7320Ǻ AIRGLOW OBSERVATIONS USING AO’s NEW LOW RESOLUTION FPI
The low resolution single etalon Fabry-Perot interferometer (FPI) in the Optical
Laboratory at the AO has been upgraded to permit remote operation, to improve FPI
sensitivity, and to permit FPI response in the near infrared. Integration of a 2048 x 2048
Andor CCD array into the existing low-resolution FPI with a new optical system is
completed. We achieved a 40-fold enhancement in sensitivity at 7320Ǻ over earlier work,
by virtue of 90-95% quantum efficiency and 4-order simultaneous sampling. Problematic
OH contamination is eliminated by use of a very narrow 3.0Ǻ FWHM interference filter,
and 0.9cm Fabry-Perot plate spacing achieves a spectral resolution of 0.032Ǻ, the
emission doublet line width at 7320Ǻ for any temperature greater than 200K. Raw
detector response is corrected using both linear (chip bias) and non-linear techniques
(flat-field) prior to ring-summing. A frequency stabilized HeNe laser at 632.8 nm is used
to establish the FPI response function. Figure 1 show an example 7320Å ring pattern
obtained at AO using this configuration.
The excited O+ (2P) atom is formed by photoionization or electron impact with energy in
excess of 18.6 eV (< 666Ǻ). With a lifetime of 4.57s in the upper state and no local
source in the earth’s shadow, the O+(2P) to O+(2D) transition produces a twilight airglow
at 7320Ǻ and 7330Ǻ very near to the solid-earth shadow line. The emission has therefore
been used in the past to determine the altitude profile of O+ temperature at the terminator,
using high spectral resolution detection. Some of these measurements have been
interpreted to imply a population of hot oxygen atoms in the upper thermosphere and
lower exosphere. That population remains speculative, but has the potential, if it exists,
to confuse ISR ion temperature fits, with hot O+ spectra having similar width to He+
spectra.
7320 Å twilight observations had been carried at AO using this new FPI configuration
since March of 2008 during new moons periods. Preliminary results show a significant
day-to-day variability. The results are still inconclusive and we cannot determine if a
population of “hot oxygen” is present or not. However, we are extending the data analysis
to significantly higher shadow heights, by co-adding weaker signals to achieve larger S:N
at higher shadow. Figure 2 shows an example of our analyses for eight days of 7320 Å
line width data acquired in April and March of 2008.
Figure 1- 7320 Å ring pattern obtained at April 9, 2008 with an integration period of 60s.
0.15
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FWHM
April 2008 - 4 days
77 Images
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18.6
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5.65
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5.85
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LOCAL TIME
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SHADOW ALTITUDE (km)
SHADOW ALTITUDE (km)
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May 2008 - 4 days
175 Images
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SHADOW ALTITUDE (km)
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SHADOW ALTITUDE (km)
Figure 2- FWHM general variability of 7320 Å spectral line at Arecibo for April and May,
2008. The red line is the polynomial fit for the data points.