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Session 48 - Workshop on the Future of Antarctic Astrophysics - I.
Topical, Oral session, Wednesday, June 10
Presidio,
Just as seeing in the atmosphere limits ground-based optical astronomy, seeing in the Antarctic ice limits high-energy neutrino astronomy. The 1989 decision to explore locating AMANDA in the deep ice at the South Pole was based on skimpy laboratory experiments on optical properties of ice and on inadequate information about air bubbles in glacial ice. Since then, using pulsed beams of laser light at wavelengths from 337 to 510 nm, we have mapped the optical properties of South Pole ice at depths from 800 to 2350 meters. For the window of transparency between 250 and 450 nm, we attribute absorption in ice entirely to the concentration of four types of "dust" from precipitation of aerosols -- insoluble mineral grains, sea salt grains, acid droplets, and soot. In this model, pure ice would be infinitely transparent in the absence of dust. At shallow depths scattering is dominated by air bubbles. Their contribution diminishes with depth due to the transition from bubbles to solid, transparent air-hydrate crystals. At depths below 1200 m scattering is dominated by the four dust components. Rigorous application of Mie theory, using concentrations and size distributions of the dust components modeled from direct measurements of ice cores at Vostok Station (east Antarctica), enables us to account for the optical properties of ice measured by AMANDA and to predict "seeing" throughout the 1300-2300 m depth relevant for AMANDA-II and for the future ICECUBE observatory. Variations in absorption and scattering lengths as a function of depth can be attributed to variations in dust concentration due to the earth's glacial stages. The background of light from radioactivity and triboluminescence is negligible. Except for bubbles at depths less than 1200 m, seeing is quite acceptable for high-energy neutrino astronomy.
