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 existing distribution of meteor radars located from high- to low-latitude regions provides a favorable temporal and spatial coverage for investigating the climatology of the global mesopause density. In this study, we report the climatology of the mesopause relative density estimated using multiyear observations from nine meteor radars, namely, the Davis Station (68.6^∘ S, 77.9^∘ E), Svalbard (78.3^∘ N, 16^∘ E) and Tromsø (69.6^∘ N, 19.2^∘ E) meteor radars located at high latitudes; the Mohe (53.5^∘ N, 122.3^∘ E), Beijing (40.3^∘ N, 116.2^∘ E), Mengcheng (33.4^∘ N, 116.6^∘ E) and Wuhan (30.5^∘ N, 114.6^∘ E) meteor radars located in the midlatitudes; and the Kunming (25.6^∘ N, 103.8^∘ E) and Darwin (12.3^∘ S, 130.8^∘ E) meteor radars located at low latitudes. The daily mean relative density was estimated using ambipolar diffusion coefficients derived from the meteor radars and temperatures from the Microwave Limb Sounder (MLS) on board the Aura satellite. The seasonal variations in the Davis Station meteor radar relative densities in the southern polar mesopause are mainly dominated by an annual oscillation (AO). The mesopause relative densities observed by the Svalbard and Tromsø meteor radars at high latitudes and the Mohe and Beijing meteor radars at high midlatitudes in the Northern Hemisphere show mainly an AO and a relatively weak semiannual oscillation (SAO). The mesopause relative densities observed by the Mengcheng and Wuhan meteor radars at lower midlatitudes and the Kunming and Darwin meteor radars at low latitudes show mainly an AO. The SAO is evident in the Northern Hemisphere, especially at high latitudes, and its largest amplitude, which is detected at the Tromsø meteor radar, is comparable to the AO amplitudes. These observations indicate that the mesopause relative densities over the southern and northern high latitudes exhibit a clear seasonal asymmetry. The maxima of the yearly variations in the mesopause relative densities display a clear latitudinal variation across the spring equinox as the latitude decreases; these latitudinal variation characteristics may be related to latitudinal changes influenced by gravity wave forcing. In addition to an AO, the mesopause relative densities over low latitudes also clearly show an intraseasonal variation with a periodicity of 30–60 d.

Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension simulations are used to investigate the solar migrating semidiurnal tide (SW2) around September equinox at middle to high latitudes in the Northern Hemisphere. A pronounced minimum in SW2 occurs around September equinox, and is characterized by a ∼50% reduction in tidal amplitudes for 20–30 days. Analysis of the simulation results indicates that the SW2 minimum occurs due to the seasonal transition of the zonal mean zonal winds, which alter the generation and propagation of different symmetric and antisymmetric modes of SW2. In particular, the antisymmetric modes notably decrease due to the more hemispherically symmetric zonal winds around equinox. It is further demonstrated that interannual variability in the timing of the SW2 minimum is related to the timing of the seasonal transition of the zonal mean zonal winds in the middle atmosphere. This leads to an earlier occurrence of the SW2 minimum during years when the seasonal transition occurs earlier, such as the recent 2019 September equinox which saw an earlier transition of the Southern Hemisphere zonal mean zonal winds following the occurrence of a sudden stratosphere warming. The connection between the timing of the SW2 minimum in the Northern Hemisphere and the timing of the seasonal transition in the middle atmosphere winds is confirmed by seasonal variability of 12‐h tides deduced from specular meteor radar observations at middle to high latitudes in the Northern Hemisphere.