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Arrival Heights Fabry-Perot

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Acknowledgments

The Arrival Heights Antarctica Fabry-Perot Spectrometer is operated by the Department of Earth and Space Sciences of the University of Washington with support from the Office of Polar Programs of the National Science Foundation. Surface observations were supplied by the Antarctic Meteorological Research Center (AMRC) of the University of Wisconsin.

Data Description

The Fabry-Perot Spectrometer at Arrival Heights, Antarctica (77.8296 S, 166.6627 E, 190.3 m ASL) has been operated since 2002 by the Department of Earth and Space Sciences, University of Washington. The apex magnetic coordinates of Arrival Heights at 250 km height in 2002 were (-80.1, -34.3) with a magnetic declination of 142.6 deg, an inclination of -81.3 deg, and 0 UT at 17.15 MLT.

The dispersing element of the spectrometer is an air-spaced, 14 cm diameter effective clear-aperture Fabry-Perot etalon, which is both self-aligning and self-stabilizing. It is operated near the optimum operational point (2.0 cm spacer) 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 (0.3 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 Arrival Heights can be the combination of any two of the following:

  1. The red line (630.0 nm, kindats=17001,17011) of atomic oxygen (OI) with a typical emission height peak in the range 210 to 300 km.
  2. The [OH] line (840.0 nm, kindats=17004,17014) of the nightglow excited hydroxyl [OH*] with an emission peak between about 87 and 91 km.
  3. The molecular oxygen [O2] Atmospheric Band lines near 866.0 nm (kindats=17007,17017). Emission peak lies in-between the [OH] and OI(557.7nm) emissions, or around 91-94 km.
  4. OI(557.7 nm). The chemically produced green line (557.7345 nm, kindats=17002,17012) of atomic oxygen (OI) has an emission height peak range near 94-98 km. However, with Arrival Heights being within the polar cap, the measurements may refer to the local -unknown- heights of auroral excitation. Observations of this wavelength are seldom made.

The spectrometer observes at 8 evenly-spaced azimuthal directions -- starting at North -- at 20-degree elevation above the horizon, as well as zenith during the polar night. Since the instrument is light-limited, the time spent in observing this 9 direction cycle can be as short as 18 minutes and the instrument is internally time-limited to spend no more than 15 minutes in any given direction. The observations are made during the austral winter observing season -- nominally March 15 through September 30. Because of the narrow filters used, operation of the instrument is not affected by moonlight. All the observations obtained are included since the thermospheric temperature observations are -statistically- undistinguishable in the absence or presence of clouds (Smith and Hernandez, 1995a). This does not apply to the wind and brightness observations, which are strongly affected by the presence of clouds. The cloud cover (code 440) observations are assumed to persist 6 hours, and are described later.

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 -observing season- of continuous observations.) Further, the determined line-of-sight (los) winds are converted to horizontal winds using a spherical Earth. The horizontal winds are reported as positive to the east and north (codes 1410 and 1420), and are positive away from the observer (code 1475), as is the convention of the CEDAR Database, which is contrary to the usual reporting of optical los winds which are positive towards the observer. For an elevation of 20 deg and an emission at 250 km height, the correct spherical horizontal distance is 587 km compared to a flat-Earth projection of 687 km. Between North and South directions this corresponds to a latitude difference of 10.57 deg for a spherical Earth versus 12.37 deg for a flat Earth approximation.

The temperatures are determined based on the daily 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 typically is much faster and robust.

Summarizing, the reported measurements are horizontal and vertical winds, kinetic temperatures and emission rates. The time between successive measurements is light-limited and has been arbitrarily set such that OI 630 nm emission measurement uncertainties (typically) do not exceed about 30 K and 10 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 determined 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.

Sky cover observations are included with Fabry-Perot (FPS) data because clouds can adversely affect ground based observations of the thermosphere. The FPS observations are made during the austral winter observing season -- nominally March 15 through September 30. Because of the narrow filters used, operation of the instrument is not affected by moonlight. All the observations obtained are included since 'statistically', the temperature observations in the thermosphere (630 nm red line) are indistinguishable in the presence or absence of clouds (Smith and Hernandez, 1995a). This does not apply to the brightness or wind observations, which are strongly affected by the clouds. Overcast conditions show no observable radial Doppler shift (very low winds in all directions), while clear conditions show a consistent wind pattern in opposite look directions. For in-between conditions (which occur most of the time), the instrument estimate of sky cover may not match that of a ground observer.

