Cold Polar Ice Research Articles

Radiofrequency ice dielectric measurements at Summit Station, Greenland

Abstract We recently reported on the radio-frequency attenuation length of cold polar ice at Summit Station, Greenland, based on bi-static radar measurements of radio-frequency bedrock echo strengths taken during the summer of 2021. Those data also allow studies of (a) the relative contributions of coherent (such as discrete internal conducting layers with sub-centimeter transverse scale) vs incoherent (e.g. bulk volumetric) scattering, (b) the magnitude of internal layer reflection coefficients, (c) limits on signal propagation velocity asymmetries (‘birefringence’) and (d) limits on signal dispersion in-ice over a bandwidth of ~100 MHz. We find that (1) attenuation lengths approach 1 km in our band, (2) after averaging 10 000 echo triggers, reflected signals observable over the thermal floor (to depths of ~1500 m) are consistent with being entirely coherent, (3) internal layer reflectivities are ≈–60 $\to$ –70 dB, (4) birefringent effects for vertically propagating signals are smaller by an order of magnitude relative to South Pole and (5) within our experimental limits, glacial ice is non-dispersive over the frequency band relevant for neutrino detection experiments.

Polarization angle dependence of vertically propagating radio-frequency signals in South Polar ice

To better understand the effect of ice properties on the capabilities of radio experiments designed to measure ultra-high energy neutrinos (UHEN), we recently considered the timing and amplitude characteristics of radio-frequency (RF) signals propagating along multi-kilometer, primarily horizontal trajectories through cold Polar ice at the South Pole. That analysis indicated satisfactory agreement with a model of ice birefringence based on ice crystal (cˆ-axis) data culled from the South Pole Ice Core Experiment (SPICE). Here we explore the geometrically complementary case of signals propagating along primarily vertical trajectories, using extant data from the Askaryan Radio Array (ARA) experiment, supplemented by a refined analysis of older RICE experimental data. The timing characteristics of the South Polar data are in general agreement with the same birefringence model, although a several nanosecond discrepancy is found in comparison to Taylor Dome data. Re-analysis of older RICE data also confirm the correlation of signal amplitudes reflected from internal-layers with the direction of ice flow, similar to previous observations made along a traverse from Dome Fuji to the Antarctic coast. These results have two important implications for radio-based UHEN experiments: (i) if birefringence can be locally calibrated, the timing characteristics of signals propagating from neutrino-ice interactions to a distant receiver might be used to infer the distance-to-vertex, which is necessary to estimate the energy of the progenitor neutrino, (ii) the measured reflectivity of internal layers may result in previously-unanticipated backgrounds to UHEN searches, requiring significantly more modeling and analysis.

In situ, broadband measurement of the radio frequency attenuation length at Summit Station, Greenland

AbstractOver the last 25 years, radiowave detection of neutrino-generated signals, using cold polar ice as the neutrino target, has emerged as perhaps the most promising technique for detection of extragalactic ultra-high energy neutrinos (corresponding to neutrino energies in excess of 0.01 Joules, or 1017 electron volts). During the summer of 2021 and in tandem with the initial deployment of the Radio Neutrino Observatory in Greenland (RNO-G), we conducted radioglaciological measurements at Summit Station, Greenland to refine our understanding of the ice target. We report the result of one such measurement, the radio-frequency electric field attenuation length $L_\alpha$. We find an approximately linear dependence of $L_\alpha$ on frequency with the best fit of the average field attenuation for the upper 1500 m of ice: $\langle L_\alpha \rangle = ( ( 1154 \pm 121) - ( 0.81 \pm 0.14) \, ( \nu /{\rm MHz}) ) \,{\rm m}$ for frequencies ν ∈ [145 − 350] MHz.

Long-baseline horizontal radio-frequency transmission through polar ice

We report on analysis of englacial radio-frequency (RF) pulser data received over horizontal baselines of 1–5 km, based on broadcasts from two sets of transmitters deployed to depths of up to 1500 meters at the South Pole. First, we analyze data collected using two RF bicone transmitters 1400 meters below the ice surface, and frozen into boreholes drilled for the IceCube experiment in 2011. Additionally, in Dec., 2018, a fat-dipole antenna, fed by one of three high-voltage (\U0001d4aa(1 kV)), fast (\U0001d4aa(1-5 ns risetime)) signal generators was lowered into the 1700-m deep icehole drilled for the South Pole Ice Core Experiment (SPICE), approximately 3 km from the geographic South Pole. Signals from transmitters were recorded on the five englacial multi-receiver ARA stations, with receiver depths between 60–200 m. From analysis of deep transmitter data, we estimate: i) the range of refractive index profiles of Antarctic ice with depth allowed by our measurements, ii) due to birefringence, a time difference between arrival times for vertically polarized vs. horizontally polarized signals (per km) for horizontally propagating signal, and iii) for the first time, the attenuation length for electromagnetic signals in the radio-frequency regime broadcast horizontally (rather than reflected vertically from bedrock). We additionally present data suggesting anomalous ice propagation effects, and contrary to expectations for a transport medium with a smoothly varying refractive index profile. Our results imply negligible uncertainty in overall neutrino detection volume due to refractive index uncertainties. Our birefringence time-difference measurements are fit to the functional form δt(H−V) [ns/km]=acosθ+b, with H/V the signal arrival times for the horizontally/vertically polarized EM signal components, and θ the opening angle in the horizontal plane between the signal propagation direction and the local ice flow direction, extracting a=8.3±1.3 ns/km, and b=-8.6±0.9 ns/km (errors combined statistical and systematic), allowing a ∼15% range estimate for future measurements of in-ice neutrino interactions. Finally, we find attenuation length values clustering around 1.5 km, with measurements from the bicone transmitters yielding Latten=1.43±0.25±0.37 km. Taken together, these measurements support cold polar ice as a near-optimal platform for ultra-high energy neutrino detectors.

