Antarctic radio frequency albedo and implications for cosmic ray reconstruction

Abstract

We describe herein a measurement of the Antarctic surface “roughness” performed by the balloon‐borne ANITA (Antarctic Impulsive Transient Antenna) experiment. Originally purposed for cosmic ray astrophysics, the radio frequency (RF) receiver ANITA gondola, from its 38 km altitude vantage point, can scan a disk of snow surface 600 km in radius. The primary purpose of ANITA is to detect RF emissions from cosmic rays incident on Antarctica, such as neutrinos which penetrate through the atmosphere and interact within the ice, resulting in signal directed upward which then refracts at the ice‐air interface and up and out to ANITA, or high‐energy nuclei (most likely irons or protons), which interact in the upper atmosphere (at altitudes below ANITA) and produce a spray of down‐coming RF which reflects off the snow surface and back up to the gondola. The energy of such high‐energy nuclei can be inferred from the observed reflected signal only if the surface reflectivity is known. We describe herein an attempt to quantify the Antarctic surface reflectivity, using the Sun as a constant, unpolarized RF source. We find that the reflectivity of the surface generally follows the expectations from the Fresnel equations, lending support to the use of those equations to give an overall correction factor to calculate cosmic ray energies for all locations in Antarctica. The analysis described below is based on ANITA‐II data. After launching from McMurdo Station in December 2008, ANITA‐II was aloft for a period of 31 days with a typical instantaneous duty cycle exceeding 95%.

Simulations of reflected radio signals from cosmic ray induced air showers

“…We adopt a refractive index of n ¼ 1:31 [26] consistent with ANITA measurements of the reflected image of the Sun [27].…”

HiCal 2: An instrument designed for calibration of the ANITA experiment and for Antarctic surface reflectivity measurements

“…To test this model, one can analyze the ratio of reflected vs. direct signal amplitudes of various signals as seen by the ANITA instrument. An analysis of surface reflectivity using the sun during the ANITA-2 flight is given in [5], and an analysis of satellite, solar, and HiCal-1 data during the ANITA-3 flight is given in [3]. The HiCal-1 data represented the first transient signals to be analyzed in such a way, but unfortunately the flight path only allowed for measurements at very large separation distances, indicating some discrepancy between model calculation and data at such glancing surface incidence angles.…”

Radio frequency ice dielectric permittivity measurements using CReSIS data

“…If there were, in fact, total decoherence at the bedrock, the scattered signal would be unobservably small.The observed variation in the measured surface return strength over a near‐GRIP echogram is shown in Figure ; the width of that distribution corresponds to a variation of order 2 dB, which should bracket the possible effects of surface roughness. We note that a balloon‐borne survey of Antarctica mapped the solar radio frequency signal (integrated from 200 MHz to 1200 MHz), as observed in its surface reflection [ Besson et al , ]. By comparing the strength of the surface reflection with the direct solar radio frequency signal strength, the reflectivity of the surface was derived.…”