Antarctic Surface Reflectivity Measurements from the ANITA-3 and HiCal-1 Experiments

Abstract

The primary science goal of the NASA-sponsored ANITA project is measurement of ultra-high energy neutrinos and cosmic rays, observed via radio-frequency signals resulting from a neutrinoor cosmic ray-interaction with terrestrial matter (atmospheric or ice molecules, e.g.). Accurate inference of the energies of these cosmic rays requires understanding the transmission/reflection of radio wave signals across the ice-air boundary. Satellite-based measurements of Antarctic surface reflectivity, using a co-located transmitter and receiver, have been performed more-or-less continuously for the last few decades. Our comparison of four different reflectivity surveys, at frequencies ranging from 2-45 GHz and at near-normal incidence, yield generally consistent maps of high vs. low reflectivity, as a function of location, across Antarctica. Using the Sun as an RF source, and the ANITA-3 balloon borne radio-frequency antenna array as the RF receiver, we have also measured the surface reflectivity over the interval 200-1000 MHz, at elevation angles of 12-30 degrees. Consistent with our previous measurement using ANITA-2, we find good agreement, within systematic errors (dominated by antenna beam width uncertainties) and across Antarctica, with the expected reflectivity as prescribed by the Fresnel equations. To probe low incidence angles, inaccessible to the Antarctic Solar technique and not probed by previous satellite surveys, a novel experimental approach ("HiCal-1") was devised. Unlike previous measurements, HiCal-ANITA constitute a bi-static transmitter-receiver pair separated by hundreds of kilometers. Data taken with HiCal, between 200-600 MHz shows a significant departure from the Fresnel equations, constant with frequency over that band, with the deficit increasing with obliquity of incidence, which we attribute to the combined effects of possible surface roughness, surface grain effects, radar clutter and/or shadowing of the reflection zone due to Earth curvature effects. We discuss the science implications of the HiCal results, as well as improvements implemented for HiCal-2, launched in December, 2016.

Measurement of the real dielectric permittivity ϵ of glacial ice

“…In this regard, within the last two years, a 1700-m deep ice core, 0.5-several km from current ARA stations, was extracted from the South Pole ice [29]. With the consent of the NSF, a piezo-electric technology radio-frequency transmitter, based on the same model as that employed in the balloon-borne HiCal experiment [30], was used in January, 2018 to sample the upper 800 meters of the SPICE borehole. The beam pattern of that transmitter is somewhat more isotropic than that used in the deep pulser studies described herein, and, by design, contains significant HPol as well as VPol content.…”

Constraints on the diffuse high-energy neutrino flux from the third flight of ANITA

“…We remove calibration pulser events from HiCal [29], a balloon-mounted calibration pulser, with an azimuth pointing cut |φ hc − φ P | < 5 • , where φ hc is the azimuth of HiCal in ANITA payload coordinates.…”

Looking at the axionic dark sector with ANITA

“…1 and 2 the reflected and incident polarization states are related through the Stokes formalism [56]. We have assumed an index of refraction for the Antarctic surface of n = 1.35 [2,10]. To estimate the experimental uncertainties, we have simulated the ANITA experimental response with a Monte Carlo, where the Stokes parameters are computed adding to the signal Gaussian white noise adjusted to reproduce the observed polarization angle uncertainty [21].…”

“…On the one hand, transition radiation predicts that events with large elevations will be anomalous, in slight tension with current data. On the other hand, sub-surface structures predict some amount of double events, which should also be generated by the calibration pulses emitted by the HiCal antenna [10,11]. Therefore, dedicated searches and/or more exposure are required to validate these possibilities.…”