Probing the firn refractive index profile using antenna response

The Radio Neutrino Observatory-Greenland (RNO-G, at Summit Station) experiment comprises an extensive fat-dipole antenna array deployed into ice boreholes over an eventual area of approximately 35 km2. Sincethe RNO-G experimental sensitivity depends on the radio-frequency properties of the firn, which are known to vary laterally on sub-km distance scalesand vertically on sub-meter distance scales, a technique for quickly extractinginformation on firn ice properties with depth (n(z)) during drilling and deployment is desirable. Given that a dipole’s resonant wavelength is fixed bygeometry, the resonant frequency fres (measured as an S-parameter reflectioncoefficient [‘S11’] minimum) scales inversely with the local refractive index, allowing a translation of a depth-dependent S11(z) profile into n(z). S11(z) datawere initially taken in August, 2024 using a dipole lowered into a newly-drilled98 ± 1-mm diameter, 350-m deep borehole at Summit Station, Greenland, approximately 1 km from the site of the original GISP-2 core; improved measurements were subsequently made in May, 2025. We conclude that S11(z)data can be used to estimate n(z) , on 50 cm vertical scales, at the per-centlevel of accuracy required by experiments such as RN0-G.