Antarctic Surface Reflectivity Calculations and Measurements from the ANITA-4 and HiCal-2 Experiments

Abstract

The balloon-borne HiCal radio-frequency (RF) transmitter, in concert with the ANITA radio-frequency receiver array, is designed to measure the Antarctic surface reflectivity in the RF wavelength regime. The amplitude of surface-reflected transmissions from HiCal, registered as triggered events by ANITA, can be compared with the direct transmissions preceding them by O(10) microseconds, to infer the surface power reflection coefficient $\mathcal{R}$. The first HiCal mission (HiCal-1, Jan. 2015) yielded a sample of 100 such pairs, resulting in estimates of $\mathcal{R}$ at highly-glancing angles (i.e., zenith angles approaching $90^{\circ }$), with measured reflectivity for those events which exceeded extant calculations. The HiCal-2 experiment, flying from Dec., 2016-Jan., 2017, provided an improvement by nearly two orders of magnitude in our event statistics, allowing a considerably more precise mapping of the reflectivity over a wider range of incidence angles. We find general agreement between the HiCal-2 reflectivity results and those obtained with the earlier HiCal-1 mission, as well as estimates from Solar reflections in the radio-frequency regime. In parallel, our calculations of expected reflectivity have matured; herein, we use a plane-wave expansion to estimate the reflectivity R from both a flat, smooth surface (and, in so doing, recover the Fresnel reflectivity equations) and also a curved surface. Multiplying our flat-smooth reflectivity by improved Earth curvature and surface roughness corrections now provides significantly better agreement between theory and the HiCal 2a/2b measurements.

arXiv commentsubmitted to Astropart. Phys

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• Coherent transition radiation from the geomagnetically-induced current in cosmic-ray air showers: Implications for the anomalous events observed by ANITA
• A search for IceCube events in the direction of ANITA neutrino candidates
• Explaining the ANITA events by a $L_e-L_{\tau }$ gauge model
• Long-baseline horizontal radio-frequency transmission through polar ice
• Experimental tests of sub-surface reflectors as an explanation for the ANITA anomalous events
• General Treatment of Reflection of Spherical Electromagnetic Waves from a Spherical Surface and its Implications for the ANITA Anomalous Polarity Events