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Mount John Fabry-Perot


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Instrument/Model Description

The Fabry-Perot Interferometric Spectrometer at Mount John Observatory, New Zealand (43.98S, 170.42E) has been operated since 1991 by the Graduate Program in Geophysics, University of Washington, in cooperation with the Physics and Astronomy Department of the University of Canterbury, NZ, and the Geophysical Institute of the University of Alaska.

The dispersing element of the spectrometer is an air-spaced, 13 cm diameter effective clear-aperture Fabry-Perot interferometer, which is both self-aligning and self-stabilizing. It is operated at the optimum operational point for kinetic temperature determinations (Hernandez, 1988).

The spectrometer operates simultaneously at two wavelengths, which are arbitrarily selected by the use of dichroic mirrors and narrow (0.2 nm wide) interference filters. The inherent stability of the spectrometer is about 0.5 m/s (632.8 nm) for periods of months, because of its self-stabilizing properties. The instrumental internal stability calibration is updated every 9 s.

The spectrometer wavelengths used at Mount John can be the combination of any two of the following:

  1. The red line (630.0 nm, kindat=17001) of atomic oxygen (OI) with a typical emission height peak in the range 210 to 300 km.
  2. The green line (557.7345 nm, kindat=17002) of atomic oxygen (OI) has an emission height peak range near 94-98 km. However, if an aurora is above the 'normal', chemically generated, emission near 96 km, then the temperatures will be larger since the auroral emission altitudes are higher, and the relative emissions will also be much larger. Instrumental start-up difficulties show high relative emissions and relatively high temperatures for Feb-Apr 1991, although winds are fine.
  3. The [OH] line (840.0 nm, kindat=17004) of the nightglow excited hydroxyl [OH*] with an emission peak between about 87 and 91 km. Had start-up difficulties so temperature and relative emission are large Feb-April, 1991, but winds are OK.
  4. The molecular oxygen [O2] Atmospheric Band lines near 866.0 nm (kindat=17007). Emission peak lies in-between the [OH] and OI(557.7nm) emissions, or around 91-94 km.

The spectrometer observes at the 4 cardinal directions at 20-degree elevation above the horizon, as well as zenith during nighttime. Since the instrument is light-limited, the time spent in observing this 5 direction cycle can be as short as 12 minutes and the instrument is internally time-limited to spend no more than 15 minutes in any given direction. Other observing protocols, such as two orthogonal directions and zenith, have also been used. The observations are made every evening and only those with clear weather -as reported by the astronomical personnel performing their observations on site- are reported. Because of the narrow filters used, operation of the instrument is not affected by moonlight. Typically, about 35% of the nights observed are clear.

Doppler shifts -winds- are determined from the displacement of the line profile relative to the long-term zenith observations, which are considered to have no long-term vertical Doppler shift. (Long-term is defined as months for continuing observations.) Further, the determined line-of-sight (los) winds are converted to horizontal winds using a spherical Earth and presuming that vertical winds are negligible. The reported winds are horizontal and vertical. The original azimuth and elevation angles are also provided, from which the geographic separation between the observing location and the ground projection of intersection at the airglow observation height can be obtained.

The temperatures are determined based on the instrumental measurements of single-wavelength laser profiles and measured instrumental parameters, such as the reflectivity. The reduction is a least-squares deconvolution in the Fourier plane (Hernandez, 1988) for single-line spectra, and steepest-descent techniques for multiple-line spectra (Conner et al, 1993). Although single-line spectra can also be reduced by the steepest-descent techniques, the Fourier deconvolution is much faster and robust. Finally, the measurements reported here have been made with a 2.0 cm air-spaced etalon.

Summarizing, the reported measurements are horizontal and vertical winds and kinetic temperatures. The time between successive measurements is light-limited and has been arbitrarily set such that OI 557.7 nm emission measurement uncertainties (for example) do not exceed about 10 K and 5 m/s respectively for temperature and winds. Emission rates are reported as counts normalized for unit time. They are not calibrated, and are given as base-10 logarithm (relative emission rate) * 1000.

The 'errors' given in the data are uncertainties of measurement, that is the statistically estimated effect that noise in the measurement will cause in the final result. This noise is inherent to the signal, since photons obey Bose-Einstein statistics. These uncertainties are 1 sigma uncertainty of the deduced horizontal wind, temperature and emission rate. There were instrumental start-up problems, in particular with [OH], so the temperatures and relative emissions should be considered as relative before June 1991 (May are borderline OK), although the winds are correct.

Summary plots of the horizontal geographic winds are determined from cardinal directions. The distance between opposite look directions is about 10.56 degrees of latitude or about 1174 km for a 250 km emission layer. Summary plots of the relative emission, neutral temperature and vertical wind are plotted for the vertical look direction, and the south direction if applicable, where there can be auroral emissions.

References for the instrument and data processing procedures

Hernandez, G., 'Fabry-Perot Interferometers', Cambridge University Press, 343 pp., 1988, second printing with corrections. (The first edition was published in 1986.)
Conner, J. F., R. W. Smith and G. Hernandez, Techniques for deriving Doppler temperatures from multiple-line Fabry-Perot profiles: An analysis, Applied Optics, 32, 4437-4444, 1993.
Smith, R. W., G. Hernandez, K. Price, G. J. Fraser, K. C. Clark, W. J. Schulz, S. Smith, and M. Clark, The June 1991 thermospheric storm observed in the southern hemisphere, J. Geophys. Res., 98, 17609-17615, 1994.
Hernandez, G., R. Wiens, R. P. Lowe, G. G. Shepherd, G. J. Fraser, R. W. Smith, L. M. LeBlanc, and M. Clark, Optical determination of the vertical wavelength of propagating 12-hour period upper atmosphere oscillations, Geophys. Res. Lett., 22, 2389-2392, 1995.

Summary Plots for Mount John Fabry-Perot in OI (630.0 nm) (peak emission ~210-300 km)

Summary Plots for Mount John Fabry-Perot in OI (557.7 nm) (peak emission ~94-98 km, or higher with aurora)

(NOTE: Had start-up problems so relative intensities and temperatures are a little high in Feb-Apr, 1991, although the winds are OK.)

Summary Plots for Mount John Fabry-Perot in OH (840 nm) (peak emission ~87-91 km)

(NOTE: Had start-up problems so relative intensities and temperatures are high in Feb-Apr, 1991, although the winds are OK.)

-Revised 02 Feb 2001 by Barbara Emery