Modeling the refractive index profile n(z) of polar ice for ultra-high energy neutrino experiments

TL;DR

This paper presents an in-situ measurement of the ice's refractive index profile at the South Pole, improving neutrino detection sensitivity by modeling radio signal propagation through ice.

Contribution

It introduces a three-phase densification model for ice, validated by radio signal timing, enhancing neutrino experiment simulations and sensitivity estimates.

Findings

The three-phase model better fits the data than a single exponential model.

Using the three-phase model increases neutrino sensitivity by 14%.

Ray tracing simulations match measured radio signal timing differences.

Abstract

We have developed an in-situ index of refraction profile n(z) for cold polar ice, using the transit times of radio signals broadcast from an englacial transmitter to 2-5 km distant radio-frequency receivers, deployed at depths up to 200 m. For propagation through a non-uniform medium, Maxwell's equations generally admit two ray propagation solutions from a given transmitter, corresponding to a direct path (D) and a refracted or reflected path (R); the measured D vs. R timing differences (dt(D,R)) are determined by the refractive index profile. We constrain n(z) near South Pole, where the Askaryan Radio Array (ARA) neutrino observatory is located, by simulating D and R ray paths via ray tracing and comparing simulations to measured dt(D,R) values. Using previous ice density data as a proxy for n(z), we demonstrate that our data strongly favors a glaciologically-motivated three-phase densification model rather than a single exponential scale height model. Effective volume simulations for a detector of ARA station antenna depths yield a 14\% increase in neutrino sensitivity over a range of $10^{17} - 10^{21}$ eV using the three-phase model compared to a single exponential.