  Ongoing experimental efforts in Antarctica seek to detect ultra-high energy
neutrinos by measurement of radio-frequency (RF) Askaryan radiation generated
by the collision of a neutrino with an ice molecule. An array of RF antennas,
deployed either in-ice or in-air, is used to infer the properties of the
neutrino. To evaluate their experimental sensitivity, such experiments require
a refractive index model for ray tracing radio-wave trajectories from a
putative in-ice neutrino interaction point to the receiving antennas; this
gives the degree of signal absorption or ray bending from source to receiver.
The gradient in the density profile over the upper 200 meters of Antarctic ice,
coupled with Fermat's least-time principle, implies ray "bending" and the
existence of "forbidden" zones for predominantly horizontal signal propagation
at shallow depths. After re-deriving the formulas describing such shadowing, we
report on experimental results that, somewhat unexpectedly, demonstrate the
existence of electromagnetic wave transport modes from nominally shadowed
regions. The fact that this shadow-signal propagation is observed both at South
Pole and the Ross Ice Shelf in Antarctica suggests that the effect may be a
generic property of polar ice, with potentially important implications for
experiments seeking to detect neutrinos.
