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South Pole Fabry-Perot

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Acknowledgments

The Amundsen-Scott South Pole Fabry-Perot Interferometric 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 South Pole Meteorological Group.

Data Description

The Fabry-Perot Interferometric Spectrometer at Amundsen-Scott South Pole Station (90.00S) has been operated since 1989 by the Department of Earth and Space Sciences, University of Washington, in cooperation with the Geophysical Institute of the University of Alaska. The instrument is located within 150 feet of the South Pole at 2835 m above mean sea level. The geographic longitude is used to reckon the azimuthal direction of observation, with 0 E corresponding to 0 deg azimuth. The apex magnetic coordinates of South Pole at 250 km height in 1992 were (-74.4, 17.7), with a magnetic declination of -21.37 deg (or 158.63 E toward the compass south pole), an inclination of -74.78 deg, and 0 UT at 2039 MLT. The invariant (apex) South Pole at 250 km height was at (74.2S, 126.0E), and the perpendicular to the magnetic latitude circle pointed to 125.6E. Hernandez et al (1991) used magnetic south towards 120 E, with 0 UT at 2023 MLT. The magnetic grid is useful for ordering the red line observations at about 250 km height, but not for other observations from emissions below 100 km height.

The dispersing element of the spectrometer is an air-spaced, 14 cm diameter effective clear-aperture Fabry-Perot interferometer, 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 (0.2 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 South Pole can be the combination of any two of the following:

  1. The red line (630.0 nm, kindats=17101,17111) 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=17104,17114) 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=17107,17117). 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=17102,17112) of atomic oxygen (OI) has an emission height peak range near 94-98 km. However, with South Pole Station being near the poleward edge of the auroral zone, the measurements 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 300-degree longitude -- at 30-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. Other observing protocols, such as two orthogonal directions and zenith, have also been used. The observations are made during the austral winter observing season -- nominally April 10 through September 8. Because of the narrow filters used, operation of the instrument is not affected by moonlight. Typically, about 35% of the nights observed are clear. 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 below. There are also ascii files and plots.

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 reported winds are positive away from the observer, 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. Since all observations obtained at the South Pole are made looking North by definition, the original azimuth (longitude) and elevation angles of observation are provided, from which the geographic separation between the observing location and the ground projection of intersection at the airglow observation height can be obtained. For an elevation angle of 30 deg and an emission at 250 km height, the correct spherical horizontal distance is 396 km compared to a flat-Earth projection of 433 km. Between two opposite viewing directions, this corresponds to a latitude difference of 7.12 deg for a spherical Earth compared to 7.79 deg for a flat-Earth approximation.

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.

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 (FPI) data because clouds can adversely affect ground based observations of the thermosphere. The FPI observations are made during the austral winter observing season -- nominally April 10 through September 8. Because of the narrow filters used, operation of the instrument is not affected by moonlight. Typically, about 35% of the nights observed are clear. 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, 1995). 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, partly because ground conditions affect the ground observer more than the instrument, which is 15 m higher. However, the simplest way to estimate the data quality is by the use of the independent sky cover data, which are taken from surface weather observations.

The FPI at Amundsen-Scott South Pole Station is on the roof (~15m above ground level) of the four story high `Skylab' building which is located within 150 feet of the geographic the South Pole -2835 m MSL. Weather observations are taken outside the building at ground level where the observing conditions are often adverse and may be dramatically different from those at the FPI height. During the observing season it is dark, often -60 F and seldom calm. When it is cold enough (any water vapor below -40 immediately changes phase into ice crystals) there is a persistent shallow ground level ice fog. Blowing snow also conspires to affect ground level visibility. The observer must go outside, wait long enough to adjust to night vision and then estimate the cloud altitude and coverage, discerning up to three layers. Contributing to inconsistency in the sky conditions report, each year there is a new observer. Therefore, partially obscured conditions are least reliable, yet most common.

Sky conditions are routinely observed every 6 hours, with more frequent observations around special campaign days. Sky conditions are assumed to persist between observations; thus, a single value may apply to 6 hours of FPI 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 changed from (0, 6, 12, 18) to (5, 11, 17, 23) in September 1997, possibly as an artifact of the changing format convention. The observations are then listed as occuring at 0550 UT etc, which is assumed to be 0600 UT etc for plotting purposes. Previous to 1997, an observation at 0 UT would be assumed to last from 0000 UT to 0559 UT, while an observation after 1997 at 23 UT would be assumed to last from 2300 UT to 0459 UT.

