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  3. Humans on the earth have had an over-inflated view of how far the planet’s atmos

Humans on the earth have had an over-inflated view of how far the planet’s atmos

游客2023-09-07  28
问题 Humans on the earth have had an over-inflated view of how far the planet’s atmosphere extends into space. It turns out that the ionosphere (电离层), the layer of electrically charged particles that comprises the outer atmosphere, is thinner than expected-and cooler.
     Understanding the shape and size of the ionosphere is an important first step in figuring out how particularly dense regions within it may distort radio, radar and navigation signals. Such distortions can make communications and satellite-based systems less reliable.
     "In order to predict how severe those distortions will be, it’s necessary to know how big those structures in the ionosphere are and where they exist," Roderick Hellis, with the Space sciences Center at the University of Texas in Dallas, told Discovery News.
     Hellis and his colleagues have been using a suite of NASA instruments called CINDI, which fly on the U.S. Air Force Communication/Navigation Outage Forecast System (C/NOFS) satellite between 250 miles and 530 miles around the planet’s equator.
    The researchers never expected to encounter the edge of Earth’s ionosphere at those altitudes, but that is exactly what happened during the summer of 2008, a time when the solar activity was unusually weak.
      "It was a real combination of low solar activity and the satellite’s range," Hellis said. "We didn’t expect to be able to look at the top of the ionosphere in all places." Computer models based on previous research had predicted the ionosphere to be about 370 miles above Earth at night and about 620 miles up during the day--the variation due to temperature and other factors.  
     Instead, the CINDI team discovered that the transition between the ionosphere and space was about 250 miles above Earth at night and about 500 miles up during the day.
     The ionosphere is primarily caused by extreme ultraviolet energy from the sun.
     "It’s powerful enough that those photons (光子) can ionize (使……电离) atoms in Earth’s upper atmosphere," said David Hathaway, a solar physicist at NASA’s Marshall Space Flight Center in Huntsville, Ala. "When you have a geomagnetic disturbance or a solar flare going off, you can get an ionosphere that is uneven, which can cause problems with radio signals and GPS satellite signals, which have to pass through."
     The findings were presented at the annual American Geophysical Union conference in San Francisco last month.
     CINDI is an acronym for Coupled Ion Neutral Dynamics Investigation. The device works by separately measuring ionized and neutral particles at altitudes where the Air Force satellite flies. A similar device is on another polar-orbiting Air Force spacecraft.
     The researchers hope to keep using CINDI as solar activity picks up in the sun’s new 11-year cycle. The next solar maximum is expected in late 2011 or 2012.   [br] In 2008, scientists encountered the top of the ionosphere all______.
选项 A、thanks to the weak solar activity
B、because of the advanced CINDI
C、by chance
D、as predicted
答案 C
解析 第5段中的never expected，but that is exactly what happened以及第6段首句中的real combination和第2句中的didn’t expect等都表明这一次能观测到电离层边沿是偶然的，是因为太阳活动和观测仪器范围的巧合，因此选C。
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