Impact of biaxial birefringence in polar ice at radio frequencies on signal polarizations in ultra-high energy neutrino detection

TL;DR

This paper investigates how biaxial birefringence in polar ice affects radio signal polarization, with implications for ultra-high energy neutrino detection, highlighting the need for a biaxial treatment to improve experimental accuracy.

Contribution

It introduces a biaxial birefringence model for radio wave propagation in ice and compares it with experimental data, revealing polarization rotation effects not previously accounted for.

Findings

High cross-polarization power observed in experiments.

Biaxial birefringence causes non-trivial polarization rotations.

Time delays may serve as neutrino signatures and distance indicators.

Abstract

It is known that polar ice is birefringent and that this can have implications for in-ice radio detection of ultra-high energy neutrinos. Previous investigations of the effects of birefringence on the propagation of radio-frequency signals in ice have found that it can cause time delays between pulses in different polarizations in in-ice neutrino experiments, and can have polarization-dependent effects on power in radar echoes at oblique angles in polar ice. I report, for the first time, on implications for the received power in different polarizations in high energy neutrino experiments, where the source of the emitted signal is in the ice, a biaxial treatment at radio wavelengths is used, and the signals propagate at oblique angles. I describe a model for this and compare with published results from the SPICE in-ice calibration pulser system at South Pole, where unexpectedly high cross-polarization power has been reported for some geometries. The data shows behaviors indicative of the need for a biaxial treatment of birefringence inducing non-trivial rotations of the signal polarization. The behaviors include, but are not limited to, a time delay that would leave an imprint in the power spectrum. I explain why this time delay has the potential to serve as both an in-ice neutrino signature and a measurement of the distance to the interaction. While further work is needed, I expect that proper handling of the effects presented here will increase the science potential of ultra-high energy neutrino experiments, and may impact the optimal designs of next-generation detectors.