Probing the Firn Refractive Index Profile Using Antenna Response

TL;DR

This work tackles the crucial problem of mapping the depth-dependent refractive index profile $n(z)$ of firn to improve Ultra-High-Energy Neutrino (UHEN) detection with RNO-G. It introduces a rapid in-situ method that links the dipole antenna resonant frequency $f_{res}$, which shifts with the local $n(z)$, to a parametric form $f(n)=\frac{a}{b+n}$, enabling conversion of depth-resolved $S_{11}(z)$ measurements into $n(z)$. By conducting two measurement campaigns at Summit Station (2024 and 2025), fitting $S_{11}$ resonances, and cross-validating with gravimetric and density-based RI data, the authors demonstrate consistent shallow firn RIP extraction with ~per-cent accuracy and identify key sources of systematic uncertainty, including axial alignment and borehole air effects. The results indicate that the proposed resonance-based RIP inference is fast, field-deployable, and sensitive to local $n(z)$ variations on ~50 cm depths, offering a practical tool to improve FIRN modeling for ray-tracing and neutrino reconstruction in UHEN experiments.

Abstract

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 ${\rm km}^2$. Since the RNO-G experimental sensitivity depends on the radio-frequency properties of the firn, which are known to vary laterally on sub-km distance scales and vertically on sub-meter distance scales, a technique for quickly extracting information on firn ice properties with depth ($n(z)$) during drilling and deployment is desirable. Given that a dipole's resonant wavelength is fixed by geometry, the resonant frequency $f_{res}$ (measured as an S-parameter reflection coefficient [`$S_{11}$'] minimum) scales inversely with the local refractive index, allowing a translation of a depth-dependent $S_{11}$(z) profile into $n(z)$. $S_{11}$(z) data were initially taken in August, 2024 using a dipole lowered into a newly-drilled $98\pm 1$-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 $S_{11}$(z) data can be used to estimate \RIP, on 50 cm vertical scales, at the per-cent level of accuracy required by experiments such as RN0-G.

Figures (14)

• Figure 1: Schematic illustrating UHEN signal detection. An incident neutrino collides with an ice molecule at a 'vertex', producing Askaryan radiation (consisting of radially outward polarized propagating electric fields) geometrically confined to the surface of an expanding Cherenkov cone. As a result of the variable refractive index in the firn (deep purple), two rays (direct ['D'; solid lines] and refracted/reflected ['R'; dashed lines]) reach each of the Surface and also Deep receiver antennas. Purple lines show the highest-amplitude, 'on-Cone' signal; Red lines show weaker, 'off-Cone' signal.
• Figure 2: Location of 2024 IDP hole at Summit Station.
• Figure 3: Top: KU-VPol antenna schematic showing dimensions of antenna employed for data-taking and feed-point detail. An N-connectorized cable is threaded through the left cylindrical chamber to the central feed-point and then connected to the Vector Network Analyzer. This antenna is similar in design to VPol antennas used for main RNO-G data-taking. Bottom: CAD model of same dipole antenna, illustrating central collar used to maintain desired spacing between cylindrical halves, and also showing endcap axial stabilizer fins, used for 2025 measurements.
• Figure 4: Beam pattern of dipole antenna used for data-taking, showing elevation (left) and azimuthal (right) gain. Measurements were taken in KU anechoic chamber.
• Figure 5: Measured 2024 $S_{11}$ at various depths in ice, illustrating anomalous response observed at depths near 80 m (bottom row, central panel). Note the abrupt change in the shape of the resonance.