Magnetic-field sensing has contributed to the formulation of the plate-tectonics theory, mapping of underground structures on Earth, and the study of magnetism of other planets. Filling the gap between space-based and near-Earth observations, we demonstrate a remote measurement of the geomagnetic field at an altitude of 85–100 km. The method consists of optical pumping of atomic sodium in the mesosphere with an intensity-modulated laser beam, and ground-based observation of the resultant magneto-optical resonance near the Larmor precession frequency. Here we validate this technique and measure the Larmor precession frequency of sodium and the corresponding magnetic field with an accuracy level of 0.28 mG Hz−1/2. These observations allow the characterization of atomic-collision processes in the mesosphere. Remote detection of mesospheric magnetic fields has potential applications such as mapping magnetic structures in the lithosphere, monitoring space weather, and electric currents in the ionosphere.
Adaptive optics is a key technology for ground-based optical and infrared astronomy, providing high angular resolution and sensitivity. Systems employing laser guide stars can achieve high sky coverage, but their performance is limited by the available return flux. Amplified spontaneous emission could potentially boost the intensity of beacons produced by resonant excitation of atomic or molecular species in the upper atmosphere. This requires the production of a population inversion in an electronic transition that is optically thick to stimulated emission. Mesospheric metals have insufficient column density for amplified spontaneous emission, but atomic oxygen and nitrogen are potential candidates. They could potentially be excited by a high-energy chirped femtosecond pulsed laser, making visible-wavelength transitions accessible. Such lasers can also generate a white-light supercontinuum in the atmosphere. In addition to providing high intensity, the broadband emission from such a source could facilitate the sensing of the tilt component of atmospheric turbulence.
The location an astronomical observatory is a key factor that affects its scientific productivity. The best astronomical sites are generally those found at high-altitudes. Several such sites in southern China and the Tibetan plateau are presently under development for astronomy. One of these is Ali, which at over 5000 m is one of the highest astronomical sites in the world. In order to further investigate the astronomical potential of Ali, we have installed a lunar scintillometer, for the primary purpose of profiling atmospheric turbulence. This paper describes the instrument and technique, and reports results from the first year of observations. We find that ground layer turbulence at Ali is remarkably weak and thin. It contributes 0.31 arcsec to the median seeing above 11 m height and just 0.19 arcsec above 50 m. At the same time, initial data obtained using a crescent moon indicate that turbulence in the free atmosphere is comparatively large, contributing a median of 0.57 arcsec to the seeing. This may be due to high-altitude turbulence induced by air flow over the crest of the Himalayas. We suggest that a modified form of ground-layer adaptive optics, in which high-altitude turbulence is compensated rather than the ground layer, could be an effective strategy for high-resolution wide-field astronomy from this site.
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