations. Notable trend with the variation of azimuth was also found in horizontal speeds at 200 and 500 km altitudes and differences between them. These results imply that (a) the global propagation of Lamb waves determined the arrival time of local ionospheric disturbances, and (b) the arriving Lamb waves caused vertical atmospheric perturbations that are not typical of GWs, resulting in local thermospheric horizontal wave propagation which is faster than the Lamb wave propagation at lower altitudes. 

Abstract Medium‐Scale Traveling Ionospheric Disturbances (MSTIDs) have long been a subject of interest in ionospheric research. However, their spatiotemporal variability across regions, local times, seasons, and solar cycles is very complicated and remains not well established. Using Total Electron Content (TEC) data from global GNSS receiver networks processed at MIT Haystack Observatory, we perform a detailed statistical analysis of MSTIDs over the Continental US (CONUS). Differential TEC data every day from 2012 to 2023 are processed using a keogram‐based image processing technique to identify MSTID wave properties, including the occurrence, propagation direction, phase speed, wavelength, and period. Focusing on eastern US midlatitudes (80°W, 40°N), we extend comparisons longitudinally and latitudinally across CONUS. Our results reveal significant variability in MSTID occurrence rates and propagation directions, notably linked to solar terminators. MSTID occurrence peaks after summer sunrise (with minor maxima near winter daytime), around summer sunset, and after summer midnight. Occurrence generally correlates positively with solar activity in summer but can become negative after winter midnight. In winter, MSTIDs propagate southeastward in the morning and rotate clockwise to west‐northwestward after midnight; in summer, propagation is more variable. Comparisons across the CONUS highlight strong regional differences. Our findings reflect complex drivers behind MSTIDs, including gravity waves, electrodynamic processes, and solar terminators. Their relative influences vary with local time, season, and location. This long‐term analysis provides critical insights into MSTID climatology and forms a basis for in‐depth investigations of MSTID generation mechanisms.