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Ann Arbor Fabry-Perot


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The Ann Arbor, Michigan Fabry-Perot Spectrometers were operated by the University of Michigan from 1986 to 1987 with support from the National Science Foundation.

Data Description

One Fabry-Perot Spectrometer was operated at the University of Michigan in Ann Arbor (42.29N, 83.71W, 276m ASL) from 1986 to 1987 with support from the National Science Foundation. The instrument was the same 630.0 nm device used at Fritz Peak Observatory, where it operated from 1972 to 1985.

Description of Instrument

The dispersing element of the spectrometer is an air-spaced, 14 cm diameter effective clear-aperture Fabry-Perot interferometer, which is self-aligning and self-stabilizing (Hernandez and Mills, 1973). The instrument was operated at the optimum point for kinetic temperature determinations (Hernandez, 1979; 1988).

The spectrometer operated with narrow (<0.3 nm wide) interference filters -a necessity, in particular, for the 630.0 nm emission, in order to avoid contamination from the nearby OH emission lines (Hernandez, 1974). 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 throughout the day, year round.

The spectrometer observed wavelength with the Fritz Peak (FPO or fpf) and Ann Arbor (AAM or aaf) instrument has been the so-called red line (630.0 nm, kindat=17001, 1973-1985 at FPO and 1986-1987 at AAM) of atomic oxygen (OI) with typical emission height peak in the range 210 to 300 km.

The spectrometer observed the night-sky at the 4 cardinal directions at 20-degree elevation above the horizon, as well as zenith. Since the instrument is light-limited, the time spent in observing this 5-direction cycle can be as short as 5 minutes during auroral activity. The instrument is internally time-limited to spend no less that one-minute and 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 were made every evening and only those with clear weather -as reported by an observer on site- are reported here. Because of the narrow filters used, operation of the instrument is not affected by moonlight. Typically, about 30% of the nights observed were 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 -meteorological convention- using a spherical Earth and presuming that vertical winds are negligible. 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 isotopic 198Hg lamps profiles and single-wavelength laser profiles and measured instrumental parameters, such as the reflectivity. The reduction is a least-squares deconvolution in the Fourier plane (Hernandez, 1979, 1988). Although single-line spectra are also reducible by steepest-descent techniques (Conner et al, 1993), the Fourier deconvolution is much faster and robust. Finally, the measurements reported here have been made with both 1.5 cm and 2.0 cm air-spaced etalons for the 630 nm emission and 3.0 cm for the 557.7 nm emission.

Summarizing, the reported measurements are horizontal and vertical winds, kinetic temperatures, and relative emissions. The time between successive measurements is light-limited and has been arbitrarily set such that OI 630 nm (OI 557.7 nm) emission measurement uncertainties do not exceed about 40 K and 10 m/s (10 K and 5 m/s) respectively for temperature and winds. Emission rates are reported as counts normalized for unit time. They are not a calibrated quantity, 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 (Hernandez, 1979, 1988)

Examples of publications using the Fritz Peak data are those of Hernandez (1976, 1977, 1982), Hernandez et al. (1980) and Hernandez and Roble (1984, 1995).

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 north direction if applicable, where there can be auroral emissions.

References for the instrument and data processing procedures

Hernandez, G., Analytical description of a Fabry-Perot spectrometer. 5: Optimization for minimum uncertainties in the determination of Doppler widths and shifts. Appl. Opt., 18, 3826 - 3834, 1979
Hernandez, G., 'Fabry-Perot Interferometers', Cambridge University Press, 343 pp., 1988, second printing with corrections. (The first edition was published in 1986.)
Hernandez, G., Contamination of the OI(^3P2-^1D2) emission line by the (9-3) band of OH X^2 II in high-resolution measurements of the night sky. J. Geophys. Res., 79, 1119 - 1123 (1974).
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.
Hernandez, G., Lower-thermosphere temperatures determined from the line profiles of the OI 17924-K (5577A) emission in the night sky. 1: Long term behavior. Geophys. Res., 81, 5165-5172, 1976.
Hernandez, G., Lower-thermosphere temperatures determined from the line profiles of the OI 17924-K (5577A) emission in the night sky. 2: Interaction with the lower atmosphere during stratospheric warmings. J. Geophys. Res., 82, 2127-2131, 1977.
Hernandez, G., Mid-latitude thermospheric neutral kinetic temperatures 1. Solar, geomagnetic and long-term effects. J. Geophys. Res., 87, 1623-1632, 1982.
Hernandez, G. and O. A. Mills, Feedback stabilized Fabry-Perot interferometer. Appl. Opt., 12, 126 - 130 (1973).
Hernandez, G., R. G. Roble and J. H. Allen, Midlatitude thermospheric winds and temperatures and their relation to the Auroral Electrojet activity index. Geophys. Res. Lett., 7, 677-680, 1980.
Hernandez, G. and R. G. Roble, The geomagnetic quiet nighttime thermospheric wind pattern over Fritz Peak Observatory during solar cycle minimum and maximum. J. Geophys. Res., 89, 327-337, 1984.
Hernandez, G., and R. G. Roble, Nighttime thermospheric neutral gas temperature and winds over Fritz Peak Observatory: observed and calculated solar cycle variation, J. Geophys. Res., 99, 14647- 14659, 1995.

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

Summary plots of the 4 cardinal directions of the horizontal winds, the vertical winds, relative emission and temperature. The distance between opposite look directions at 45 degrees elevation angle is about 500 km for a 250 km emission layer. The relative emission and relative neutral temperature show values for the vertical look direction, and the N direction.

-Revised 30 Apr 2007 by Barbara Emery