Scientists watched the Arctic with particular interest in the summer of 2012, when Arctic sea ice set a new record low. The behavior of sea ice following such a low extent also interests scientists, but as Arctic sea ice was advancing in the autumn of 2012, so was polar darkness.
Fortunately, the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite can see in the dark. The VIIRS “day-night band” detects light in a range of wavelengths from green to near-infrared and uses filtering techniques to observe signals such as gas flares, auroras, wildfires, city lights, and reflected moonlight. VIIRS acquired this nighttime view of sea ice north of Russia and Alaska on October 30, 2012.
The day-night band takes advantage of moonlight, airglow (the atmosphere’s self-illumination through chemical reactions), zodiacal light (sunlight scattered by interplanetary dust), and starlight from the Milky Way. By using these dim light sources, the day-night band can detect changes in clouds, snow cover, and sea ice. The VIIRS day-night band offers a unique perspective because once polar night has descended, satellite sensors relying on visible light can no longer produce photo-like images. And although passive microwave sensors can monitor sea ice through the winter, they offer much lower resolution.
Steve Miller of the Cooperative Institute for Research in the Atmosphere at Colorado State University has used the day-night band to study nighttime behavior of weather systems and sees advantages in studying the polar regions. “There’s a lot of use with these measurements as we look back at a season of record ice melt in the Arctic,” Miller says. “We can observe areas where there is ice melt and reformation, where there’s clear water and ships can pass through—especially as the ‘great darkness’ approaches with winter.”
Ted Scambos of the National Snow and Ice Data Center at the University of Colorado concurs. “Things start changing rapidly in the late fall: sea ice formation and snow cover extent at the highest latitudes. This lets us see rapid-growth areas in detail.”
The day-night band is also useful for following weather systems, including severe storms, which can develop and strike populous areas at night as well as day. Geostationary Operational Environmental Satellites orbit the Earth’s equator. The satellites offer uninterrupted observations of North America, but high-latitude areas such as Alaska may benefit more from polar-orbiting satellites. Miller explains, “In the high latitudes, the orbits begin to overlap considerably, which gives you a lot more passes in Alaska. If you start to look at multiple passes and stitch them together, you can get a version of a poor man’s geostationary time loop of the weather.”
NASA Earth Observatory image by Jesse Allen and Robert Simmon, using VIIRS Day-Night Band data from the Suomi National Polar-orbiting Partnership. Suomi NPP is the result of a partnership between NASA, the National Oceanic and Atmospheric Administration, and the Department of Defense. Caption by Michon Scott.
Satellite images of Earth at night have been a curiosity for the public and a tool of fundamental research for at least 25 years. They have provided a broad, beautiful picture, showing how humans have shaped the planet and lit up the darkness.