\cite{Narayanan_2024}Gravity waves (GWs) play an important role in the dynamics and energetics of the mesosphere. Geomagnetic activity is a known source of GWs in the upper atmosphere. However, how deep the effects of geomagnetic activity induced GWs penetrate into the mesosphere remains an open question. We use temperature measurements from the SABER/TIMED instrument between 2002 - 2018 to study the variations of mesospheric GW activity following intense geomagnetic disturbances identified by AE and Dst indices. By considering several case studies, we show for the first time that the GWs forced by geomagnetic activity can propagate down to about 80 km in the high latitude mesosphere. Only regions above 55° latitudes show a clear response. The fraction of cases in which there is an unambiguous enhancement in GW activity following the onset of geomagnetic disturbance is smaller during summer than other seasons. Only about half of the events show an unambiguous increase in GW activity during non-summer periods and about one quarter of the events in summer show an enhancement in GWs. In addition, we also find that the high latitude mesopause is seen to descend in altitude following onset of geomagnetic activity in the non-summer high latitude region.

During winter, the latitude belt at 60S is one of the most intense hotspots of stratospheric gravity wave (GW) activity. However, producing accurate representations of GW dynamics in this region in numerical models has proved exceptionally challenging. One reason for this is that questions remain regarding the relative contributions of different orographic and non-orographic sources of GWs here. We use 3-D satellite GW observations from the Atmospheric InfraRed Sounder (AIRS) from winter 2012 in combination with the Gravity-wave Regional Or Global Ray Tracer (GROGRAT) to backwards ray trace GWs to their sources. We trace over 14.2 million rays, which allows us to investigate GW propagation and to produce systematic estimates of the relative contribution of orographic and non-orographic sources to the total observed stratospheric GW momentum flux in this region. We find that in winter 56% of momentum flux (MF) traces back to the ocean and 44% to land, despite land representing less than a quarter of the region’s area. This demonstrates that, while orographic sources contribute much more momentum flux per unit area, the large spatial extent of non-orographic sources leads to a higher overall contribution. The small islands of Kerguelen and South Georgia specifically contribute up to 1.6% and 0.7% of average monthly stratospheric MF, and the intermittency of these sources suggests that their short-timescale contribution is even higher. These results provide the important insights needed to significantly advance our knowledge of the atmospheric momentum budget in the Southern polar region.

The solar tides of the mesosphere and lower thermosphere (MLT) show great variability on timescales of days to years, with significant variability at interannual timescales. However, the nature and causes of this variability remain poorly understood. Here, we present measurements made over the interval 2005 to 2020 of the interannual variability of the 12-hour tide as measured at heights of 80 to 100 km by a meteor radar over Rothera (68S, 68W). We use a linear regression analysis to investigate correlations between the 12-hour tidal amplitudes and several climate indices, specifically the solar cycle (as measured by F10.7 solar flux), El Nino Southern Oscillation (ENSO), the Quasi-Biennial Oscillation (QBO) at 10 hPa and 30 hPa and the Southern Annular Mode (SAM). Our observations reveal that the 12-hour tide has a large amplitude and a clearly defined seasonal cycle with monthly mean values as large as 35 ms−1. We observe substantial interannual variability, exhibiting 2σ range in monthly mean 12-hour tidal amplitudes at the height of 95 km in spring of 13.4 ms−1, 11.2 ms−1 in summer, 18.6 ms−1 in autumn and 7.0 ms−1 in winter. We find that F10.7, QBO10, QBO30, SAM and time all have significant correlations to the 12-hour tidal amplitudes at the 95% level, with a linear trend also present. Whereas we detect very minimal correlation with ENSO. These results suggest that variations in F10.7, the QBO and SAM may contribute significantly to the interannual variability of 12-hour tidal amplitudes in the Antarctic MLT.