Propagating Air Shower Radio Signals to In-ice Antennas

In-ice Askaryan radio experiments like ARA, RNO-G, and ARIANNA have deployed radio
antennas inside the Greenlandic and Antarctic ice sheets [1–3]. These in-ice radio experiments
are looking to measure the flux of cosmogenic and astrophysical neutrinos at even higher energies
than IceCube (> 1017 eV). The in-ice radio detectors are designed to record and measure coherent
radio Cherenkov emission, known as Askaryan emission, from the particle cascade induced by the
neutrino-ice nuclei collision.
Radio emissions from particle cascades, dubbed extensive air showers, induced by ultra-high
energy cosmic rays interacting in the atmosphere serve as essential backgrounds for Askaryan radio
detectors. Arriving with a relatively high flux, cosmic ray particle cascades produce radio signals
with many similar features compared to neutrino signals. As such, cosmic rays provide the ideal
calibration signal if their signal is understood and can be separated from the neutrino signal. The
effort to fully characterize cosmic-ray signals observed by in-ice radio detectors is underway and
will be the goal of this work. However, it should be noted that this work will be limited to in-air
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radio emissions. In contrast, the in-ice emission when the cosmic-ray shower propagates into the
ice sheet will be discussed in another work [4, 5].
In this work, cosmic-ray air shower radio emission will be simulated with CoREAS (CORSIKA-
based Radio Emission from Air Showers) [6]. CoREAS is a sub-module of the CORSIKA (COsmic
Ray SImulations for KAscade) air shower simulation [7]. CoREAS uses the endpoint formalism to
calculate the electric field emission from each shower particle (as simulated by CORSIKA) as the
shower propagates and makes no assumptions regarding the emission mechanism. However, the
CoREAS code has only been designed to work in air using straight-line ray propagation. In reality,
air and ice have density-dependent exponential refractive index profiles that cause radiowave rays
to curve as they propagate toward an observer. The ray-bending effect is much less pronounced for
air than ice since, in polar ice, the refractive index refractive has an exponential profile which has a
large gradient in the first 100 − 200 m. Polar ice refractive index changes from 1.35 at the surface
to 1.78 within 100 − 200 m, and the sharp change in the refractive index causes the rays to bend
significantly near the ice surface.