During the summer of 2002/2003, the AAD's Space and Atmospheric Physics group commissioned a VHF (Very High Frequency) atmospheric radar on the outskirts of Davis station in Antarctica. This is the first time a radar of this kind has been operated on the Antarctic continent and represents a great opportunity to help unravel the mysteries of the climate of the remotest parts of our atmosphere.
VHF radar with Davis station in the background. Photo by D. Murphy.
VHF radars (otherwise known as Mesosphere-Stratosphere-Troposphere or MST radars) are used to measure wind speed and direction over a wide range of heights in the middle atmosphere. The wind plays a critical role in the state of the atmosphere: It is the wind that makes the summer polar mesosphere (near 86 km in altitude) the coldest region of the earth's environment at around -130°C, and it is the spring-time wind that holds the ozone hole together.
The behaviour of the wind in the middle atmosphere is as varied as it is important. It changes from season to season, week to week and from day to day. Fine scale atmospheric waves can alter the wind dramatically in a few minutes or a few hours, or cause turbulence that leads to more chaotic behaviour, and on scales as big as the earth and as small as a few metres.
Despite the insight that measurements of a simple parameter like the wind can bring, the middle atmosphere is poorly understood because of the difficulty in making these measurements. A VHF radar can investigate the region below 20 km all year but must use technological tricks and seasonal phenomena to expand its capability. A persistent rain of meteoric dust can be detected between around 85 and 100 km and used by the VHF radar. And the unusual behaviour of the summer polar mesosphere can lend a hand.
Early atmospheric research using MST radars in the northern polar regions led to the discovery of an atmospheric phenomena called Polar Mesosphere Summer Echo (PMSE). It was found that the VHF radar return from this part of the atmosphere was much stronger than at other times of the year. The root cause is the extremely cold temperatures that exist in the summer polar mesosphere. Although the mechanisms of PMSE formation are part of an active field of research, PMSE are known to be related to ice crystal formations that can sometimes be observed as noctilucent clouds.
