The Fabry-Perot Interferometer

The atomic oxygen lines O1D and O1S give optical emissions at 630.0 nm and 557.7 nm with peak intensities normally occurring around 240 km and between 90 and 130 km altitude respectively. These emissions arise both from airglow and auroral processes (Chamberlain, 1961). Precise measurements of the wavelengths of the emissions allow the Doppler shift, related to the source gas velocity, to be obtained. A very high resolution instrument is needed to detect the relatively small Doppler shift (a few fm). With a single etalon interferometer, it is possible to make these measurements only during the hours of darkness. At high latitudes in winter operation can extend to complete 24 hour periods, but the intensity of the emission can diminish to around 10-50 Rayleighs. Therefore a high sensitivity instrument is required. It is a combination of these two factors that make the Fabry-Perot interferometer (FPI) the most suitable instrument for thermospheric studies (Smith, 1980). Other requirements necessary for successful operation of an FPI in the Antarctic environment include a high degree of instrument stability, reliability, automation and a specialist housing.

In 1982, an FPI, constructed by Ulster Polytechnic and University College London, was installed at Halley. This instrument was operated for three years after which it was returned to the UK to undergo a complete refurbishment, which included improvements to the level of automation, and to the system sensitivity. The instrument was redeployed at Halley early in 1987, and has operated throughout the austral winters thereafter.

The FPI comprises five main optical components. They are the scanning mirror system, the etalon, the cassegrain telescope, the filter wheel and the imaging photon detector.

The prime function of the scanning mirror system is to steer the optic axis of the FPI to any chosen direction. For operation at Halley, the instrument views in the four cardinal directions at 30 elevation, and vertically. The scanning mirror also provides a means of reflecting light from a calibration lamp into the instrument. It has an efficiency of about 94%.

The Fabry-Perot etalon consists of a pair of quartz plates, 150 mm in diameter and separated by three 15 mm spacers. The inner surfaces which form the cavity are polished flat to better than ~3 nm rms, corresponding to /200 at 630 nm. Deposited onto these surfaces is a multi-layer dielectric coating which gives a peak reflectivity of 84% at 630 nm. Multiple reflections of monochromatic light between the two surfaces result in the emerging rays having a different phase shift and thus, when they are focused, a circular interference pattern is produced (fringes). The diameter of a given fringe is directly related to the wavelength of incident light. Although the etalon mount is designed to be thermally stable by careful selection of materials used, it is still necessary to maintain a high degree of thermal control to maximise the system performance. The air temperature surrounding the interferometer is controlled to within 1.5-2.0 C. A separate system keeps the etalon temperature constant to within 0.1 C.

The light from the etalon is focused by a 1 m focal length Cassegrain telescope onto the detector. Before reaching the latter, the light passes through a 1 nm bandwidth interference filter, which is used to select individual emission lines. Each filter has a transmission value of around 40% at peak wavelength. A filter selector allows a choice of one out of three 50 mm diameter filters to be placed immediately in front of the detector, with an additional shutter position to block all light (necessary for accurate calibration of the instrument).

A cooled imaging photon detector of very high sensitivity is used. This device has been developed at University College London, and is described in further detail by Rees et al. (1980, 1981) and McWhirter et al. (1982).

An RF-excited neon calibration lamp provides a spectral line close to the wavelength of the oxygen red line (630.4 nm). This is usually sampled at hourly intervals to provide a base-line from which the sky emission Doppler shifts can be measured.

All functions of the FPI are controlled by a PC which allows considerable flexibility in the recording schedule. The normal programme involves sequential observations at 630 nm in north, east, west, south, and zenith directions, followed by a calibration. The integration time for each position is approximately 5 minutes. The FPI is operated only when the solar zenith angle exceeds 99 .

Immediately after collection, the interference pattern image is reduced to a single radial profile. The position of the peak of the profile is then determined together with an estimate of the goodness of fit.

Each look direction provides a line-of-sight velocity measurement which contains a component of vertical and horizontal velocity. Two, or more, look directions can be combined to produce a neutral wind vector provided that the following assumptions hold:

In general these assumptions are believed to hold (Smith et al., 1982), but recent studies by Crickmore (1993) have shown that there are circumstances in the vicinity of the auroral oval where they may not.

Comparison of F-region observations from the FPI at Halley and output from a sophisticated global three-dimensional, time-dependent model of the thermosphere (Stewart et al., 1985) developed at UCL, London show a significant difference near 0800 UT. This discrepancy appears to arise because the model does not properly describe the geomagnetic field in the southern hemisphere. The neutral wind patterns in the vicinity of the auroral oval (Crickmore et al., 1991), and associated with the Harang discontinuity ( Stewart et al., 1988) have been determined.

Vertical motions in the E-region have also been studied with the Halley FPI using 557.7 nm observations. Surprisingly, the variations in vertical winds appear greater during geomagnetically quiet periods, than during active periods (Rodger and Stewart, 1990).

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