Instruments:pkf

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Poker Flat ASI FPI

Contents


Contact Persons

Web Sites

Acknowledgments

The Poker Flat scanning imaging Fabry-Perot Spectrometer is operated by the Geophysical Institute of the University of Alaska. Initial funding came from the Aeronomy Program of the National Science Foundation (NSF), with additional support from the joint TIMED/CEDAR program of the National Aeronautics and Space Administration (NASA) and NSF.

Data Description

The all-sky scanning imaging Fabry-Perot Spectrometer (ASIFPS) at Poker Flat, Alaska (65.12N, 147.43W) is also known as a Scanning Doppler Instrument (SDI). It is an extremely high resolution optical imaging spectrometer. It measures the angular distribution of Doppler shifts and of Doppler broadening of optical airglow and auroral light emitted from the Earth's upper atmosphere, across the entire sky scene that is visible to the gorund-based instrument, down to a zenith angle of ~75 deg. The distributions of Doppler shifts and of Doppler broadening across the sky are used to infer geographic maps of the wind and temperature distributions prevailing within the emission layer.

The instrument is located on a ridge-top, 50 km ENE from Fairbanks and at 440 m above mean sea level. It has been operated since November 1994 by the Geophysical Institute of the University of Alaska as part of the optical diagnostic instruments at the Poker Flat Research Range. The building is aligned in the magnetic north direction at 28.5 degrees azimuth to NE. The apex magnetic coordinates of Poker Flat in December 2001 at 240 km were (65.25, -95.41), with a magnetic declination of 23.56 deg to NE, an inclination of 77.36 deg to SW, and 0 UT at 1231 MLT. At 96 km, apex coordinates are (65.23, -95.45), with a magnetic declination of 24.28 deg, an inclination of 77.37 deg, and 0 UT at 1231 MLT. The observations are reported in approximate local magnetic coordinates, rotated 28.5 degrees clockwise from geographic, and also in geographic coordinates. The magnetic skew angle (code 1020) is about 4.6 deg, which is negligible.

The instrument is based on a 100-mm aperture capacitance-stabilized Fabry-Perot etalon, the plates of which are piezoelectrically scannable in spacing over approximately 1.5 orders of interference (at 630-nm) about a nominal 20-mm gap. Skylight is coupled into the etalon through an all-sky lens and optical relay system which maps an approximately 75-degree half-angle field-of-view onto 6 orders of interference at the etalon. Interference fringes formed by the etalon are conjugate with the sky. The sky, modulated by this fringe pattern, is re-imaged onto an intensified CCD detector, after first passing through a narrow-band interference filter.

The key difference between this and previous all-sky imaging Fabry Perot spectrometer (ASIFPS) systems is that here the etalon plates are rapidly scanned in separation. A spectral bin is calculated for each CCD pixel at each etalon scan step using both the pixel position and the plate spacing prevailing at that scan step. The plates scan through one whole order of interference in 128 steps or channels every 12.8 seconds.

Prior to sky observations the instrument records a calibration map of the detector which encodes, for each pixel location, the etalon gap required to maximize the transmitted intensity at that pixel of a monochromatic source at the nominal observing wavelength. The spectral channel appropriate to a pixel's measurement at a given scan step is then given by the difference between the current plate spacing and that spacing at which transmission maximizes for the detection location. The measured signal from each pixel after each scan step is added to the total in the corresponding spectral channel, so that, at the end of each scan through one interference order in plate spacing, the photon totals represent a spectrum spanning one free spectral range in wavelength.

Since spectral accumulation is valid for any subset of the detector's pixels it is possible to divide the detector into any number of zones, of arbitrary shape(s), and accumulate an independent spectrum for each zone. Since October 2000, there are 47 zones allocated, mapping onto the sky as sectors of six concentric, annular, rings centered about the zenith. The ring edges are spaced uniformly in zenith angle, with the outermost being at 75 degrees. The rings contain 1,4, 6, 8, 12, and 16 sectors respectively.

Other ASIFP spectrometers operate with a fixed plate spacing and form static Fabry-Perot fringes optically conjugate with the sky. In this configuration the shapes of the Fabry-Perot fringes are modulated by variations across the sky of both the intensity and spectral content of the emission. The intensities of aurorally-excited emissions may vary significantly over angular distances comparable to the widths of the Fabry-Perot fringes projected onto the sky. This would distort spectra inferred from static fringe patterns, which may ultimately appear as small-scale artifacts in derived wind fields. The Poker Flat SDI instrument is not subject to this potential source of error.

