In situ and laboratory studies of radiofrequency propagation through ice and implications for siting a large-scale Antarctic neutrino detector

摘要

We report on two studies of radiowave ice response, relevant to the construction of a large-scale, future ultra-high energy neutrino detector in Antarctica. We are specifically interested in the relative merits of South Pole as a detection site. First, using a bistatic radar system on the ice surface, we have studied radiofrequency reflections off internal layers in Antarctic ice at the South Pole. In contrast to nearly all previous measurements, our measurements are conducted exclusively in the time-domain. The total propagation time of similar to ns-duration, vertically broadcast radio signals, as a function of polarization axis in the horizontal plane, provides a direct probe of the geometry-dependence of the ice permittivity to depths of 1-2 km. Previous work has inferred birefringent asymmetries based on the elliptical polarization of signal returns at a fixed frequency as a function of depth. However, such polarization is not an unambiguous signature of birefringence, and could be due to anisotropic reflecting layers and/or Faraday rotation. We do not observe direct evidence for birefringent-induced effects at fractional levels less than 10(-4). Second, we have performed a laboratory study of microwave Faraday rotation through ice. Although expected to be small. (to our knowledge) there are no previous measurements of this parameter, which, if substantial, would tend to reduce the neutrino detection estimates for existing and planned experiments. Our results indicate that the signal rotation through 3 km of ice in the South Polar geomagnetic field corresponds to a de-polarization of less than 10% in transmitted power.