The actual surface observations where sky conditions were available have been condensed into yearly files of winter months (April through August or September) for 12 years from 1989 to 2000. These are located at /instruments/spf.html. The record format changed six times and sky cover was described using two nomenclatures. Before addition to the CEDAR Database, these cloud coverage data were first converted to a standard format: octas (eighths). Prior to the institution of the METAR format in 1997, sky cover estimates were recorded in two fields: SKY/CEILING and REMARKS. For each observed cloud deck, the SKY field contains a three-letter designator and additional qualifier (-) which were translated to octas as follows:

  1. 0 = CLR = clear
  2. 2 = -SCT = scattered
  3. 4 = SCT = scattered clouds (5/10 or less)
  4. 5 = -BKN = few broken clouds
  5. 6 = BKN = broken clouds
  6. 7 = -OVC = not completely overcast with clouds
  7. 8 = OVC = completely overcast with clouds
  8. 9 = W#X = extreme conditions, vertical visibility of # 100s ft

The REMARKS field often contained a quantitative octas estimate for each cloud deck; the largest of these numbers was taken in preference to the most obscured of the three letter designations. In 1997 this redundancy was eliminated and a new nomenclature was imposed:

  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. The complete description of surface observations is available at the web site for before and after the major format change starting 1 Sep 1997.

REMARKS also includes notes of aurora sighting for the format previous to September 1997. The aurora are reported as a flag (parameter code 445) which may be no report (0), aurora (1), or missing (9 or -32767), where missing can be no surface observation was taken, or when there is ground obscuration (W#X or VV) when the sky cover is 9, or after 1 Sep 1997. The auroral sightings are also assumed to persist over 6 hours.

The sky cover and auroral sightings that are converted from the surface observations are available at the CEDAR website along with another sky cover set obtained from the Data Support Section (DSS) at the National Center for Atmospheric Research (NCAR). They receive this data from the Climate Prediction Center (CPC) of the National Centers for Environmental Prediction (NCEP) who extract surface synoptic weather observations exchanged on the Global Telecommunications System (GTS). The URL at DSS is http://dss.ucar.edu/datasets/ds512.0. This represents 3-h surface observations, or 6-h for the South Pole. The original METAR observations from May 2003 are in netcdf files at http://dss.ucar.edu/datasets/ds336.0. Other datasets including cloud cover from the lowest clouds (not always the total sky cover) are from April 2000 at http://dss.ucar.edu/datasets/ds461.0 for 1-h data and from Feb 1975 at http://dss.ucar.edu/datasets/ds464.0/ for 3-h data.

The GTS data are eight 3-hour observations of current conditions and then eight 3-hour GTS conversions of surface observations to sky cover in octas. Skycover 0 is clear, except prior to April 1997, 0 also meant missing. The sky cover is reported between Jan 2, 1994 and Feb 28, 2001. However, the first report of meaningful sky cover is Nov 8, 1995, while the second report of meaningful sky cover is Sep 18, 1997. Days with missing current conditions (--------) are absent except in the austral winter months of April-September 1998-2000 where these days were filled in to make comparison plots easier with the sky cover deduced from surface observations provided by Gonzalo Hernandez.

Both cloud cover data sets were plotted for the austral winter periods of April 3 to September 9, 1989-2000, and are available below. Comparisons between the GTS set and the converted surface observations in 1998-2000 in the period of overlap revealed that the GTS set was often the same, and one octa less. There were also occasions when the GTS conversion set the sky conditions to clear when the clouds (FEW-OVC) were at the ground (clouds 0 hundred feet above the ground).

To summarize, sky cover is a subjective evaluation of the skyward visibility which conservatively characterizes actual FPI viewing conditions at the South Pole. FPI 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., F. G. McCormac and R. W. Smith. Austral thermospheric wind circulation and IMF orientation. J. Geophys. Res., 96, 5777-5783, 1991.
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., J. M. Forbes, R. W. Smith, Y. Portnyagin, J. F. Booth and N. Makarov, Simultaneous mesospheric wind measurements near South Pole by optical and meteor radar methods. Geophys. Res. Lett., 23, 1079-1082, 1996.
Hernandez, G., R. W. Smith, J. M. Kelley and K. C. Clark. Mesospheric standing waves near South Pole, Geophys. Res. Lett., 24, 1987-1990, 1997.
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.
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 South Pole cloud cover in octas

Summary Plots for South Pole cloud cover in octas

These are separte summary plots of the cloud cover from two sources, using the same surface observations. The solid lines refer to a CEDAR Database conversion of surface observations during the winter, while the dotted line refers to a conversion of real-time surface observations from the Global Telecommunications System (GTS) which provides near real time data as input to forecast models. The 2 sets are nearly equivalent, where the CEDAR Database is more conservative.

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

Summary plots of the cloud cover, the winds, relative emission and temperature. The horizontal winds are positive in the directions of 15W, 30E (magnetic east), 75E and 120E (magnetic south). Plots before May 2005 mislabelled winds positive to magnetic north instead of south. Three days were added to the older set from 1989-1995: 1989.06.06, 1995.04.23, 1995.09.07. The distance between opposite look directions is about 7.12 degrees of latitude or about 792 km for a 250 km emission layer. The relative emission and neutral temperature show values for the vertical look direction, and the directions towards (120E to polar cap) and away (60W to auroral zone) from the south magnetic pole.


-Revised 31 May 2005 by Barbara Emery

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