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Register a new userModelling ice birefringence and oblique radio wave propagation for neutrino detection at the South Pole
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The Askaryan Radio Array (ARA) experiment at the South Pole is designed to detect high-energy neutrinos which, via in-ice interactions, produce coherent radiation at frequencies up to 1000 MHz. In Dec. 2018, a custom high-amplitude radio-frequency transmitter was lowered into the 1700 m SPICE ice core to provide test sources for ARA receiver stations sensitive to vertical and horizontal polarizations. For these tests, signal geometries correspond to obliquely propagating radio waves from below. The ARA collaboration has recently measured the polarization-dependent time delay variation, and report more significant time delays for trajectories perpendicular to ice flow. Here we use fabric data from the SPICE ice core to construct a bounding model for the ice birefringence and the polarization time delays across ARA. The data-model comparison is consistent with the vertical girdle fabric at the South Pole having the prevailing horizontal crystallographic axis oriented near-perpendicular to ice flow. This study presents the possibility that ice birefringence can be used to constrain the range to a neutrino interaction, and hence aid in neutrino energy reconstruction, for in-ice experiments such as ARA.
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The detection of acoustic signals from ultra-high energy neutrino interactions is a promising method to measure the tiny flux of cosmogenic neutrinos expected on Earth. The energy threshold for this process depends strongly on the absolute noise level in the target material. The South Pole Acoustic Test Setup (SPATS), deployed in the upper part of four boreholes of the IceCube Neutrino Observatory, has monitored the noise in Antarctic ice at the geographic South Pole for more than two years down to 500 m depth. The noise is very stable and Gaussian distributed. Lacking an in-situ calibration up to now, laboratory measurements have been used to estimate the absolute noise level in the 10 to 50 kHz frequency range to be smaller than 20 mPa. Using a threshold trigger, sensors of the South Pole Acoustic Test Setup registered acoustic pulse-like events in the IceCube detector volume and its vicinity. Acoustic signals from refreezing IceCube holes and from anthropogenic sources have been used to localize acoustic events. Monte Carlo simulations of sound propagating from the established sources to the SPATS sensors have allowed to check corresponding model expectations. An upper limit on the neutrino flux at energies $E_ u > 10^{11}$ GeV is derived from acoustic data taken over eight months.
We report on studies of the viability and sensitivity of the Askaryan Radio Array (ARA), a new initiative to develop a Teraton-scale ultra-high energy neutrino detector in deep, radio-transparent ice near Amundsen-Scott station at the South Pole. An initial prototype ARA detector system was installed in January 2011, and has been operating continuously since then. We report on studies of the background radio noise levels, the radio clarity of the ice, and the estimated sensitivity of the planned ARA array given these results, based on the first five months of operation. Anthropogenic radio interference in the vicinity of the South Pole currently leads to a few-percent loss of data, but no overall effect on the background noise levels, which are dominated by the thermal noise floor of the cold polar ice, and galactic noise at lower frequencies. We have also successfully detected signals originating from a 2.5 km deep impulse generator at a distance of over 3 km from our prototype detector, confirming prior estimates of kilometer-scale attenuation lengths for cold polar ice. These are also the first such measurements for propagation over such large slant distances in ice. Based on these data, ARA-37, the 200 km^2 array now under construction, will achieve the highest sensitivity of any planned or existing neutrino detector in the 10^{16}-10^{19} eV energy range.
Experimental efforts to measure neutrinos by radio-frequency (RF) signals resulting from neutrino interactions in-ice have intensified over the last decade. Recent calculations indicate that one may dramatically improve the sensitivity of ultra-high energy (UHE; >EeV) neutrino experiments via detection of radio waves trapped along the air-ice surface. Detectors designed to observe the Askaryan effect currently search for RF electromagnetic pulses propagating through bulk ice, and could therefore gain sensitivity if signals are confined to the ice-air boundary. To test the feasibilty of this scenario, measurements of the complex radio-frequency properties of several air-dielectric interfaces were performed for a variety of materials. Two-dimensional surfaces of granulated fused silica (sand), both in the lab as well as occurring naturally, water doped with varying concentrations of salt, natural rock salt formations, granulated salt and ice itself were studied, both in North America and also Antarctica. In no experiment do we observe unambiguous surface wave propagation, as would be evidenced by signals travelling with reduced signal loss and/or superluminal velocities, compared to conventional EM wave propagation.
IceCube is currently being built deep in the glacial ice beneath the South Pole. In its second year of construction, it is already larger than its predecessor, AMANDA. AMANDA continues to collect high energy neutrino and muon data as an independent detector until it is integrated with IceCube. After introducing both detectors, recent results from AMANDA and a status report on IceCube are presented.
The IceCube Neutrino Observatory, approximately 1 km^3 in size, is now complete with 86 strings deployed in the Antarctic ice. IceCube detects the Cherenkov radiation emitted by charged particles passing through or created in the ice. To realize the full potential of the detector, the properties of light propagation in the ice in and around the detector must be well understood. This report presents a new method of fitting the model of light propagation in the ice to a data set of in-situ light source events collected with IceCube. The resulting set of derived parameters, namely the measured values of scattering and absorption coefficients vs. depth, is presented and a comparison of IceCube data with simulations based on the new model is shown.
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