Bolometric Night Sky Temperature and Subcooling of Telescope Structures

Context. The term sky temperature is used in the literature in different contexts which often leads to confusion. In this work, we study $T_\text{sky}$, the effective bolometric sky temperature at which a hemispherical black body would radiate the same power onto a flat horizontal structure on the ground as the night sky, integrated over the entire thermal wavelength range of $1-100\,\mu$m. We then analyze the thermal physics of radiative cooling with special focus on telescopes and discuss mitigation strategies. Aims. The quantity $T_\text{sky}$ is useful to quantify the subcooling in telescopes which can deteriorate the image quality by introducing an Optical Path Difference (OPD) and induce thermal stress and mechanical deflections on structures. Methods. We employ the Cerro Paranal Sky Model of the European Southern Observatory to derive a simple formula of $T_\text{sky}$ as a function of atmospheric parameters. The structural subcooling and the induced OPD are then expressed as a function of surface emissivity, sky view factor, local air speed and structure dimensions. Results. At Cerro Paranal (2600 m) and Cerro Armazones (3060 m) in the Atacama desert, $T_\text{sky}$ towards the zenith mostly lies $20-50$ Kelvin below the ambient temperature near the ground, depending strongly on the precipitable water vapor (PWV) column in the atmosphere. The temperature difference can decrease by several Kelvin for higher zenith distances. The subcooling OPD scales linearly to quadratically with the telescope diameter and is inversely proportional to the local air speed near the telescope structure.Comment: 14 pages, 16 figure

Modeling of pulsed laser guide stars for the Thirty Meter Telescope project

The Thirty Meter Telescope (TMT) has been designed to include an adaptive optics system and associated laser guide star (LGS) facility to correct for the image distortion due to Earth's atmospheric turbulence and achieve diffraction-limited imaging. We have calculated the response of mesospheric sodium atoms to a pulsed laser that has been proposed for use in the LGS facility, including modeling of the atomic physics, the light-atom interactions, and the effect of the geomagnetic field and atomic collisions. This particular pulsed laser format is shown to provide comparable photon return to a continuous-wave (cw) laser of the same average power; both the cw and pulsed lasers have the potential to satisfy the TMT design requirements for photon return flux.Comment: 16 pages, 20 figure

Magnetometry with mesospheric sodium

Measurement of magnetic fields on the few 100-km length scale is significant for many geophysical applications including mapping of crustal magnetism and ocean circulation measurements, yet available techniques for such measurements are very expensive or of limited accuracy. We propose a method for remote detection of magnetic fields using the naturally occurring atomic sodium-rich layer in the mesosphere and existing high-power lasers developed for laser guide star applications. The proposed method offers a dramatic reduction in cost and opens the way to large-scale, parallel magnetic mapping and monitoring for atmospheric science, navigation, and geophysics. atomic physics | geomagnetism | optical pumping M easurements of geomagnetic fields are an important tool for peering into the Earth's interior, with measurements at differing spatial scales giving information about sources at corresponding depths. Mapping of fields on the few meter scale can locate buried ferromagnetic objects (e.g., unexploded ordnance or abandoned vessels containing toxic waste), whereas maps of magnetic fields on the kilometer scale are used to locate geological formations promising for mineral or oil extraction. On the largest scale, the Earth's dipole field gives information about the geodynamo at depths of several thousand kilometers. Magneticfield variations at intermediate length scales, in the range of several tens to several hundreds of kilometers likewise offer a window into important scientific phenomena, including the behavior of the outer mantle, the solar quiet dynamo in the ionosphere (1), and ionic currents as probes of ocean circulation (2), a major actor in models of climate change. To avoid contamination from local perturbations, measurements of such slowly varying components of the magnetic field must typically be made at a significant height above the Earth's surface (e.g., measurements of components with a spatial-variation scale of 100 km require an altitude of approximately 100 km) and with high sensitivity (on the order of 1 nT). Though magnetic mapping at high altitude has been realized with satellite-born magnetic sensors (3-5), the great expense of multisatellite missions places significant limitations on their deployment and use. Here, we introduce a high-sensitivity ground-based method of measuring magnetic fields from sources near Earth's surface with 100 km spatial resolution.* The method exploits the naturally occurring atomic sodium layer in the mesosphere and the significant technological infrastructure developed for astronomical laser guide stars (LGS). This method promises to enable creation of geomagnetic observatories and of regional or global sensor arrays for continuous mapping and monitoring of geomagnetic fields without interference from ground-based sources. Overview of Technique The measurement we envisage is a form of atomic magnetometry, adapted to the conditions of the mesosphere. The principle is to measure spin precession of sodium atoms by spin-polarizing them, allowing them to evolve coherently in the magnetic field, and determining the postevolution spin state. Spin polarization of mesospheric sodium is achieved by optical pumping, as proposed in the seminal paper on sodium LGS by Happer et al. (6). In the simplest realization, the pumping laser beam is circularly polarized and is launched from a telescope at an angle nearly perpendicular to the local magnetic field, as shown i

Satellite-assisted laser magnetometry with mesospheric sodium

Magnetic field sensing provides crucial insights into various geophysical phenomena such as atmospheric currents, crustal magnetism, and oceanic circulation. In this paper, a method for remote detection of magnetic fields using mesospheric sodium with an assisting satellite is proposed. Sodium atoms in the mesosphere are optically pumped with a ground-based laser beam. A satellite-borne detector is used to measure magneto-optical rotation of the polarization of a probe laser beam by the sodium atoms. This sensitive magnetometry method benefits from direct detection of laser photons and complements existing space- and aircraft-borne techniques by probing magnetic fields at upper-atmospheric altitudes inaccessible to those

Remote sensing of geomagnetic fields and atomic collisions in the mesosphere

Remote sensing of geomagnetic fields in mesosphere is both challenging and interesting to explore the magnetic field structures and atomic collision processes. Here the authors demonstrate an atomic magnetometer that utilizes the Larmor frequency in sodium atoms and operates in kilometers range

Polarization-driven spin precession of mesospheric sodium atoms

We report experimental results on the first on-sky observation of atomic spin precession of mesospheric sodium driven by polarization modulation of a continuous-wave laser. The magnetic resonance was remotely detected from the ground by observing the enhancement of induced fluorescence when the driving frequency approached the precession frequency of sodium in the mesosphere, between 85 km and 100 km altitude. The experiment was performed at La Palma, and the uncertainty in the measured Larmor frequency ($\approx$260 kHz) corresponded to an error in the geomagnetic field of 0.4 mG. The results are consistent with geomagnetic field models and with the theory of light-atom interaction in the mesosphere

Magnetometry with mesospheric sodium

Measurement of magnetic fields on the few 100-km length scale is significant for many geophysical applications including mapping of crustal magnetism and ocean circulation measurements, yet available techniques for such measurements are very expensive or of limited accuracy. We propose a method for remote detection of magnetic fields using the naturally occurring atomic sodium-rich layer in the mesosphere and existing high-power lasers developed for laser guide star applications. The proposed method offers a dramatic reduction in cost and opens the way to large-scale, parallel magnetic mapping and monitoring for atmospheric science, navigation, and geophysics