Fabry-Perot spectrometers

A glass dome housing the spectrometer sits in the roof at Davis.
The Fabry-Perot Spectrometer dome on the roof at Davis (Photo: Theo Davies)
The filter or 'etalon' at the heart of the Davis Fabry-Perot Spectrometer showing multiple reflections.

Fabry-Perot Spectrometers* are optical instruments that allow accurate measurements to be made of atmospheric winds and temperatures from the ground. The technique is based on making very precise measurements of small Doppler shifts of the light emitted by the aurora australis and atmospheric airglow. Everyone knows how the pitch (or frequency) of a car horn or siren changes when it passes you. This is because the sound waves appear to be slightly compressed when approaching (higher frequency) and slightly expanded when receding (lower frequency). The amount of shift depends on the speed of approach and recession. The same occurs for light waves too. We make measurements of these very small shifts to find wind speeds and temperatures in the atmosphere.

The principle of operation is to use two semi-reflecting glass plates, one above the other, separated by a few millimetres, to provide a very good 'colour filter' by multiple reflections and what is known as 'constructive and destructive interference of light'. This filter (called an 'etalon') only allows an extremely narrow range of wavelength (colour) of the auroral or airglow light to pass through to a CCD (charge coupled device) camera, which photographs a pattern of circular 'interference fringes'. Small shifts in the colour of the light (due to the Doppler shift) can be detected by measuring the change in diameter of the fringes. A new Fabry-Perot Spectrometer (FPS) was installed at Davis in 2003. The specrometer was built at La Trobe University in Australia and is operated remotely from La Trobe University by the Space Physics Group.

The thermosphere is the part of the atmosphere above about 100 km altitude, up to 500 to 1000 km or so, which is reaching out into space. The atmosphere throughout this range is extremely thin compared to ground level, but never-the-less it is in the 'front line' when radiation and particles from the Sun arrive at the Earth. Humans make use of this part of the atmosphere. Short-wave radio communications rely on 'bouncing' radio of signals off 'electrical layers' (the ionosphere) around 100 to 300 km above the Earth, in order to travel large distances. Many communication and other satellites are in orbit in the upper atmosphere at altitudes of 800-1000 km. Other satellites, such as those in the GPS constellation, transmit radio waves through the region and their signals can be disrupted causing, for example, errors in positioning and navigation. When the Sun gives off a burst of energy in a solar storm it can not only disrupt radio communication, but it also heats the upper atmosphere. A hotter, more expanded, atmosphere increases the frictional drag on satellites, and affects their orbits. Additionally, high-energy particles from the sun may damage sensitive spacecraft electronics. Knowledge of how the upper atmosphere behaves in both 'quiet' and 'active' times is of large practical importance in today's technological world.

Measurements of the winds and temperatures over Davis, and how they change, tells us how energy from the Sun is redistributed through the atmosphere. The diagram below shows the thermospheric wind field above Davis and Mawson on 8 April 2008. Yellow and green arrows indicate horizontal winds inferred from measurements by the Mawson (which was operational at that time) and Davis FPSs respectively, while red arrows indicate the average horizontal winds over the two stations. While the data in this figure show a relatively uniform wind field, these instruments allow scientists to measure small-scale spatial structures in the thermospheric wind and temperature fields, which often occur in the presence of auroral arcs. As there are few observing locations in the southern hemisphere compared to the north, observations from Australia's Antarctic stations are extremely important for testing global models of how winds and energy flow around the atmosphere.

*The Fabry-Perot Spectrometer at Davis is operated by La Trobe University. This work forms part of Australian Antarctic Science project 4130 “How do Antarctic space-weather disturbances propagate northward to influence ionospheric density and structure above Australia and the Southern Ocean?”, PI Dr. Mark Conde.