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High-energy particles from the magnetic anomaly of the South Atlantic cheat satellite sensors
On July 14, 2017, the Visible Infrared Imaging Radiometer Suite ( VIIRS ) of the Suomi NPP satellite captured this nocturnal image of the South Atlantic. The red dot several hundred kilometres off the coast of Brazil is a thermal anomaly, an area of the Earth’s surface indicated by the satellite as unusually warm.
Of the thousands of thermal anomalies that VIIRS detects every 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.
The eruptions of natural gas also cause thermal anomalies, but only found in shallow waters near the shore. 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 South Atlantic Magnetic Anomaly. This weakness in the Earth’s magnetic field, centered in South America and the South Atlantic, allows one of the Van Allen radiation belts of Earth (areas of energetic particles trapped by the magnetic field) to sink closer to the Earth’s surface, 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 armour due to SAMA, and the Hubble Space Telescope turns off its scientific instruments when it passes through the region.
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 developed the VIIRS active fire data product was surprised at the frequency with which the particles appeared as fires when they began processing the data.
“Every 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 VIIRS active fire product. The scientists were aware of this type of anomaly because it was found by researchers working with the NASA MODIS sensor and the Advanced Along Track Scanning Radiometer ( ATSR ) satellite of the European Space Agency. 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 trigger algorithm and eliminate false signals in this region. They are eliminated by the algorithm suspicious thermal anomalies that are especially weak, on the ocean and short duration (all signs that were caused by SAMA instead of a real fire).
But every once in a while a lost SAMA pixel slides 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. “Detections of false fires are quite 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 eliminate real fires from the data record,” 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 these kinds of problems.”
Source: Vista al Mar