Mesosphere Sodium Column Density and the Sodium Laser Guide Star Brightness

TL;DR

This study simultaneously measured sodium column density and laser guide star brightness, revealing polarization effects and seasonal/diurnal variations, to optimize laser power for astronomical adaptive optics.

Contribution

First simultaneous measurements of sodium column density and laser guide star flux, including polarization effects and seasonal/diurnal variations, at a mid-latitude site.

Findings

Circular polarization increases fluorescence by about 30%.

Sodium column density varies diurnally by up to a factor of two.

Seasonal variation amplitude is smaller than at higher latitudes.

Abstract

The first time simultaneous measurements of sodium column density and the absolute flux from a sodium laser guide star, created by a monochromatic 3 W cw laser, tuned to the peak of the sodium D2 hyperfine structure, were conducted at the MMT and CFA 60 inch telescope in 1997. The results show that linearly and circularly polarized laser returns are proportional to the simultaneous sodium column density. Moreover, circularly polarized laser provides about 30% increase in fluorescent return over linearly polarized laser. A laser guide star with R = 10.3 mag. or absolute flux of 8.4x10^5 photons/s/m^2, could be formed from a 1 watt projected circularly polarized sodium laser beam when sodium layer abundance N(Na) = 3.7x10^9 /cm^2. Together with the distributed column density measurements (e.g. seasonal and diurnal variations), we can project laser power requirements for any specified guide star brightness. The mesosphere sodium column density variation was measured above Tucson sky throughout the year, through sodium absorption line measurements in stellar and solar spectra. Previous measurements, e.g. Papen et al, 1996, have not been made at this latitude (32 degrees). Further, our absorption method is more direct and may be more accurate than the lidar methods normally used. The seasonal variation amplitude is smaller than that at higher latitudes. While the annual mean sodium column density tends to be lower than at higher latitudes. Diurnal sodium column density tends to vary by as much as a factor of two within an hour.