3 SCIENTIFIC ENDEAVOURS AND ACHIEVEMENTS

In this section we describe the two major optical experiments performed, followed by a brief description of various auxiliary experiments intended to monitor the physical condition of the ice.

The experiment developed in 1985 consists of a monochromatic source which introduces light into a spot on the surface of the ice and detectors that measure the light emerging from the top and bottom surfaces. Since sea ice is a very turbid medium, light emerges over a large area centred on the source, and it is the intensity profile of this spot that is measured in our experiment. These profiles can be interpreted in terms of the light scattering properties at various depths. The apparatus, methods, and interpretation scheme were developed entirely by this research group. This year's use of the equipment has permitted us to extend our measurements to about two weeks earlier in the season than we had managed previously, and has confirmed that the ice is even clearer in early October than we had predicted based on our measurements of 1986 and 1989. This experiment was performed as often as convenient over the entire period in the field, 16 October - 8 November.

The second major experiment was a measurement of the distribution of path lengths taken by light as it scatters randomly through sea ice. For this purpose we introduce a very short (3 × 10⁻⁹ sec) burst of light into the surface, and measure the straggling in the emergent pulse at some distance (100-500 mm) from the source. This very demanding experiment was deployed for the first time on 22 October, and we collected many spectra then and on a page 4 number of later dates until our departure from the field. We can clearly see the effects of straggling in the pulse shape, although it still remains to be seen how well they can be interpreted. A number of items used in this experiment had to be kept above 0°C and during measurements we kept them in carefully insulated and heated boxes. At other times we had to keep them in a dry heated wannigan, and for this purpose the oil heated wannigan NZ8 was invaluable.

In addition to the two primary experiments, we continuously monitored the ice with measurements of the salinity, density, and temperature at various depths. These experiments were all operated successfully as they have been in earlier seasons. Changes in these quantities, particularly near the surface, correlate with changes in the optical behaviour.