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June - LIDAR

June 2003 - Midwinter

Rothera Lidar Facility

During the 2002/2003 summer season a new atmospheric research laboratory was installed at Rothera. This facility houses a single piece of equipment, the Iron Boltzmann Temperature Lidar.

Lidar is an acronym for 'light detection and ranging' and is basically the same as a radar except that instead of using radio waves, lasers are used to 'fire' a pulse of light into the sky and this light is reflected off molecules and atoms in the atmosphere. The following is a summary of how our lidar works;

  1. Each laser emits a pulse of UV light. This is reflected off a 45° mirror and out through a window in the roof of the laboratory. This is defined as time = 0 (ie we start the stopwatch).
  2. Each pulse of light contains ~250×109 photons and each photon may collide with a molecule (eg nitrogen molecule) or an atom (eg argon) and be reflected back towards the ground (time = t1 in the Fig. 1).
  3. In the laboratory we use a 40cm reflective telescope to collect the light that comes back. These are the two white upright cylinders at the back of the room in Fig. 2. When a photon is detected we stop the stopwatch.
  4. A photon counting technique is used to determine the number of photons that come back and their time of arrival relative to time = 0 (time = 2×t1).
  5. Once we know how long each photon took to go up into the atmosphere and then back down again we can work out how far they went (as we know the speed of light) and thus the altitude of the molecule or atom they collided with in the first place.
  6. We collect back-reflected light for approximately 2 minutes and build up a picture of the density of the atmosphere as a function of altitude.
Principle of lidar operation - Click to enlarge
Figure 1
Lidar optical table - Click to enlarge
Figure 2

Above: Principle of lidar operation (left) and right: the lidar optical table while both lasers are running. The beam path from laser #1 has been marked in blue. Pictured is Xinzhao Chu from the University of Illinois.

The system at Rothera has an added complication in that we use two lasers. These are very precisely tuned to two different wavelengths which are resonant with two transitions in atomic iron. Using this system we have the ability to measure the temperature of a thin iron-rich layer that is only a couple of kilometres thick at approximately 90km high.

I'm but one person in a team of researchers at BAS and the University of Illinois working on measuring the temperature and dynamics of the middle atmosphere above Rothera with the lidar (as well as other atmospheric instruments that BAS operates here). The temperature structure of the northern hemisphere middle atmosphere at mid and high latitudes has been studied for a long time. However, due to the inaccessibility of Antarctica until recent times the equivalent southern hemisphere atmosphere is less well understood. The current lidar was operated at the South Pole for 2 years and data from this site and other lidars around the perimeter of Antarctica indicate that there are important differences between the northern and southern hemisphere middle atmospheres. By running this lidar system at Rothera we will be able to build on the body of knowledge that exists and so more fully understand the dynamics of the southern hemisphere atmosphere. Another aspect of the project is to study high altitude ice clouds called polar mesospheric clouds (PMC). Also called noctilucent clouds, these have been observed and studied in the high latitude northern hemisphere and present thinking indicates that these clouds could be an important pointer to global atmospheric changes such as the 'greenhouse effect'.

My principle job is to run the system, collect the data and make sure that the data being collected actually makes sense. It is often exciting but often very frustrating as well. Exciting; as I get to see what is happening high in the atmosphere in near real-time, especially activity in the iron layer that can vary on a scale of minutes. Frustrating; because two things are required to get quality data, a well performing system and clear skies. UV light is heavily absorbed by water vapour and even clouds that are invisible to the eye can affect the data. So whenever the sky is clear, day or night, I walk up the hill from the base to the laboratory to run the system. I don't have the worst job on base (Eric is uncontested for that title!) but it does mean that whenever the weather's good I've got to work. Still, as you can see from Fig 3, there's worse places to be....

The lidar lab - Click to enlarge

Above: The lidar lab looking towards the rest of the base and the Northern horizon.

Graeme Nott
Lidar Research Engineer