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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. 

Abstract In Vadas et al. (2024,https://doi.org/10.1029/2024ja032521), we modeled the atmospheric gravity waves (GWs) during 11–14 January 2016 using the HIAMCM, and found that the polar vortex jet generates medium to large‐scale, higher‐order GWs in the thermosphere. In this paper, we model the traveling ionospheric disturbances (TIDs) generated by these GWs using the HIAMCM‐SAMI3 and compare with ionospheric observations from ground‐based Global Navigation Satellite System (GNSS) receivers, Incoherent Scatter Radars (ISR) and the Super Dual Auroral Radar Network (SuperDARN). We find that medium to large‐scale TIDs are generated worldwide by the higher‐order GWs from this event. Many of the TIDs over Europe and Asia have concentric ring/arc‐like structure, and most of those over North/South America have planar wave structure and occur during the daytime. Those over North/South America propagate southward and are generated by higher‐order GWs from Europe/Asia which propagate over the Arctic. These latter TIDs can be misidentified as arising from geomagnetic forcing. We find that the higher‐order GWs that propagate to Africa and Brazil from Europe may aid in the formation of equatorial plasma bubbles (EPBs) there. We find that the simulated GWs, TIDs and EPBs agree with EISCAT, PFISR, GNSS, and SuperDARN measurements. We find that the higher‐order GWs are concentrated at N at 200 km, in agreement with GOCE and CHAMP data. Thus the polar vortex jet is important for generating TIDs in the northern winter ionosphere via multi‐step vertical coupling through GWs. 

Abstract This study investigates the ionospheric total electron content (TEC) responses in the 2‐D spatial domain and electron density variations in the 3‐D spatial domain during the annular solar eclipse on 14 October 2023, using ground‐based Global Navigation Satellite System (GNSS) observations, a novel TEC‐based ionospheric data assimilation system (TIDAS), ionosonde measurements, and satellite in situ data. The main results are summarized as follows: (a) The 2‐D TEC responses exhibited distinct latitudinal differences. The mid‐latitude ionosphere exhibited a more substantial TEC decrease of 25%–40% along with an extended recovery time of 3–4 hr. In contrast, the equatorial and low‐latitude ionosphere experienced a smaller TEC reduction of 10%–25% and a faster recovery time of 20–50 min. The minimal eclipse effect was observed near the northern equatorial ionization anomaly crest region. (b) The ionospheric electron density variations during the eclipse were effectively reconstructed by TIDAS data assimilation in the 3‐D domain, providing important altitude information with validity. (c) The ionospheric electron density variations showed a notable altitude‐dependent feature. The eclipse led to a substantial electron density reduction of 30%–50%, with the maximum depletion occurring around the ionospheric F2‐layer peak height (hmF2) of 250–350 km. The post‐eclipse recovery of electron density exhibited a relatively slower pace near the F2‐layer peak height than that at lower and higher altitudes. 

Abstract The high latitude ionospheric evolution of the May 10‐11, 2024, geomagnetic storm is investigated in terms of Total Electron Content and contextualized with Incoherent Scatter Radar and ionosonde observations. Substantial plasma lifting is observed within the initial Storm Enhanced Density plume with ionospheric peak heights increasing by 150–300 km, reaching levels of up to 630 km. Scintillation is observed within the cusp during the initial expansion phase of the storm, spreading across the auroral oval thereafter. Patch transport into the polar cap produces broad regions of scintillation that are rapidly cleared from the region after a strong Interplanetary Magnetic Field reversal at 2230UT. Strong heating and composition changes result in the complete absence of the F2‐layer on the eleventh, suffocating high latitude convection from dense plasma necessary for Tongue of Ionization and patch formation, ultimately resulting in a suppression of polar cap scintillation on the eleventh. 

Abstract This work investigates mid‐ and low‐latitude ionospheric disturbances over the American sector during a moderate but geo‐effective geomagnetic storm on 13–14 March 2022 (π‐Day storm), using ground‐based Global Navigation Satellite System total electron content data, ionosonde observations, and space‐borne measurements from the Global‐scale Observations of Limb and Disk (GOLD), Swarm, the Defense Meteorological Satellite Program (DMSP), and the Ionospheric Connection Explorer (ICON) satellites. Our results show that this modest but geo‐effective storm created a number of large ionospheric disturbances, especially the dynamic multi‐scale electron density gradient features in the storm main phase as follows: (a) The low‐latitude equatorial ionization anomaly (EIA) exhibited a dramatic storm‐time deformation and reformation, where the EIA crests evolved into a bright equatorial band for 1–2 hr and then quickly separated back into the typical double‐crest structure with a broad crest width and deep equatorial trough. (b) Strong equatorial plasma bubbles (EPBs) occurred with an abnormally high latitude/altitude extension, reaching the geomagnetic latitude of ∼30°, corresponding to an Apex height of 2,600 km above the dip equator. (c) The midlatitude ionosphere experienced a conspicuous storm‐enhanced density (SED) plume structure associated with the subauroral polarization stream (SAPS). This SED/SAPS feature showed an unusual temporal variation that intensified and diminished twice. These distinct mid‐ and low‐latitude ionospheric disturbances could be attributed to the storm‐time electrodynamic effect of electric field perturbation, along with contributions from neutral dynamics and thermospheric composition change. 

Given the existential threat of climate change, we urge the heliophysics scientific community to consider ways in which we might further contribute to global efforts to address climate change. Whole atmosphere studies reveal that climate change processes impact even the uppermost regions of the atmosphere. The heliophysics research community now has models spanning the surface through the upper thermosphere and a diversity of observational datasets of the middle and upper atmosphere that span multiple decades. These studies indicate that the middle and upper atmosphere provide multiple vertical footprints for climate change and thus can contribute to an understanding of whole atmosphere climate change processes in the complex atmosphereland- ocean system. This white paper outlines recommendations for expansion of long-term data sets; simulations of climate with whole atmosphere models; engagement in collaborations with the tropospheric research community; and exploration of the possibility of heliophysics contributions to climate assessment efforts. Additionally, we recommend education and outreach efforts to help members of the wider community become more knowledgeable about climate change; support for efforts to increase the diversity of the heliophysics science community; support for international collaborations, and climate mitigation measures that our science community can implement to reduce greenhouse gas emissions from our research, education, and outreach activities.