Experimental detection of freshwater ice clarification in the optical range near 0 °C

Abstract

The article presents the results of laboratory measurements of the transparency of blocks of fresh ice in the optical range in the temperature range from –15 °C to 0 °C. The studies were carried out at two wavelengths in the visible range (at a wavelength of 535 nm) and in the ultraviolet range (at a wavelength of 370 nm). Ice from a natural freshwater reservoir with a characteristic predominant spatial orientation of the main optical axis of crystals was studied. It was found that as the temperature approaches 0 °C, the sample becomes clearer (attenuation decreases). The effect occurs in the range of –0.5 °C…0 °C. It is determined by the occurrence of plastic deformation due to thermal stresses caused by the initial stage of the ice-water phase transition. At this point, ice crystals begin to flow along their basal planes, along which, as is known, they deform at minimum values of shear stresses. It was found that the effect is most pronounced when the direction of the electric field vector of waves coincides with the basal plane of ice crystals. Possible reasons for clarification are considered. The most likely of them is associated with the formation of a conductive layer of nanometer thickness, along which parts of ice crystals slide. In such a structure, a significant enhancement of electric fields is possible due to plasmon resonances. If the assumed mechanism takes place, then clarification is also possible in other directions in the crystal in case significant mechanical stresses are applied to them, creating shear deformations. Clarification in experiments with ice blocks ~10 cm thick was 3%…25%. The obtained results are of interest for solving remote sensing tasks, since melting ice is a widespread object due to its significant heat of phase transition. Previously, the effect of clarification of fresh ice was also found in the microwave range when approaching the sample temperature to the phase transition point.