In the depths of Norwegian winter, when the sun refuses to rise above the horizon for weeks at a time, something remarkable happens. Two extraordinary light shows perform simultaneously: northern lights dancing overhead while marine organisms glow in the waters below. Welcome to polar night, where darkness reveals illumination on both sides of the surface.

When researchers deployed autonomous underwater vehicles equipped with bathyphotometers into Kongsfjord, Svalbard, during January’s perpetual twilight, they discovered something unexpected: the water was alive with light. Not reflected light from above, but bioluminescence produced by marine organisms themselves, flashing and glowing in patterns that rival the auroras overhead.

At approximately 30 meters depth, something extraordinary occurs. This is what scientists call the bioluminescence compensation depth, the point where biological light exceeds atmospheric light in the underwater photon budget. Here, tiny copepods named Metridia longa pulse with luminescence as they migrate vertically through the water column. Dinoflagellates create their own light show, contributing up to 96 percent of the total underwater light budget in Norwegian fjords. Arctic krill sport small photophores on their undersides, essentially built-in light bulbs that help them blend into background light through counter-illumination.

The choreography is precise. During the dim midday hours of polar night, when faint atmospheric light penetrates the surface, zooplankton retreat deeper into darkness where predators can’t use that light to hunt. As true night arrives and surface light vanishes completely, these organisms rise toward shallower waters, their bioluminescence becoming the dominant light source in a world evolved for darkness.

Scientists discovered that this bioluminescent activity doesn’t follow circadian rhythms tied to day and night, because there is no day. Instead, organisms respond to subtle changes in the already dim light field, vertical migrations continuing throughout the polar night in patterns that acoustic backscattering confirms. The ecosystem is basically performing an invisible ballet lit by its own biological fireworks.

Meanwhile, 80 to 500 kilometers overhead, charged particles from solar winds collide with oxygen and nitrogen in Earth’s atmosphere, creating the aurora borealis. Green curtains ripple across the sky when solar particles strike oxygen molecules. Red auroras glow at the highest atmospheric levels where oxygen atoms have nearly two minutes between collisions. The northern lights reach their peak intensity during the same polar night period when underwater bioluminescence thrives below.

Both phenomena flourish when darkness provides the stage. Without competing daylight, bioluminescence becomes visible and functionally important to marine organisms. Without light pollution from the sun, auroras paint the sky in colors impossible to appreciate during summer’s midnight sun. Polar night transforms Norway’s waters into a theater lit from both directions.

Researchers studying this double illumination aren’t just documenting pretty lights. Understanding how bioluminescence structures Arctic food webs during winter darkness has implications for commercially important species like cod and herring that depend on these zooplankton. The knowledge matters for predicting how climate change might alter these light-dependent interactions.

But science doesn’t diminish the wonder. Stand on Norway’s Arctic coast during polar night and you might witness both shows at once: green auroras overhead, bioluminescent plankton glowing in the wave breaks at your feet. Two forms of light, one atmospheric and one biological, both thriving in what should be the darkest season.

Winter darkness is no void at all. It’s a quiet chamber, holding its breath, waiting to reveal the kind of light that needs no sun to exist.

Written by: Junior Thanong Aiamkhophueng.

Attribution: This article draws on research from studies of bioluminescence during Arctic polar night in Kongsfjord, Svalbard using autonomous underwater vehicles; findings on bioluminescence compensation depth and vertical community structure published in Scientific Reports and Marine Biology; research on Arctic krill counter-illumination from the University of Delaware; analysis of vertical migration patterns from ScienceDaily; transition depth studies published in the Journal of Geophysical Research; and information on northern lights formation, timing, and viewing conditions from National Geographic, Visit Norway, and Visit Svalbard.