A fire in the middle of the Atlantic Ocean?
A fire in the middle of the Atlantic Ocean?
On July 14, 2017, the Radiometer visible for infrared images (VIIRS) on the Suomi NPP satellite captured this nocturnal image of the South Atlantic. The red dot several hundred kilometers off the coast of Brazil is a thermal anomaly, an area of the Earth's surface designated by the satellite as exceptionally warm.
Of the thousands of thermal anomalies that VIIRS detects each night, the vast majority are caused by fires. "But, obviously, a fire is not burning in the middle of the ocean," said Patricia Oliva, a senior university scientist who helped develop a fire detection algorithm for VIIRS when she was at the University of Maryland. natural gas flares They also cause thermal anomalies, but they are only found in shallow waters near the coast. Volcanic activity can also illuminate the satellite, but there are no volcanoes near this area.
"It's almost certainly SAMA," said Oliva, using an acronym for the Magnetic anomaly of the South Atlantic. This weakness in the Earth's magnetic field, centered on South America and the South Atlantic, allows one of the Van Allen radiation belts- Zones of energetic particles trapped by the magnetic field - to submerge closer to the atmosphere. As a result, much of South America and part of the South Atlantic Ocean receive an extra dose of radiation.
While the atmosphere blocks most high-energy particles and does not cause surface problems, there are enough of them in near-Earth space to cause problems for the electronic systems of spacecraft. The International Space Station has additional armor due to SAMA, and the Hubble Space Telescope turns off its scientific instruments when it passes through the region.
[youtube https://www.youtube.com/watch?v=iNmTsd6vZqE]
In the case of VIIRS, there are enough energetic particles circulating in the atmosphere around South America for the highly sensitive radiometer to detect some of them. In fact, the team that develops the Active fire data product VIIRS I was surprised at how often the particles. it appeared as fires When they started processing the data.
"Each night, the sensor detected several dozen thermal anomalies over the Atlantic Ocean in places that did not make sense," said Wilfrid Schroeder, the principal investigator of the Product of active fire VIIRS.. Scientists were aware of this type of anomaly because researchers working with the MODIS sensor from NASA and the European Space Agency Advanced along the track scanning radiometer (ATSR) satellite had found it. But the VIIRS team did not anticipate the detection of so many false fire signals.
His response was to build a series of filters in his active fire algorithm and eliminate false signals in this region. The suspicious thermal anomalies that are especially weak, over the ocean and of short duration (all signs that were caused by SAMA instead of a real fire) are eliminated by the algorithm.
But every once in a while a lost SAMA pixel slips through the filters. "We probably see one or two of these spurious fire detections per night, but remember it's in comparison to the thousands of real thermal anomalies that the satellite detects every night," Schroeder said. "False fire detections are pretty rare."
"By developing an algorithm like this for a global data product, we had to find a balance. "If we are too aggressive with our filtering, there is a risk that we will eliminate the real fires of data logging," Oliva said. "I do not think people realize that most satellite data products go through a full battery of calibration and validation tests to address problems like this."
Image of the NASA Earth Observatory by Lauren Dauphin, using data from the VIIRS day-night band Suomi National Polar-orbiting Partnership. Story of Adam Voiland.
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