Measurement of the real dielectric permittivity ϵr of glacial ice

Owing to their small interaction cross-section, neutrinos are unparalleled astronomical tracers. Ultra-high energy (UHE; E > 10 PeV) neutrinos probe the most distant, most explosive sources in the Universe, often obscured to optical telescopes. Radio-frequency (RF) detection of Askaryan radiation in cold polar ice is currently regarded as the best experimental measurement technique for UHE neutrinos, provided the RF properties of the ice target can be well-understood. To that end, the Askaryan Radio Array (ARA) experiment at the South Pole has used long-baseline RF propagation to extract information on the index-of-refraction (n=ϵr) in South Polar ice. Owing to the increasing ice density over the upper 150–200 m, rays are measured along two, nearly parallel paths, one of which refracts through an inflection point, with differences in both arrival time and arrival angle that can be used to constrain the neutrino properties. We also observe (first) indications for RF ice birefringence for signals propagating along predominantly horizontal trajectories, corresponding to an asymmetry of order 0.1% between the ordinary and extra-ordinary birefringent axes, numerically compatible with previous measurements of birefringent asymmetries for vertically-propagating radio-frequency signals at South Pole. Qualitatively, these effects offer the possibility of redundantly measuring the range from receiver to a neutrino interaction in Antarctic ice, if receiver antennas are deployed at shallow (z ∼ −25 m) depths. Such range information is essential in determining both the neutrino energy, as well as the incident neutrino direction.

Design and initial performance of the Askaryan Radio Array prototype EeV neutrino detector at the South Pole

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 describe measurements 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 2.5km 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 ∼200km2 array now in its initial construction phase, will achieve the highest sensitivity of any planned or existing neutrino detector in the 1016–1019eV energy range.

AURA—A radio frequency extension to IceCube

The excellent radio frequency (RF) transparency of cold polar ice, combined with the coherent Cherenkov emission produced by neutrino-induced showers when viewed at wavelengths longer than a few centimeters, has spurred considerable interest in a large-scale radio-wave neutrino detector array. The AURA (Askaryan Under-ice Radio Array) experimental effort, within the IceCube collaboration, seeks to take advantage of the opportunity presented by IceCube [A. Karle, Nucl. Instr. and Meth. A (2009), this issue, doi: 10.1016/j.nima.2009.03.180. [1]; A. Achtenberg et al., The IceCube Collaboration, Astropart. Phys. 26 (2006) 155 [2]] drilling through 2010 to establish the RF technology needed to achieve 100– 1000 km 3 effective volumes. In the 2006–2007 Austral summer, three deep in-ice RF clusters were deployed at depths of ∼ 1300 and ∼ 300 m on top of the IceCube strings. Additional three clusters will be deployed in the Austral summer of 2008–2009. Verification and calibration results from the current deployed clusters are presented, and the detector design and performances are discussed. Augmentation of IceCube with large-scale ( 1000 km 3 sr ) radio and acoustic arrays would extend the physics reach of IceCube into the EeV–ZeV regime and offer substantial technological redundancy.

In situ radioglaciological measurements near Taylor Dome, Antarctica and implications for ultra-high energy (UHE) neutrino astronomy

Radiowave detection of the Cherenkov radiation produced by neutrino-ice collisions requires an understanding of the radiofrequency (RF) response of cold polar ice. We herein report on a series of radioglaciological measurements performed approximately 10km north of Taylor Dome Station, Antarctica from December 6, 2006–December 16, 2006. Using RF signals broadcast from: (a) an englacial discone, submerged to a depth of 100m and broadcasting to a surface dual-polarization horn receiver, and (b) a dual-polarization horn antenna on the surface transmitting signals which reflect off the underlying bed and back up to the surface receiver, we have made time-domain estimates of both the real (index-of-refraction “n”) and imaginary (attenuation length “Latten”) components of the complex ice dielectric constant. We have also measured the uniformity of ice response along two orthogonal axes in the horizontal plane. We observe an apparent wavespeed asymmetry of order 0.1% between two orthogonal linear polarizations projected into the horizontal plane, consistent with some previous measurements, but somewhat lower than others.

Performance and simulation of the RICE detector

The Radio Ice Cherenkov Experiment at the South Pole, co-deployed with the AMANDA experiment, seeks to detect ultra-high energy electron neutrinos interacting in cold polar ice. Such interactions produce electromagnetic showers, which emit radio-frequency Cherenkov radiation. We describe the experimental apparatus and the procedures used to measure the neutrino flux.

Modelling fabric development along the GRIP ice core, central Greenland

The preferred c-axes orientation (fabric) observed in cold polar ice is induced by intracrystalline slip only when the grain-boundary migration rate is low enough (i.e. corresponding to grain growth or rotation recrystallization regimes). Fabrics reflect the entire thermomechanical history of the ice and strongly influence its mechanical behaviour. Large viscoplastic anisotropy is always associated with pronounced fabrics. We use a viscoplastic self-consistent (VPSC) polycrystal deformation model to calculate fabric development. In this model, stress- and strain-rate fields are not uniform within the polycrystal and both equilibrium and compatibility conditions are fulfilled. We compare fabrics measured on thin sections along the GRIP ice core (central Greenland) with those calculated down to a depth of 2800 m. Behaviours predicted by uniform stress and uniform strain bounds are presented for comparison. Predictions of the VPSC model are in close agreement with measurements within the upper 650 m, which corresponds to the entire grain-growth zone. Deeper clown, the simulated fabric strength appears to be too high. A simple calculation shows that this discrepancy may be fully attributed to the effects of rotation recrystallization.

Modelling fabric development along the GRIP ice core, central Greenland

The preferredc-axes orientation (fabric) observed in cold polar ice is induced by intracrystalline slip only when the grain-boundary migration rate is low enough (i.e. corresponding to grain growth or rotation recrystallization regimes). Fabrics reflect the entire thermomechanical history of the ice and strongly influence its mechanical behaviour. Large viscoplastic anisotropy is always associated with pronounced fabrics. We use a viscoplastic self-consistent (VPSC) polycrystal deformation model to calculate fabric development. In this model, stress- and strain-rate fields are not uniform within the polycrystal and both equilibrium and compatibility conditions are fulfilled. We compare fabrics measured on thin sections along the GRIP ice core (central Greenland) with those calculated down to a depth of 2800 m. Behaviours predicted by uniform stress and uniform strain bounds are presented for comparison. Predictions of the VPSC model are in close agreement with measurements within the upper 650 m, which corresponds to the entire grain-growth zone. Deeper clown, the simulated fabric strength appears to be too high. A simple calculation shows that this discrepancy may be fully attributed to the effects of rotation recrystallization.

Sulphuric acid at grain boundaries in Antarctic ice

It has been suggested1,2 that acids in the cold polar ice sheets may exist as aqueous mixtures at grain boundaries. This assumption can correctly predict the d.c. conductivity of polar ice2, but this does not prove the existence of acids or liquid veins at grain boundaries, and this remains controversial3–5. In this study we used a scanning electron microscope (SEM), equipped with a cold stage and an energy-dispersive X-ray microanalysis facility, to determine the location of sulphur in ice from the Antarctic Peninsula. As expected, sulphur was undetectable in the bulk of the ice. However, at the junctions where three grains met (triple-junctions), sulphur was found in concentrations greater than 1 M in areas of < 1 μm2. Calculations show that between 40 and 100% of the sulphuric acid present in this ice was found at the triple-junctions, and would have been liquid at ice-sheet temperatures. This finding, if general, has considerable implications for many of the physical properties of polar ice.

Seismic refraction measurements of internal friction in Antarctic ice

Seismic refraction measurements were carried out in 1970–1971 near Byrd Station, Antarctica, with a specific emphasis on the determination of seismic wave attenuation. The primary difficulty in interpreting field results is the extreme sensitivity of the observed attenuation to small variations in the seismic velocity gradient and its first derivative. Detailed velocity analysis, based on multiple surface-reflected arrivals, has made possible an estimate of the average internal friction for compressional waves between 100- and 500-m depth (temperature, −28°C): QP−1 = 0.0014 at 136 Hz, with upper and lower standard error estimates of +0.0005 and −0.0008, respectively. This value has been compared with laboratory measurements by Kuroiwa (1964) on cold slightly impure polycrystalline ice. The comparison required extrapolation of Kuroiwa's results to lower frequency, a correction for the difference between shear and compressional wave absorption, and an estimate of the amount of salt contained in ice frozen from a known solution. The agreement between field and laboratory results is good for the ionic impurity concentration measured in the ice near Byrd Station. All field measurements of QP−1 by reflection shooting in Antarctica and Greenland, although they cannot be assigned to a particular temperature, are also in satisfactory agreement, as are some previous determinations by refraction shooting in Greenland, although some from the high interior part of the Greenland ice sheet are seriously discrepant, probably because of inadequate knowledge of the seismic velocity gradient. We conclude that with the possible (but unlikely) exception of the central Greenland ice sheet, cold polar ice sheets exhibit an internal friction in the frequency range 50–200 Hz that is completely explained by the two fundamental mechanisms in polycrystalline ice: molecular relaxation (dominant at relatively cold temperatures) and grain boundary viscosity (dominant near the melting point). There is no evidence of any additional mechanism associated with internal structures such as bubbles or microcracks. Appendices 1 and 2 are available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J76-003; $1.00. Payment must accompany order.