The instrument records a complete emission spectrum for each allocated viewing zone. To derive estimates of Doppler shift, Doppler width, and optical emission intensity, numerical model spectra are fitted to the observed sky spectra. The model consists of a Gaussian curve convolved with an experimentally-determined instrumental wavelength response function. A DC background term also appears in the model, to account for the (largely instrumental) background that is superimposed upon the recorded spectra. The fitting program adjusts four model parameters until the best fit is found. These four parameters correspond to the emission intensity, the background intensity, the Doppler shift, and the Doppler width of the emission line.

Images from a frequency stabilized neon (atomic mass 20) laser are used to characterize the instrument function, and to correct for the etalon drift due to temperature changes during the night. The calibration wavelength used is 632.8165 nm.

The first filter used was the atomic oxygen [OI] red line at 630.0311 nm (kindat=17001) with airglow emission between about 210 and 300 km, peaking around 240 km. Auroral emissions are lower, with peak emissions as low as 180 km. A green line filter at the [OI] line at 557.7 nm will also be used (kindat=17002), where airglow emissions peak near 94-98 km, but auroral emissions can peak as high as 120 km. The green line filter was put into place April 12, 2002.

Observations are affected by moonlight and clouds, which usually increase the background and noise levels due to scattered light. With complete cloud cover, the velocity appears to be very small in all directions. If there is a bright auroral arc in part of the sky, the instrument is biased towards that bright area, and the winds across the sky will be about the same as those near the arc. Temperature observations are statistically OK as found by Smith and Hernandez (1995), except when the S/N is very low. Cloud cover can be determined by the all-sky camera (ASC) and meridian scanning photometer (MSP). Notes about conditions are added by hand later to the electronic notebook are included as an accompanying catalog record to each night of data when available.

The `errors' given in the data are uncertainties resulting from errors in the fit to the spectra peak position, and in errors from the drift due to the pressure and temperature changes in the air spacing of the etalon. Additional errors arise from fitting the width of the spectra to determine the neutral temperature of the [OI] emitting species. However, fits to the sky spectra are usually good, resulting in small error bars. The actual geophysical error bars due to uncertainties in knowing the altitude of the emission range are larger.

A computer runs the FPS and stores the data, which is downloaded to a web server. Plots can be viewed at: http://thing.pfrr.alaska.edu/conde/sdi_arc.asp. Digital data are also available in netCDF format upon request from the P.I. Dr. Mark Conde. An electronic log book is maintained, that comments on observing conditions and data quality. The log entries are determined by looking at other instruments such as the all-sky camera (ASC) and meridian scanning photometer (MSP). Links to the log entries for each day are given in the plot archive page (http://thing.pfrr.alaska.edu/conde/sdi_arc.asp). Alternatively, the logbook itself can be accessed directly at: http://thing.pfrr.alaska.edu/conde/log_form.asp This is a web-based data entry page, that is also suitable for viewing existing entries. Only users connecting from authorized IP subnets may change the logbook entries, but all users can view them. There is also a page that will generate monthly summaries of the log entries, that can be accessed at http://thing.pfrr.alaska.edu/conde/sum_form.asp Finally, a directory listing of the actual logbook entry files (which are stored as ascii text) is available at: http://thing.pfrr.alaska.edu/conde/sdiplots/log/

References for the instrument and data processing procedures

Conde, M., and R. W. Smith, Mapping thermospheric winds in the auroral zone, Geophys. Res. Lett., 22, 3019-3022, 1995.
Conde, M., and R. W. Smith, Phase compensation of a separation scanned, all-sky imaging Fabry-Perot spectrometer for auroral studies, Appl. Opt., 36, 5441-5450, 1997.
Conde, M., and R. W. Smith, Spatial structure in the thermospheric horizontal wind above Poker Flat, Alaska, during solar minimum, J. Geophys. Res., 103, 9449-9472, 1998.
Conde, M., and R. W. Smith, Simultaneous Observations of the Aurora and of Non-Uniform Thermospheric Winds, from Poker Flat, Alaska, Proc. NIPR Symp. Upper Atmos. Phys., 12, 30-38, 1998.
Conde, M., J. D. Craven, T. Immel, E. Hoch, H. Stenbaek-Nielsen, T. Hallinan, R. W. Smith, J. Olson, Wei Sun, L. A. Fank, and J. Sigwarth, Assimilated observations of thermospheric winds, the aurora, and ionospheric currents over Alaska, J. Geophys. Res., 10493-10508, 2001.
Smith, R. W. and G. Hernandez, Upper thermospheric temperatures at South Pole, Adv. Space. Res., 16(5), 31-39, 1995.

Summary Plots for Poker Flat ASI FPI

Summary plots of the counts under the peak area, neutral temperature and neutral winds are plotted for the vertical look direction, and the most N, E, S and W regions of the other 46 locations.

Red Line (~240 km) Summary Plots

Green Line (~96 km) Summary Plots

The green line filter was installed April 12, 2002, but the data were not very good in the end of this first observing season.


-Revised 05 Oct 2004 by Barbara Emery