Polar Mesosphere Summer Echoes above Antarctica
In the summer of 2003–04, the first observations of Polar Mesosphere Summer Echoes (PMSE) above Antarctica were recorded using the Very High Frequency (VHF) radar at Davis.
PMSE are strong radar echoes associated with extremely cold temperatures – as low as -140°C – that occur in the ‘mesosphere’, some 50–92 km above the Earth, during summer. PMSE are also associated with the presence of increased water vapour content in the cold summer mesopause region. This water vapour originates from natural processes, such as the oxidation of methane, meteor ablations and volcanic eruptions, and anthropogenic emissions, such as rocket and space-shuttle exhaust gases. In fact, ‘noctilucent’ or night shining clouds, which are also associated with increased water vapour and lower temperatures in the mesosphere, were first observed in 1885, some two years after the spectacular eruption of Krakatoa and corresponding to the time needed to transport water vapour to the mesopause.
The seasonal occurrence of PMSE is dependent in part on a critical mesosphere temperature threshold (less than 150 K or -123°C). In 2004–05, observations of PMSE and noctilucent clouds will be undertaken at Davis using the VHF radar and LIDAR, to investigate the threshold temperature dependencies of each.
PMSE were first observed in 1979 using a VHF radar at Poker Flat, Alaska. The first observations of PMSE in the southern hemisphere were conducted at Machu Picchu base on King George Island during the summer of 1992–93. It has been suggested that southern hemisphere PMSE differ from their northern hemisphere counterparts, however, the first summer of PMSE observations at Davis revealed their intensity to be similar to those reported at equivalent northern hemisphere latitudes.
International interest is now focused on whether there is any measurable difference in summer mesopause temperatures between the northern and southern hemispheres. A proposed interhemispheric comparison of the properties of PMSE will contribute to this effort.
PMSE events provide a monitor of the background wind and a proxy for the temperature and water vapour content in the little-studied mesosphere. Research using the Davis VHF radar will make an important contribution to the long-term investigation of these phenomena and will add to the limited knowledge of polar mesosphere climatology; and thus to our capacity to detect climate change in the mesosphere.
Ray Morris, Damian Murphy, Andrew Klekociuk and John French
Ice, Oceans, Atmosphere and Climate Program,
Australian Antarctic Division