Summary
The polar upper atmosphere is receiving increasing interest as a natural laboratory to investigate processes and phenomena under extreme conditions, and to detect atmospheric change. During summer, temperatures in the high-latitude mesopause region (80–95 km altitude) are the coldest in any part of the Earth’s environment, reaching as low as ~120 K [1]. These low temperatures arise because of a general upwelling and resulting adiabatic cooling over the summer pole that offsets heating by solar radiation and turbulence. The upwelling is associated with a residual meridional circulation from the summer pole to the winter pole that is driven by the breaking of atmospheric gravity waves in the summer mesosphere [2]. Associated with the low summer mesopause temperatures is the formation of water-ice aerosols that are intimately associated with Polar Mesospheric Clouds (PMC; also known as noctilucent clouds) and Polar Mesosphere Summer Echoes (PMSE). There has been considerable interest in these phenomena owing to their likely sensitivity to long-term changes in temperature and trace gas concentrations in the upper atmosphere caused by human activities [3,4]. The relationship between PMC and PMSE has largely been investigated with ground-based lidars and radars at northern high latitudes due to the lack of suitable collocated instrumentation in the Southern Hemisphere. Over the last decade, improvements in ground-based and satellite instrumentation have started to give a clearer picture of conditions in the Antarctic summer mesopause, revealing evidence of inter-hemispheric differences in the climate and structure of the polar atmosphere [5,6,7]. In this paper we review the salient features of the PMC and PMSE phenomena, with emphasis on studies undertaken in the Southern Hemisphere. We also present new results from coordinated ground-based and satellite measurements undertaken during the first Southern Hemisphere season of the Aeronomy of Ice in the Mesosphere (AIM) mission.