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Abstract Hypotheses concerning processes related to medium‐scale traveling ionospheric disturbances (MSTIDs) are investigated with the application of models and the analysis of observational data. Wave‐packet parameters for MSTIDs from 2011 through 2022 are obtained from OI 6300 Å observations from the Boston University all‐sky imager (ASI) at the Millstone Hill Observatory during periods for which concurrent Millstone Hill (MH) incoherent scatter radar (ISR) observations are available. A combination of a numerical multi‐layer (NML) model for gravity waves (GW) in the thermosphere with the Field‐Line Interhemispheric Plasma (FLIP) model for ionospheric processes and upper‐atmospheric emissions is applied to generate perturbation electron‐density values, which are compared with ISR‐observed perturbation electron‐density values. A detailed comparison is made between model‐generated and ISR‐observed electron density for two cases, and the comparisons show notably good agreement. Twelve other MSTID cases are also described, giving a total of 14 cases. The results confirm that some nighttime MSTIDs at midlatitudes directly correspond to local GWs. They also suggest that some MSTIDs occurring over MH primarily consist of plasma fluctuations without corresponding local neutral fluctuations and that such MSTIDs are more common during winter months. The phase relationship between electron density and neutral vertical velocity variations is examined for two cases. Additionally, the hypothesis that standard thermospheric dynamic molecular viscosity values should be reduced is evaluated, and it is found that this is not supported by the results. 

Abstract This paper conducts a multi‐instrument and data assimilation analysis of the three‐dimensional ionospheric electron density responses to the total solar eclipse on 08 April 2024. The altitude‐resolved electron density variations over the continental US and adjacent regions are analyzed using the Millstone Hill incoherent scatter radar data, ionosonde observations, Swarm in situ measurements, and a novel TEC‐based ionospheric data assimilation system (TIDAS) with SAMI3 model as the background. The principal findings are summarized as follows: (a) The ionospheric hmF2 exhibited a slight enhancement in the initial phase of the eclipse, followed by a distinct reduction of 20–30 km in the recovery phase of the eclipse. The hmF2 in the umbra region showed a post‐eclipse fluctuation, characterized by wavelike perturbations of 10–25 km in magnitude and a period of 30 min. (b) There was a substantial reduction in ionospheric electron density of 20%–50% during the eclipse, with the maximum depletion observed in the F‐region around 200–250 km. The ionospheric electron density variation exhibited a significant altitude‐dependent feature, wherein the response time gradually delayed with increasing altitude. (c) The bottomside ionospheric electron density displayed an immediate reduction after local eclipse began, reaching maximum depletion 5–10 min after the maximum obscuration. In contrast, the topside ionospheric electron density showed a significantly delayed response, with maximum depletion occurring 1–2.5 hr after the peak obscuration.