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A strong Southern Hemisphere (SH) sudden stratospheric warming event occurred in September 2019 and significantly weakened the stratospheric polar vortex. Due to the positive zonal wind anomalies in the troposphere, the barotropic/baroclinic instability, primarily controlled by the horizontal/vertical wind shear, weakened in the upper troposphere at midlatitudes in late September and early October. As a result, planetary waves (PWs) were deflected equatorward near the tropopause rather than upward into the stratosphere, resulting in less perturbation to the stratospheric polar vortex. After October 15, the westward zonal wind anomalies propagate downward and reach the troposphere, increasing the tropospheric barotropic/baroclinic instability. This benefits the propagation of PWs into the stratosphere, leading to the early breaking of the stratospheric polar vortex. In turn, the SH mesosphere becomes anomalously cold due to the stratospheric wind filtering on the gravity waves, leading to the much earlier onset of SH polar mesospheric clouds.

Previous work suggested that the peak response time of the mass densities of atomic oxygen (O) and molecular nitrogen (N₂) in the thermosphere had more than a 1‐day difference relative to the peak of the 27‐day periodic variation of solar extreme ultraviolet (EUV) flux. In this study, we used the Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) to explore the physical mechanisms responsible for the different peak response times of the daytime thermospheric neutral species. It was found that the peak response time of O or N₂mass density corresponds to the time of equilibrium between the contributions from the barometric effect and the change in its abundance. The peak response time of O is shorter than that of thermospheric temperature Tn, due to a dynamic change in the circulation that acts to cancel out the contribution from the barometric process prior to the peak of Tn. On the contrary, the change of N₂abundance contributes further to a decrease of N₂mass density on a constant pressure surface when the thermosphere is expanding. The change of chemical loss leads to a longer peak response time of N₂abundance than that due to barometric motion. Therefore, an equilibrium is reached after the barometric effect turns from expansion (contraction) to contraction (expansion), so that the peak response time of N₂is longer than that of Tn. Moreover, the meridional circulation in the thermosphere modulates the latitudinal dependence of the peak response time of thermospheric neutral species.

It is well‐known that solar eclipses can significantly impact the ionosphere and thermosphere, but how an eclipse influences the magnetosphere‐ionosphere system is still unknown. Using a coupled magnetosphere‐ionosphere‐thermosphere model, we examined the impact on geospace of the northern polar‐region eclipse that occurred on June 10, 2021. The simulations reveal that the eclipse‐induced reduction in polar ionospheric conductivity causes large changes in field‐aligned current, cross‐polar cap potential and auroral activity. While such effects are expected in the northern hemisphere where solar obscuration occurred, they also occurred in the southern hemisphere through electrodynamic coupling. Eclipse‐induced changes in monoenergetic auroral precipitation differ significantly between the northern hemisphere and southern hemisphere while diffuse auroral precipitation is interhemispherically symmetric. This study demonstrates that the geospace response to a polar‐region solar eclipse is not limited just to the eclipse region but has global implications.

The effect of the Madden‐Julian Oscillation (MJO) on springtime Antarctic ozone variations is revealed for the first time from multi‐satellite reanalysis and model simulations. Twenty to 30 days after MJO Phase 8 (P8), Antarctic total column ozone (TCO) anomalies significantly decrease by up to −15 DU, associated with a wave‐1 response at around 60°S. After MJO P8, MJO‐related geopotential height anomalies in the southern hemispheric (SH) Indian Ocean emanate from subtropics to polar regions, leading to suppressed upward and poleward propagation of planetary waves (PWs) and weakened Brewer‐Dobson circulation in the SH stratosphere. This in turn results in less ozone transport from midlatitudes into the polar region and thus a negative polar TCO response. Dynamical transport plays a dominant role in modulating the Antarctic TCO after MJO P8. The magnitude of transient changes due to chemical processes is relatively weak than that caused by dynamical transport.