IceCube ice anisotropy could be due to birefringent polycrystals

IceCube is a neutrino-detecting telescope installed in the ice at the U.S. Amundsen-Scott South Pole Station at the geographic South Pole. The cube-shaped telescope is 1 km on a side and consists of many strings of digital optical modules (DOMs), containing large photomultiplier tubes (PMTs), that were installed by melting holes in the super-clear ice, lowering the 1-km-long strings in, and letting the ice refreeze. Photons produced by neutrinos creating Cherenkov radiation as they strike the ice are captured by the PMTs, allowing the direction and energy of the neutrinos to be determined. But there’s a problem: IceCube data shows that the ice that serves as the medium for the experiment has an anisotropy in its light propagation properties at large scales. A test revealed that measurements taken about 125 m from an isotropic emitter in the ice shows about twice as much light received along the ice’s flow direction than on the perpendicular tilt axis.

Dmitry Chirkin from the Wisconsin IceCube Particle Astrophysics Center, University of Wisconsin (Madison, WI), Martin Rongen from RWTH Aachen University (Aachen, Germany), and others in the IceCube Collaboration have looked into the idea that the microstructure of the ice as it has been affected by ice flow has led to the formation of a birefringent polycrystal structure, which can explain the direction-dependent differences in attenuation. The researchers carried out an exact calculation and simulation of the “fabric” of the polycrystal structure, showing diffusion that is highest along the flow, growing smaller in a continuous fashion at angles farther away toward the tilt direction. Interestingly, at intermediate angles, the diffusion distribution is slightly asymmetric, which produces a mean deflection towards the ice flow axis. Reference: The IceCube Collaboration, arXiv:1908.07608v1 [astro-ph.HE] (Aug. 20, 2019).