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Uranus' signature in a radio signal.

Uranus' signature in a radio signal

Less than a week before Voyager 2 flew past Uranus in 1986, two of the craft's instruments detected bursts of radio signals in an extremely narrow band of frequencies. They seemed to be the first signs of radio emissions from the planet, making them the first clear evidence that Uranus has a magnetic field (SN: 2/1/86, p.72). However, the frequency band was so narrow -- the emissions showed up in one of the instruments' channels but not in an adjacent one -- that scientists initially wondered whether these signals might be not from Uranus at all, but instead due to electronic noise created when the instruments were periodically "cycled" through their operating ranges.

Now three scientists involved with the Voyager mission report that the data may have been confusing because the "instrument cycling periods" used before the flyby were based on a Uranian day then assumed to be 15.57 hours long. They conclude that the narrowband bursts reflect the planet's actual 17.24-hour rotation period, a sign that they are indeed "planetary in origin." Michael D. Desch and Michael L. Kaiser of the NASA Goddard Space Flight Center in Greenbelt, Md., with William S. Kurth of the University of Iowa in Iowa City, describe the work in the May 1 JOURNAL OF GEOPHYSICAL RESEARCH.

In addition to the narrowband bursts detected just before Voyager flew past Uranus, the researchers have since identified another batch from about three weeks earlier -- in other words, a total of two major episodes, each about 10 days long. Comparing the timing of the burst episodes with Voyager's solar-wind measurements near Uranus indicates, they say, that "the radiated power [of the radio emissions] is greatly enhanced when the solar-wind density is enhanced. When the density is very low, few if any narrowband bursts are emitted." The two "bursty" periods thus provide what the scientists call the first evidence of "solar-wind-driven control" of Uranus radio emissions.
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Title Annotation:Space Sciences
Publication:Science News
Date:May 20, 1989
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