Monitoring of the mesospheric sodium layer using a magneto-optical filter

Adaptive optics (AO) systems for atmospheric turbulence compensation require a bright reference star for measuring and correcting wavefront distortion. For these reason a general use of AO requires a laser guide star (LGS) to provide the wavefront information. An artificial guide star can be created by focusing a laser beam tuned to the sodium D-2 line at 5890 Angstrom on the mesospheric sodium layer at about 90 km altitude and observing the resonant scattering. The D-1 line of the sodium layer can be observed in absorption on early type unreddened (or slightly reddened) stars using a high resolution spectroscopic system. Recent studies have reported that the column density of the layer is temporally variable with timescales from a year down to tens of minutes. Therefore a monitoring of the column density is necessary when observing with sodium LGS. It is demonstrated that, with a sufficiently high spectral resolution, a relatively small (50 cm) telescope observing a bright star can collect in minutes the number of photons necessary to measure the equivalent width of the sodium line with a good signal-to-noise ratio, provided that an efficient spectroscopic device of suitable resolution is attached to it. We propose here a system to monitor the column density of the mesospheric sodium layer which is based on the sodium magneto-optical filter developed by A. Cacciani and already widely used in solar observations since the 70's. This filter is compact, stable in wavelength and profile shape, with high efficiency (up to 40 %) and narrow profile width (20 m Angstrom). It is based on the magneto-optical activity of the sodium gas embedded in a strong transverse magnetic field. The filter has a central bandpass that can be fitted to the width of the D-1 mesospheric line, plus two lateral symmetric bandpasses that observe the adjacent stellar continuum. The D-1 equivalent width carl be derived by observing the mesospheric sodium line in absorption on the spectrum of a bright early type (O, B, A) star.