The mesosphere/lower thermosphere (MLT, 80–100 km) region is an important boundary between Earth's atmosphere below and space above and may act as a sensitive indicator for anthropogenic climate change. Existing observational and modeling studies have shown the middle atmosphere and the MLT is cooling and contracting because of increasing greenhouse gas emissions. However, trend analyses are highly sensitive to the time periods covered, their length, and the measurement type and methodology used. We present for the first time the linear and 11‐year solar cycle responses in the meteor ablation altitude distributions observed by 12 meteor radars at different locations. Decreasing altitudes were seen at all latitudes (linear trends varying from −10.97 to −817.95 m dec ⁻¹ ), and a positive correlation with solar activity was seen for most locations. The divergence of responses at high latitudes indicates an important and complex interplay between atmospheric changes and dynamics at varying time scales.

This study analyzes strong sporadic E layer (Es) formation in Boa Vista (BV, 2.8°N, 60.7°W, dip: 18°), a low‐latitude region in the Brazilian sector, which occurred far after the onset of a magnetic storm recovery phase. Such occurrences were observed during seven magnetic storms with available data for BV. Thus, the ionospheric behavior on days around the magnetic storm that occurred on 20 January 2016 was investigated to search for possible explanations. This analysis indicated that the probable mechanism acting during the Es layer strengthening is the zonal westward electric field caused by a disturbance dynamo. The same evidence was also observed in two other magnetic storms at the same location. Hence, a numerical model of the E region dynamics, called MIRE (Portuguese acronym for E Region Ionospheric Model), was used to confirm whether the disturbance dynamo could cause the Es layer intensification. The inputs for the model were the electric field deduced from the vertical drift and the wind components provided by GSWM‐00 model. The simulations indicate that the Es layer density is significantly enhanced when the zonal electric field is present compared to the reference scenario with only the winds. Therefore, it is concluded that the disturbance dynamo electric field is the likely cause of the strong Es layers in the analyzed cases. Finally, the combined results from the model and observational data seem to contribute significantly to advance our understanding of the role of the electric fields in the Es layer formation at low latitudes.

The Hunga Tonga–Hunga Ha′apai volcano erupted on 15 January 2022, launching Lamb waves and gravity waves into the atmosphere. In this study, we present results using 13 globally distributed meteor radars and identify the volcanogenic gravity waves in the mesospheric/lower thermospheric winds. Leveraging the High-Altitude Mechanistic general Circulation Model (HIAMCM), we compare the global propagation of these gravity waves. We observed an eastward-propagating gravity wave packet with an observed phase speed of 240 ± 5.7 m s⁻¹ and a westward-propagating gravity wave with an observed phase speed of 166.5 ± 6.4 m s⁻¹. We identified these waves in HIAMCM and obtained very good agreement of the observed phase speeds of 239.5 ± 4.3 and 162.2 ± 6.1 m s⁻¹ for the eastward the westward waves, respectively. Considering that HIAMCM perturbations in the mesosphere/lower thermosphere were the result of the secondary waves generated by the dissipation of the primary gravity waves from the volcanic eruption, this affirms the importance of higher-order wave generation. Furthermore, based on meteor radar observations of the gravity wave propagation around the globe, we estimate the eruption time to be within 6 min of the nominal value of 15 January 2022 04:15 UTC, and we localized the volcanic eruption to be within 78 km relative to the World Geodetic System 84 coordinates of the volcano, confirming our estimates to be realistic.

The present work shows for the first time the study of morphology of the mesopause sodium (Na) and potassium (K) layers simultaneously observed by a dual‐beam LIDAR in the Southern Hemisphere. We analyze these two alkali metal layers from November 2016 to February 2019 measured at São José dos Campos (23.1°S, 45.9°W) and present their nocturnal and seasonal behavior. The night of 25 April 2017 was investigated as a representative of the vertical descending structure frequently observed in the metal density data at this location. We discuss here the chemical and dynamical contribution in the formation of these distinct layers, using simultaneous meteor radar wind and ionosonde data. These downward structures are seen in 66% of Na/K LIDAR data and seem to occur preferentially around April. The good agreement with diurnal tide and the presence of Es layer, suggest a combining mechanism in the formation of these structures. Moreover, semiannual variations are observed in both layers; however, they present different maxima location. The Na density presents its maxima around May and September; whereas, K density shows a strong maximum around July and a weaker one around December. Likewise, semiannual variation with maxima at the equinoxes is observed in the centroid height for both layers. However, the column abundances of these two metals show distinct seasonal variation: Annual variation peaking from one equinox to the other is observed in Na and semiannual with maxima in the solstices in K. The same behavior of centroid height for both layers indicates the same mechanism acting in the seasonal variation, which is not yet completely understood.