Modelling ice birefringence and oblique radio wave propagation for neutrino detection at the South Pole

TL;DR

This paper models ice birefringence at the South Pole to improve understanding of radio wave propagation for neutrino detection, potentially enhancing neutrino energy reconstruction in in-ice experiments.

Contribution

It introduces a bounding model for ice birefringence based on fabric data, linking ice crystal orientation to polarization time delays in radio signals.

Findings

Ice fabric data aligns with the horizontal crystallographic axis orientation.

Birefringence causes measurable polarization-dependent time delays.

Model suggests birefringence can help constrain neutrino interaction range.

Abstract

The Askaryan Radio Array (ARA) experiment at the South Pole is designed to detect high-energy neutrinos which, via in-ice interactions, produce coherent radiation at frequencies up to 1000 MHz. In Dec. 2018, a custom high-amplitude radio-frequency transmitter was lowered into the 1700 m SPICE ice core to provide test sources for ARA receiver stations sensitive to vertical and horizontal polarizations. For these tests, signal geometries correspond to obliquely propagating radio waves from below. The ARA collaboration has recently measured the polarization-dependent time delay variation, and report more significant time delays for trajectories perpendicular to ice flow. Here we use fabric data from the SPICE ice core to construct a bounding model for the ice birefringence and the polarization time delays across ARA. The data-model comparison is consistent with the vertical girdle fabric at the South Pole having the prevailing horizontal crystallographic axis oriented near-perpendicular to ice flow. This study presents the possibility that ice birefringence can be used to constrain the range to a neutrino interaction, and hence aid in neutrino energy reconstruction, for in-ice experiments such as ARA.