Sky conditions are routinely observed every 6 hours at nearby McMurdo. Sky conditions are assumed to persist between observations; thus, a single value may apply to 6 hours of FPS data, even though the sky cover observations may cover but a few minutes. Observations are taken during the hour indicated (e.g., 0 UT applies to 0:00 to 0:59). The routine observing hours are (0, 6, 12, 18).

The actual surface observations where sky conditions were available have been condensed into yearly files of winter months (March through September) 2002 to 2005. These are linked in this file at

  1. Cloud Cover Data
  2. Cloud Cover Plots

The sky data contains a quantitative octas (eighths of the sky covered) estimate for each cloud deck for up to 5 cloud decks. The total cloud amount ('CA' in the original files) is either the largest of these cloud cover numbers, or even larger since the various cloud decks can overlap giving a great obscuration than each of the parts, ranging from 0 to 8. When 'CA' is blank, the base of the lowest level clouds ('ZCB[M]') is always low (e.g. 25 m) and additionally 47% of the time the octal reading for the next cloud level ('CAI1') is 9 ('obscured'). Therefore, the total cloud cover is set to 9 ('obscured'). When no observations are made, the cloud cover is set to -32767 (missing).

The subjective translation of the octa scale is:

  1. 0 = SKC = clear
  2. 2 = FEW = a few clouds
  3. 4 = SCT = scattered clouds
  4. 6 = BKN = broken clouds
  5. 8 = OVC = completely overcast with clouds
  6. 9 = VV = no vertical visibility

These are conservative numbers since FEW=1-2, SCT=3-4, BKN=5-6, OVC=7-8.

To summarize, sky cover is a subjective evaluation of the skyward visibility which conservatively characterizes actual FPS viewing conditions at the Arrival Heights. FPS winds are most trustworthy under clearest conditions (0-2 octas), perhaps still okay under partly cloudy conditions (up to 4 octas), and unreliable when cloudier (more than 4 octas).

References for the instrument and data processing procedures:

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., 'Fabry-Perot Interferometers', Cambridge University Press, 343 pp., 1988, second printing with corrections.
Hernandez, G., R. W. Smith, G. J. Fraser and W. L. Jones, Large-scale waves in the upper-mesosphere at Antarctic high-latitudes. Geophys. Res. Lett., 19, 1347-1350, 1992.
Hernandez, G., G. J. Fraser and R. W. Smith, Mesospheric 12-hour oscillations near South Pole, Antarctica, Geophys. Res. Lett., 20, 1787-1790, 1993.
Hernandez, G., R. W. Smith, and G. J. Fraser, Antarctic high-latitude mesospheric dynamics, Adv. Space. Res., 16(5), 71-80, 1995.
Hernandez, G., and R. G. Roble, Simultaneous thermospheric observations during the geomagnetic storm of April 2002 from South Pole and Arrival Heights, Antarctica. Geophys. Res. Lett., 30(10), 1511, 2003.
Hernandez, G., Winter mesospheric temperatures above South Pole (90S) and their relationship to the springtime ozone hole size. Geophys. Res. Lett., 31(7), L071109, 2004.
Smith, R. W. and G. Hernandez, Upper thermospheric temperatures at South Pole, Adv. Space. Res., 16(5), 31-39, 1995a.
Smith, R. W., and G. Hernandez, Vertical winds in thermosphere within the polar cap, J. Atmos. Terr. Phys., 57, 611-620, 1995b.

Data Files for Arrival Heights cloud cover in octas

Summary Plots for Arrival Heights cloud cover in octas

These are summary plots of the cloud cover from the Antarctic Meteorological Research Center (AMRC) at the University of Wisconsin-Madison.

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

Summary plots of the 4 cardinal directions of the horizontal winds (ignoring the 4 off-cardinal directions), the vertical winds, relative emission and temperature. The distance between opposite look directions at 20 degrees elevation angle is about 10.6 degrees of latitude or about 587 km for a 250 km emission layer. The relative emission and neutral temperature show values for the vertical look direction, and the N direction. Cloud cover observations are usually every 6 hours and are assumed to persist for 6 hours.


-Revised 28 Dec 2005 by Barbara Emery