Telluric sodium layer temporal variations

Abstract

We present observations of the high-speed variations of the altitude of the telluric sodium layer. In this experiment we observed the Gemini-North sodium laser guide star from approximately 80 meters off-axis using the UH-2.2m telescope on Mauna Kea, Hawaii. Observations were made using an electron-multiplying camera at a rate of about 100Hz. The temporal power spectrum of the layer centroid follows a power law between 0.001 and 1Hz and we find that the exponent of the power law (α=-1.8) is similar to that found at lower temporal frequencies from lidar experiments. This data set taken with the lidar results shows that the power spectrum of the sodium layer mean altitude follows a simple power law over 5 orders of magnitude from 10^(-4.5) Hz to 1Hz. The approach taken in this experiment is difficult due to telescope jitter in any of the three telescopes (Gemini-N, Gemini-N LGS launch telescope, or from the observing UH2.2m) and atmospheric tip/tilt wave front aberrations. We circumvented these problems by analyzing the differential motion between two distinct features that appeared in the sodium layer during that night.

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