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 Oð10Þ microseconds, to infer the surface power reflection coefficient R. The first HiCal mission (HiCal-1, Jan. 2015) yielded a sample of 100 such pairs, resulting in estimates of R at highly glancing angles (i.e., zenith angles approaching 90°), with measured reflectivity for those events which exceeded extant calculations [P. W. Gorham et al., Journal of Astronomical Instrumentation, 1740002 (2017)]. The HiCal-2 experiment, flying from December 2016-January 2017, provided an improvement by nearly 2 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 [D. Z. Besson et al., Radio Sci. 50, 1 (2015)]. 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-2 measurements. DOI: 10.1103/PhysRevD.98.042004 PHYSICAL REVIEW D 98, 042004 (2018) 2470-0010=2018=98(4)=042004 (19) 042004-1 © 2018 American Physical Society I. OVERVIEWThe NASA-sponsored ANITA project [1][2][3][4] has the goal of detecting the highest-energy particles incident on the Earth. Although designed for measurement of ultra-highenergy neutrinos interacting in-ice, the first ANITA flight also demonstrated (unexpectedly) excellent sensitivity to primary ultra-high-energy cosmic rays (UHECR) with energies exceeding 1 EeV (10 18 eV) [5] interacting in the Earth's atmosphere. These are assumed to be charged nuclei (likely protons), given the lack of efficient acceleration mechanisms for electrically uncharged particles, and the long lifetimes required to traverse megaparsec-scale distances. Through interactions with terrestrial matter, both neutrinos and charged cosmic-rays produce observable radio-frequency (RF) emissions via the Askaryan effect [6][7][8], with three important distinctions between the two experimental signatures:(1) as viewed from the airborne ANITA gondola, charged primary cosmic ray interactions in the atmosphere generally produce down-coming signals, which subsequently reflect off the surface and up to the gondola, whereas neutrinos interacting in-ice produce up-coming signals which refract through the surf...

Constraints on the ultrahigh-energy cosmic neutrino flux from the fourth flight of ANITA

“…ANITA detected over 10,000 pulses from the HiCal payloads, both direct signals from the HiCal itself, and companion signals that were reflected off of the surface of the ice be-fore being recorded by ANITA. Pairs of direct and reflected pulses provide measurements of Antarctic ice surface Fresnel coefficients at a variety of angles important for both analysis and simulation [18].…”

Coherent Transition Radiation from the Geomagnetically Induced Current in Cosmic-Ray Air Showers: Implications for the Anomalous Events Observed by ANITA

“…To investigate in more detail if the ANITA anomalous events lie within the region of interest for coherent transition radiation to be significant, we ran 10 Monte-Carlo showers for each anomalous event within their given reconstruction errors. These errors are obtained by using the event param-eters published by the ANITA collaboration [23], including energy (the reported energies for the anomalous events, provided for a direct shower, have been scaled by the empirically determined surface reflectivity coefficients attained on recent ANITA flights [38]), zenith angle, and surface elevation. A similar procedure has been made for the normal cosmic-ray air shower events detected during the ANITA-1 flight [39,40].…”

The Calibration of the Geometry and Antenna Delay in Askaryan Radio Array Station 4 and 5

“…The study of ultra high energy (UHE) neutrinos (neutrino energy, > 10 17 eV) is intriguing as they carry exclusive information about the ultra high energy cosmic rays. Several ongoing experiments such as the IceCube [1], ARA [2], ANITA [3], ARIANNA [4], and also the RNO-G [5], currently under construction at Greenland, aim to detect ultra high energy neutrinos.…”