The Davis meteor radar
Shooting stars (also known as meteors) are a common example of the disintegration of interplanetary dust as it falls through our atmosphere. Earth is subject to a constant influx of many tonnes of this dust and it provides us with an opportunity to study the outer reaches of our atmosphere.
During the summer of 2004–2005, a 33.2 MHz meteor detection radar was added to the suite of middle-atmosphere observing instruments at Davis.
Meteor detection radars, such as the one supplied by Atmospheric Radar Systems, make it possible to measure components of the wind and temperature near the top of the middle atmosphere (85–95 km). They also allow us to learn more about the quantity of meteoric material that is entering that part of the atmosphere and the rate at which it is deposited.
The meteor detection radar contributes to the Australian Antarctic program in the Global System theme area by improving our knowledge of the polar middle atmosphere and allowing us separate long-term changes from the other short-term processes at play there. It is supported through the Antarctic Science Advisory Committee project entitled “Gravity wave drag parameterization in climate models” (project number 4025). Technical support at Davis is provided by Antarctica in the Global System program electronics engineers.
What the radar does
Dust particles and small rock fragments float in space near the earth’s orbit. Some of these are captured by the earth's gravity and fall rapidly into our atmosphere as meteors. As they fall, the friction between the atmosphere and the meteor creates a trail: a ‘shooting star’ in the case of the bigger fragments. Although short lived, these trails can be detected with a radar.
The weaker trails, that are due to small dust particles, are of interest to atmospheric physicists because they do not change the atmosphere much as they disintegrate. Instead they create a passive ‘target’ or tracer that a radar can detect. The target exists long enough for the radar to be able to trace the movement of air and the signal from the meteor decays in a way that tells us about the temperature and pressure. As in most remote sensing experiments, these little pieces of information are enough to build up a picture of what is going on in places we can’t easily get to: in this case, the outer reaches of our atmosphere.
Meteor radars continuously send radio pulses into the sky and look for returned signals with the characteristics of a meteor trail. These include a rapid rise in the returned signal from heights near 80–100 km, and a slower (but still rapid) decay back to the starting signal level. The trails last long enough for multiple pulses to be reflected from them and for its motion in the wind to be detected. And by measuring the time of arrival of the signal at five receiving antennas, the direction from which it came can be calculated. These yield a direction and wind speed at the height of reflection.
The meteor trail is most visible to the radar after its diameter is half of the transmitted wavelength (a characteristic of radio waves related to their frequency). The meteor trail expands from its dust particle sized beginning through this width in a manner that is dependent on the pressure and temperature. So the speed with which the meteor signal decays can be related to these atmospheric parameters at the height of the trail (again, between 80 and 100 km altitude).
Six antennas sit in Heidemann valley behind Davis and enable us to make meteor measurements. One is used to transmit pulses over a broad part of the sky and five (such as the one pictured here) are used for detecting the returned signal.
Radar reflections from meteor trails are strongest perpendicular to the trail. Because most meteors fall to earth at a shallow angle to the horizontal, meteor trails are generally detected a long way from overhead at zenith angles of about 30 to 40 degrees. The graph of echo directions of arrival shows each meteor detection as a dot.
The range, time of day and height of meteor echoes for the 33 MHz radar show most of the echoes come from near 90 km altitude. Using the rate of meteor detections, it is possible to learn about the flux of meteoric material into the earth’s atmosphere. This is of interest in studies of atmospheric composition as well as in meteor astronomy.
The rate at which meteors are detected is high enough to allow hourly determinations of the wind between 80 and 100 km. A plot of the wind speed and direction shows the speed and the direction of the wind (which corresponds to a compass dial with North to the top) as coloured arrows.