While the sun never rises above the horizon up here during polar night, it does get a little brighter in the southern sky at midday, around the hours of 10am and 2pm. Add to that some light from the moon which is getting fuller every day, a little light from the stars, and a dash of aurora (northern lights), and there are quite a few photons here. In all, this makes for an interesting mix of skylight that still appears pretty dim to the human eye, but can be measured with instruments we have brought along. So each day at noon, Kim Last and I dress up in our warmest clothing and measure these photons in the sky. We have made a great set of measurements so far, starting as we left the dock in Tromsø, and continuing through today here in Krossfjorden. A big theme of the Arctic ABC project is light, and these measurements will be useful for interpreting the data generated by ice-tethered POPEs to be deployed in the near future.
But there is another bit to all of this. Many zooplankton swimming around in the waters up here produce their own light – they are bioluminescent. We have also brought along instruments to measure bioluminescence in the ocean. After Kim and I measure light from the sky, I run down to the CTD garage and lower these instruments, which we call the “light cage”. It takes about 30 minutes for the cage to descend to around 100 meters, stopping at intervals along the way to slurp up luminescent animals and record the light that they emit. Once the cage is back on the ship, I recover the data and after the cruise will examine how much bioluminescence exits at our stations, and determine the identity of the organisms doing it, which can be done by looking at the speed that the flashes of light occur.
In all, measuring light in both the sky and underwater is providing us with information on quantities of light that are not obvious to the human eye, but are important parts of marine ecology in the polar night.
-Jon Cohen (Assistant Professor, University of Delaware)