[Paper Review] Mesosphere Sodium Column Density and the Sodium Laser Guide Star Brightness

Jian Ge, J. R. P. Angel+8 more|arXiv (Cornell University)|Aug 29, 1997

Adaptive optics and wavefront sensing3 citations

TL;DR

This study presents the first simultaneous measurements of mesospheric sodium column density and sodium laser guide star (LGS) brightness at 32°N latitude, demonstrating that circularly polarized laser light increases fluorescent return by ~30% compared to linear polarization. At a sodium column density of 3.7×10⁹ cm⁻², a 1 W circularly polarized laser produces an LGS with magnitude R = 10.3 mag (8.4×10⁵ photons/s/m²), enabling accurate laser power predictions for desired guide star brightness based on seasonal and diurnal column density variations.

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.

Motivation & Objective

• To measure sodium column density in the mesosphere at 32°N latitude using stellar and solar absorption spectroscopy.
• To correlate sodium column density with the absolute flux of a sodium laser guide star created by a 3 W cw laser.
• To determine the impact of laser polarization (linear vs. circular) on laser guide star brightness.
• To provide a direct, accurate method for predicting required laser power to achieve specific guide star magnitudes based on column density variations.
• To characterize seasonal and diurnal variations in sodium column density at mid-latitude sites.

Proposed method

• Measured sodium column density via absorption line spectroscopy in stellar and solar spectra observed from the MMT and CFA 60-inch telescope.
• Conducted simultaneous laser guide star experiments using a 3 W continuous-wave laser tuned to the peak of the sodium D2 hyperfine structure.
• Compared fluorescent return from linearly and circularly polarized laser beams under identical column density conditions.
• Used the measured column density to project required laser power for achieving target guide star magnitudes.
• Applied a direct absorption method, avoiding reliance on lidar techniques, to improve accuracy in column density estimation.
• Analyzed data over a full year to assess seasonal and diurnal variations in sodium abundance.

Experimental results

Research questions

• RQ1 How does sodium column density at 32°N vary seasonally and diurnally?
• RQ2 What is the relationship between sodium column density and the absolute flux of a sodium laser guide star?
• RQ3 Does circularly polarized laser light produce a significantly brighter laser guide star than linearly polarized light?
• RQ4 Can the measured column density be used to accurately predict the laser power needed for a desired guide star magnitude?
• RQ5 How does the absorption method compare in accuracy to lidar for measuring mesospheric sodium column density?

Key findings

• The sodium column density at 32°N exhibits diurnal variations of up to a factor of two within a single hour.
• Seasonal variation amplitude at 32°N is smaller than at higher latitudes, and the annual mean column density is lower than at higher latitudes.
• Laser return flux is linearly proportional to the simultaneous sodium column density for both linear and circular polarization.
• Circularly polarized laser light produces approximately 30% higher fluorescent return than linearly polarized light under the same conditions.
• A 1 W circularly polarized laser beam produces a laser guide star with magnitude R = 10.3 mag and an absolute flux of 8.4×10⁵ photons/s/m² at a sodium column density of 3.7×10⁹ cm⁻².
• The direct absorption method provides a more accurate and straightforward measurement of column density compared to traditional lidar techniques.