Night-time Treats

This image of Rio de Janeiro was acquired on the night of July 20, 2012 by the VIIRS instrument aboard the Suomi NPP satellite. Data courtesy of NASA/NASA’s Earth Observatory.

This image of Rio de Janeiro was acquired on the night of July 20, 2012 by the VIIRS instrument aboard the Suomi NPP satellite. Data courtesy of NASA/NASA’s Earth Observatory.

The Opening Ceremony of the Rio Olympics featured a plane taking off from the Maracanã Stadium and treating us to a fantastic night flight over Rio. It was a beautiful sequence to celebrate the famous Brazilian aviator Alberto Santos-Dumont, for us at Pixalytics it led to a conversation about the beauty of night-time satellite imagery!

Currently, the best source of night-time imagery comes from Visible Infrared Imaging Radiometer Suite (VIIRS) which is one of five instruments aboard the Suomi National Polar-orbiting Partnership satellite launched on 28 October 2011. Although, if you look on Twitter you’ll also see a huge number of night-time images taken by astronauts aboard the International Space Station. This data has been used as the basis of the Cities at Night citizen science project whose aim is to create a Google maps style map of the world – as the astronauts are using cameras to take photos of the places that interest them, and there is no georeferencing information, citizens identify the cities pictures.

In contrast VIIRS is an orbiting satellite and so continually collecting calibrated and georeferenced data of the whole globe. In the day VIIRS is collecting optical and temperature data over both the land and ocean, while at night it collects temperature data and the night-time imagery using the 750 m spatial resolution Day/Night Band (DNB). Working through both the night and day, the DNB needs to be calibrated through several orders of magnitude in brightness to accommodate the dramatic contrast between solar reflection and the darkness of night. Its forerunner was the uncalibrated Operational Linescan System (OLS) on the Defense Meteorological Satellite Program (DMSP) satellites, whose primary aim was to study clouds, but when its data was declassified in the 1970s it generated a lot of interest in low light night-time observations.

The DNB VIIRS images, like the one at the top of the blog, show hubs of human activity and the road arteries that connect them, and so are of special interest to the Campaign for the Protection of Rural England who use these types of maps to protect dark skies. It also enables calculations of light pollution to be made, together with indications of the associated carbon emissions. The DNB can pick up many different phenomena. For example, aurorae are visible, as well as gas flares, volcanic activity, the lights of ships, sea ice and climatological monitoring of clouds. It’s even possible to see thunderstorms, although individual lightning flashes are hard to make out in these snapshots, the glow inside clouds caused by them are evident as bright strips with DNB imagery as seen in this image from over Louisiana, USA on 4 April 2012 (Miller et al., 2013).

Another interesting discovery in 2012 was the presence of a faint ‘nightglow’ in the upper atmosphere on moonless night over the Pacific. The DNB team were aiming to collect scenes of complete darkness for calibration purposes, but they found clouds were still clearly visible. This was due to an assortment of photochemical reactions, especially of the molecule fragment hydroxyl, which allows this nightglow to pick up subtle atmospheric phenomena such as gravity waves and the tops of anvil clouds.

Here we’ve gone from an aviation image inspired from 1903 to modern satellites, all via the Rio Olympics. It’s amazing where space can take you!

 

Blog written by Dr Louisa Reynolds and Andrew Lavender from Pixalytics Ltd.

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