Observation of an optical anisotropy in the deep glacial ice at the geographic South Pole using a laser dust logger

Martin Rongen,Summer Blot,Ryan Carlton Bay

doi:10.5194/tc-14-2537-2020

Martin Rongen, Summer Blot + Show 1 more

Open Access

https://doi.org/10.5194/tc-14-2537-2020

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Abstract

Abstract. We report on a depth-dependent observation of a directional anisotropy in the recorded intensity of backscattered light as measured by an oriented laser dust logger. The measurement was performed in a drill hole at the geographic South Pole about a kilometer away from the IceCube Neutrino Observatory. The drill hole has remained open for access since the SPICEcore collaboration retrieved a 1751 m ice core. We find the anisotropy axis of 126±3∘ as measured below 1100 m to be compatible with the local flow direction. The observation is discussed in comparison to a similar anisotropy observed in data from the IceCube Neutrino Observatory and favors a birefringence-based scenario over previously suggested Mie-scattering-based explanations. In the future, the measurement principle, when combined with a full-chain simulation, may have the potential to provide a continuous record of fabric properties along the entire depth of a drill hole.

Highlights

The viscosity of an individual ice crystal strongly depends on the direction of the applied strain
The oriented dust logger was deployed down the SPICEcore hole twice during the 2016/2017 season, both times using the Intermediate Depth Logging Winch provided by the Ice Drilling Program (IDP)
We have presented the first direction-dependent measurement of the intensity of backscattered, optical light in deep glacial ice

Summary

Introduction

The viscosity of an individual ice crystal strongly depends on the direction of the applied strain. Ice crystals are a birefringent material, with any incoming electromagnetic radiation being separated into ordinary and extraordinary rays of perpendicular polarization with respect to the c axis, which propagate with different refractive indices This is classically observed as a direction-dependent delay in the propagation of radio waves, as, for example, described by Fujita et al (2006). First attempts have been made by Chirkin and Rongen (2019) by attributing the effect to the cumulative diffusion that a light beam experiences as it is refracted or reflected on many grain boundary crossings in a birefringent polycrystal with a preferential c-axis distribution In this scenario, the diffusion is found to be strongest when photons initially propagate along the flow and smallest when initially propagating orthogonal to the flow. If the anisotropy is caused by Mie scattering, a reduced return signal is expected when the light source points along the flow, while more light is expected to return in the case of the birefringence and absorption explanations

The oriented dust logger

SPICEcore deployments

Anisotropy signature

Depth evolution

Conclusions