In this study, we use measurements from over 4,735 globally distributed Global Navigation Satellite System receivers to track the progression of traveling ionospheric disturbances (TIDs) associated with the 15 January 2022 Hunga Tonga‐Hunga Ha'apai submarine volcanic eruption. We identify two distinct Large Scale traveling ionospheric disturbances (LSTIDs) and several subsequent Medium Scale traveling ionospheric disturbances (MSTIDs) that propagate radially outward from the eruption site. Within 3,000 km of epicenter, LSTIDs of >1,600 km wavelengths are initially observed propagating at speeds of ∼950 and ∼555 ms⁻¹, before substantial slowing to ∼600 and ∼390 ms⁻¹, respectively. MSTIDs with speeds of 200–400 ms⁻¹ are observed for 6 hrs following eruption, the first of which comprises the dominant global ionospheric response and coincides with the atmospheric surface pressure disturbance associated with the eruption. These are the first results demonstrating the global impact of the Tonga eruption on the ionospheric state.

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.

We present here the Empirical Canadian High Arctic Ionospheric Model (E‐CHAIM) quiet N_mF₂, perturbation N_mF₂, and quiet h_mF₂ models. These models provide peak ionospheric characteristics for a domain above 50°N geomagnetic latitude. Model fitting is undertaken using all available ionosonde and radio occultation electron density data, constituting a data set of over 28 million observations. A comprehensive validation of the model is undertaken, and performance is compared to that of the International Reference Ionosphere (IRI). In the case of the quiet N_mF₂ model, the E‐CHAIM model provides a systematic improvement over the IRI Union Radio Scientifique Internationale maps. At all stations within the polar cap, we see drastic RMS error improvements over the IRI by up to 1.3 MHz in critical frequency (up to 60% in N_mF₂). These improvements occur primarily during equinox periods and at low solar activities, decreasing somewhat as one tends to lower latitudes. Qualitatively, the E‐CHAIM is capable of representing auroral enhancements in N_mF₂, as well as the location and extent of the main ionospheric trough, not reproduced by the IRI. The included N_mF₂ storm model demonstrates improvements over the IRI by up to 35% and over the quiet time E‐CHAIM model by up to 30%. In terms of h_mF₂, over the validation periods used in this study, we found overall RMS errors of ~13 km for E‐CHAIM, with IRI2007 overall h_mF₂ errors ranging between 16 km and 22 km. The E‐CHAIM performs comparably to or slightly better than the IRI within the polar cap; however, significant improvements are found within the auroral oval.

The analytical description of the topside ionosphere included in the NeQuick model is studied in detail. First, the modeled scale height behavior is analyzed at infinity and for the lowest part of the topside region; in the latter case, the analysis is done through an expansion in Taylor series near the F2-layer peak. Moreover, the significant influence of the three NeQuick topside parameters in the modeling of the topside profile is carefully investigated and, in particular, it is shown that for the lowest part of the topside the model assumes a linearly increasing trend of the topside scale height. Second, the topside formulation is inverted to derive a fully analytical expression of the topside scale height as a function of the electron density and F2-layer peak parameters. This expression has been applied to a selected and very reliable dataset of COSMIC/FORMOSAT-3 (Constellation Observing System for Meteorology, Ionosphere, and Climate) Radio Occultation profiles. Statistical analyses strongly support the hypothesis made by NeQuick regarding the linear trend of the topside scale height for the lowest part of the topside.

Monthly median values of ionospheric peak height (hmF₂) and density (NmF₂), derived from ionosonde measurements at four Canadian High Arctic Ionospheric Network (CHAIN) stations situated within the polar cap and Auroral Oval, are used to evaluate the performance of the International Reference Ionosphere (IRI) 2007 empirical ionospheric model during the recent solar minimum between 2008 and 2010. This analysis demonstrates notable differences between IRI and ionosonde NmF₂ diurnal and seasonal behavior over the entire period studied, where good agreement is found during summer periods but otherwise errors in excess of 50% were prevalent, particularly during equinox periods. hmF₂ is found to be marginally overestimated during winter and equinox nighttime, while also being underestimated during summer and equinox daytime by in excess of 25%. These errors are shown to be related to significant mismodeling of the M(3000)F₂ propagation factor. The ionospheric bottomside thickness parameter (B0) is also evaluated using ionosonde measurements. It is found that both of the IRI's internal B0 models significantly misrepresent both seasonal and diurnal variations in bottomside thickness when compared to ionosonde observations, where errors at times exceed 40%. A comparison is also presented between IRI and Resolute (74.75N, 265.00E) Advanced Modular Incoherent Scatter Radar (AMISR)‐derived topside thickness. It is found in this comparison that the IRI is capable of modeling ionospheric topside thickness exceptionally well during winter and summer periods but fails to represent significant diurnal variability during the equinoxes and seasonal variations.