Optical Properties of the South Pole Ice at Depths Between 0.8 and 1 Kilometer

Diffuse optics for glaciology

Optical probing of glaciers has the potential for tremendous impact on environmental science. However, glacier ice is turbid, which prohibits the use of most established optical measurements for determining a glacier's interior structure. Here, we propose a method for determining the depth, scattering and absorption length based upon diffuse propagation of short optical pulses. Our model allows us to extract several characteristics of the glacier. Performing Monte Carlo simulations implementing Mie scattering and mixed boundary conditions, we show that the proposed approach should be feasible with current technology. The results suggest that the optical properties and geometry of the glacier can be extracted from realistic measurements, which could be implemented with a low cost and small footprint.

Rapid optical method for logging dust concentration versus depth in glacial ice

We describe the design and simulated response of a dust logger consisting of a downward-pointing phototube, ~2 m below side-directed light-emitting diodes (LEDs), attached to a cable that can lower the device down a 3-in. (7.5-cm) borehole filled with butyl acetate. LED photons that enter the ice are scattered or absorbed by dust grains, and those that reach the phototube provide a measure of dust or volcanic ash concentration at a given depth. An increased dust concentration associated with an ancient colder climate will usually result in an increase in collected light, but may decrease collected light if air bubbles are present. Centimeter-thick volcanic ash bands can also be detected. The concept is based on six years of experience with pulsed light sources used to measure optical properties of deep Antarctic ice.

Observation of high-energy neutrinos using Cerenkov detectors embedded deep in Antarctic ice

Neutrinos are elementary particles that carry no electric charge and have little mass. As they interact only weakly with other particles, they can penetrate enormous amounts of matter, and therefore have the potential to directly convey astrophysical information from the edge of the Universe and from deep inside the most cataclysmic high-energy regions. The neutrino's great penetrating power, however, also makes this particle difficult to detect. Underground detectors have observed low-energy neutrinos from the Sun and a nearby supernova, as well as neutrinos generated in the Earth's atmosphere. But the very low fluxes of high-energy neutrinos from cosmic sources can be observed only by much larger, expandable detectors in, for example, deep water or ice. Here we report the detection of upwardly propagating atmospheric neutrinos by the ice-based Antarctic muon and neutrino detector array (AMANDA). These results establish a technology with which to build a kilometre-scale neutrino observatory necessary for astrophysical observations.

In situ measurements of optical parameters in Lake Baikal with the help of a Neutrino telescope

We present results of an experiment performed in Lake Baikal at a depth of approximately 1 km. The photomultipliers of an underwater neutrino telescope under construction at this site were illuminated by a distant laser. The experiment not only provided a useful cross-check of the time calibration of the detector but also allowed us to determine inherent optical parameters of the water in a way that was complementary to standard methods. In 1997 we measured an absorption length of 22 m and an asymptotic attenuation length of 18 m. The effective scattering length was measured as 480 m. By use of (cos theta) = 0.95 (0.90) for the average scattering angle, this length corresponds to a geometric scattering length of 24 (48) m.

Optical properties of deep ice at the South Pole: absorption

We discuss recent measurements of the wavelength-dependent absorption coefficients in deep South Pole ice. The method uses transit-time distributions of pulses from a variable-frequency laser sent between emitters and receivers embedded in the ice. At depths of 800-1000 m scattering is dominated by residual air bubbles, whereas absorption occurs both in ice itself and in insoluble impurities. The absorption coefficient increases approximately exponentially with wavelength in the measured interval 410-610 nm. At the shortest wavelength our value is approximately a factor 20 below previous values obtained for laboratory ice and lake ice; with increasing wavelength the discrepancy with previous measurements decreases. At ~415 to ~500 nm the experimental uncertainties are small enough for us to resolve an extrinsic contribution to absorption in ice: submicrometer dust particles contribute by an amount that increases with depth and corresponds well with the expected increase seen near the Last Glacial Maximum in Vostok and Dome C ice cores. The laser pulse method allows remote mapping of gross structure in dust concentration as a function of depth in glacial ice.

Optical properties of deep ice at the South Pole: scattering

Recently, absorption and scattering at depths 800-1000 m in South Pole ice have been studied with transit-time distributions of pulses from a variable-frequency laser sent between emitters and receivers embedded in the ice. At 800-1000 m, scattering is independent of wavelength and the scattering centers are air bubbles of size ? wavelength. At 1500-2000 m it is predicted that all bubbles will have transformed into air-hydrate clathrate crystals and that scattering occurs primarily at dust grains, at liquid acids concentrated along three-crystal boundaries, and at salt grains. Scattering on decorated dislocations, at ice-ice boundaries, and at hydrate-ice boundaries will be of minor importance. Scattering from liquid acids in veins at three-crystal boundaries goes as ~lambda(-1) to ~lambda(-2) and should show essentially no depth dependence. Scattering from dust grains goes as ~lambda(-2) and should show peaks at depths of ~1050, ~1750, and ~2200 m in South Pole ice. If marine salt grains remain undissolved, they will scatter like insoluble dust grains. Refraction at ice-ice boundaries and at hydrate-ice boundaries is manifested by a multitude of small-angle scatters, independent of wavelength. The largest contribution to Rayleigh-like scattering is likely due to dislocations decorated discontinuously with impurities. Freshly grown laboratory ice exhibits a large Rayleigh-like scattering that we attribute to the much higher density of decorated dislocations than in glacial ice.

Implications of optical properties of ocean, lake, and ice for ultrahigh-energy neutrino detection

The collecting power and imaging ability of planned ultrahigh-energy neutrino observatories depend on wavelength-dependent absorption and scattering coefficients for the detector medium. Published data are compiled for deep ice at the South Pole, for deep fresh water at Lake Baikal, and for deep seawater. The effective scattering coefficient is smallest for the clearest deep ocean sites, whereas the absorption coefficient is an order of magnitude smaller for deep ice than for the ocean and lake sites. The effective volume per detector element as a function of energy is calculated for electromagnetic cascades produced by electron neutrinos interacting at the various sites. It is largest for deep bubble-free ice, smallest for shallow bubbly ice, and intermediate for lake and seawater. The effective volume per element is calculated for detection of positrons resulting from the capture of a few megaelectron volt supernova neutrinos by protons in the medium. This volume is proportional to the absorption length and independent of the scattering length; it is larger for ice than for seawater or lake water.

Kinetics of Conversion of Air Bubbles to Air Hydrate Crystals in Antarctic Ice

The depth dependence of bubble concentration at pressures above the transition to the air hydrate phase and the optical scattering length due to bubbles in deep ice at the South Pole are modeled with diffusion-growth data from the laboratory, taking into account the dependence of age and temperature on depth in the ice. The model fits the available data on bubbles in cores from Vostok and Byrd and on scattering length in deep ice at the South Pole. It explains why bubbles and air hydrate crystals coexist in deep ice over a range of depths as great as 800 meters and predicts that at depths below approximately 1400 meters the AMANDA neutrino observatory at the South Pole will operate unimpaired by light scattering from bubbles.