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Millstone Hill Fabry-Perot


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The Millstone Hill Fabry-Perot is supported by the National Science Foundation.

Instrument/Model Description

The Millstone Hill Fabry Perot interferometer is operated by MIT in cooperation with the University of Pittsburgh. The interferometer is located near the Millstone Hill incoherent scatter radar at latitude 42 degrees 37 minutes North (42.62) and longitude 71 degrees 27 minutes West (-71.45). Mean local solar time differs from UT by -(4 hour 46 minutes). The local magnetic field has a 15 degree variation to the West and an inclination of 72 degrees.

Analysis of the data is based on the methods used at the University of Pittsburgh. The analysis is a three step process. First, all the data from the frequency stabilized laser are fit to a parameterized Airy function, producing a table of the instrumental parameters as a function of time throughout the night. Second, a parabolic numerical least squares fitting process is then used on the nightglow data, based on the measured instrumental parameters. This method gives 4 parameters: a doppler shift of the nightglow from the shift in the measured peak, a relative intensity of the nightglow from the signal integrated over the peak, an effective temperature of the neutral atmosphere from the doppler width of the measured spectrum and a continuum background signal from the baseline of the profile. In the third analysis step, the doppler shift of the nightglow line is interpreted in terms of a wind.

Log10 relative emission intensity (parameter 2506) is the integration of the fitted line profile over the free spectral range of the instrument. This is only a relative intensity parameter, intended for comparison of intensities during a single night, or perhaps over periods of a week or two. Changes or drifts in sensitivity are not removed from this number, so comparisons between different nights are not advised. An order of magnitude estimate of the calibration is 10 of these units per rayleigh, however, this is only a very rough approximation.

Typically, a 5 position scan is used: a vertical measurement and 4 measurements at an elevation of 30 degrees from the horizon looking at either the cardinal points or at 45 degrees from the cardinal points. Winds are calculated by measuring the difference in line position between the fitted line and a zero velocity reference. The zero velocity reference is generated by taking all the fitted lines from vertical measurements and smoothing and interpolating them as a function of time. This assumes that the vertical velocity is small compared to the resolution of the interferometer. For nights in which the quality of the vertical measurements is poor, or in which there are not enough vertical measurements for a good smoothed reference, the vertical measurements may be supplemented by an average of measurements in opposite directions. This assumes that the wind field is uniform over the observation points, i.e. without divergences. The method used to obtain the vertical reference is flagged by KINDAT (7001 and 17001 for vertical measurements only, 7002 and 17002 for combined measurements).

Uncertainties in the derived parameters are purely statistical, and do not reflect possible systematic errors. The wind uncertainty is calculated from the data by considering the accuracy of determination of the center of gravity of a line. Temperatures and temperature errors are estimated by taking a fourier cosine transform of the data and fitting the logarithm of the coefficients to a straight line.

When data are taken in the cardinal directions, the winds derived from the measurements are either geographic meridional or zonal. To get values of the horizontal winds at the same time, the measurements are smoothed and interpolated. When data are taken at 45 degrees to the cardinal points, two measurements from orthogonal directions are used to define a vector, from which both geographic and geomagnetic winds are determined. When there are significant latitudinal gradients, the winds are often determined from combining SE and SW and NE and NW so as to keep the latitudinal gradients intact.

References for the instrument and data processing procedures

Biondi et. al., Appl. Opt. 24, 232, 1985.
Hernandez, "Fabry-Perot Interferometers", Cambridge University Press, 1986.
Sipler et. al., Planet. Space Sci., 31, 53, 1985.

Summary Plots for Millstone Hill Fabry-Perot in OI (6300 A)

Summary plots of the relative emission, neutral temperature and vertical wind are plotted for the vertical look direction. Summary plots of the horizontal geographic winds are either smoothed and interpolated or determined from non-cardinal directions. Occasionally, the geographic winds are absent in the analysis and the plots. Blank plots do not always mean no data. For example, only temperatures in the vertical are plotted.

1980's Plots

1990's Plots

2000's Plots

-Revised 04 Feb 2003 by Barbara Emery