Artificial cloud created at the edge of space

 作者:褚殛瘰     |      日期:2019-03-08 10:08:10
By Rachel Courtland Update on 23 September: The dust released in the CARE experiment scattered sunlight, creating a bright cloud that could be seen at visible wavelengths for several minutes before it dispersed. The cloud’s effects on the ionosphere lingered longer, however, and were detected by radar for about four hours after the dust’s release. Researchers are still analysing the data and expect to know more about the results in the coming weeks. Watch a video of the cloud See a gallery of noctilucent clouds The study of Earth’s mysterious noctilucent clouds got a boost on Saturday, when a rocket was launched to create an artificial cloud at the edge of space. “Noctilucent”, or night-shining, clouds float dozens of kilometres higher than other clouds, at an altitude of about 80 kilometres. Because of their height, they can be seen glowing before sunrise or after sunset as the sun illuminates them from below the horizon. The clouds were first observed above polar regions in 1885 – suggesting they may have been caused by the eruption of Krakatoa two years before. But in recent years they have spread to latitudes as low as 40°, while also growing in number and getting brighter – a change that some attribute to global warming. The ice-covered particles in these clouds are commonly thought to get their start as tiny dust particles that had been sloughed off comets and meteoroids. See a gallery of noctilucent clouds These particles build up a charge as they are bombarded by fast-moving electrons and ions. Since charged particles reflect radio waves, they are good candidates for radar studies, which could help measure the dust at an early stage in the clouds’ formation. But the radar data is hard to interpret. To get a better sense of how high-flying dust reflects radio waves, researchers launched the Charged Aerosol Release Experiment (CARE) at 2346 GMT on Saturday from NASA’s Wallops Flight Facility in Virginia. The experiment, which blasted off on a Black Brant XII suborbital sounding rocket, spewed more than 100 kilograms of aluminium oxide into the atmosphere at an altitude of about 280 km. A combination of ground- and space-based instruments will watch this cloud for days and perhaps months as its particles become charged, sink, and disperse. Laser pulses will be used to measure the density of the particles in the cloud. These will be compared with radar measurements of the plume. “Our primary science goal is to understand [how] radar [scatters] from a dusty plasma,” says CARE’s principal investigator, Paul Bernhardt of the Naval Research Laboratory in Washington, DC. “We want to understand what kind of information you get back when you send out radar pulses.” Radar has been used to examine icy dust that hovers some 90 km above the Earth and can grow to become the ice particles inside noctilucent clouds, says Scott Bailey of Virginia Tech in Blacksburg. He is a lead scientist for NASA’s Aeronomy of Ice in the Mesosphere spacecraft, which launched in 2007 to study noctilucent clouds. But at the moment, interpreting radar measurements of these proto-cloud particles is difficult. Although they reflect radar brightly, it is difficult to say how large the particles are or how many there might be, Bailey says. Studying an artificial cloud created in a controlled manner could help resolve these uncertainties. “If the radar could be used to say exactly what the population of particles is up there that the ice is forming around, that would be a major step forward,” Bailey told New Scientist. The CARE experiment could also pave the way for future launches that would use the uppermost part of Earth’s atmosphere as a large physics laboratory for studying charged dust, Bernhardt says. Dusty plasmas, like those that will be created in the CARE experiment, are seen in a range of environments, from Saturn’s rings and comet tails to the dust hovering above the lunar surface. See a gallery of noctilucent clouds More on these topics: