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Scientists identify origin of hiss in upper atmosphere (Forwarded)



 
 
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Old March 6th 08, 04:43 AM posted to sci.astro
Andrew Yee
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Default Scientists identify origin of hiss in upper atmosphere (Forwarded)

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March 05, 2008

Scientists identify origin of hiss in upper atmosphere

Scientists have solved a 40-year-old puzzle by identifying the origin of
the intense radio waves in the Earth's upper atmosphere that control the
dynamics of the Van Allen radiation belts -- belts consisting of
high-energy electrons that can damage satellites and spacecraft and pose a
risk to astronauts performing activities outside their spacecraft.

The source of these low-frequency radio waves, which are known as
plasmaspheric hiss, turns out to be not lightning or instabilities from a
plasma, as previously proposed, but an intense electromagnetic wave type
called "chorus," which energizes electrons and was initially thought to be
unrelated to hiss, said Jacob Bortnik, a researcher with the UCLA
Department of Atmospheric and Oceanic Sciences.

The findings appear March 6 in the journal Nature.

"That chorus waves are the dominant source of plasmaspheric hiss was a
complete surprise," said Bortnik, whose research was federally funded by
the National Science Foundation.

"Numerous theories to explain the origin of hiss have been proposed over
the past four decades, but none have been able to account fully for its
observed properties," Bortnik said. "Here, we show that a different wave
type, called chorus, can propagate into the plasmasphere from tens of
thousands of kilometers away and evolve into hiss. Our new model naturally
accounts for the observed frequency band of hiss, its incoherent nature,
its day-night asymmetry in intensity, its association with solar activity
and its spatial distribution.

"The connection between chorus and hiss is very interesting because chorus
is instrumental in the formation of high-energy electrons outside the
plasmasphere, while hiss depletes these electrons at lower equatorial
altitudes," he said.

Beginning in the late 1960s, spacecraft observations of wideband
electromagnetic noise at frequencies below a few kilohertz established the
presence of a steady, incoherent noise band in the frequency range between
200 Hz and 1 kHz. This emission was dubbed plasmaspheric hiss because of
its unstructured nature, its spectral resemblance to audible hiss and its
confinement to the plasmasphere, a dense plasma region around the Earth.

Bortnik was initially studying chorus, not hiss, when he made the
discovery -- one of many examples of serendipity in science.

Hiss tends to be confined inside of the plasmasphere, and chorus outside
of it. Bortnik was modeling chorus because he knew it was important in
creating high-energy electrons in space. While chorus occurs outside the
plasmasphere, it leaks inside of it.

A better understanding of plasmaspheric hiss will help scientists to more
accurately model the behavior of the high-energy electrons in the Van
Allen radiation belts and thus improve their forecasts of space
conditions, Bortnik said.

Co-authors of the research are Richard M. Thorne, UCLA professor of
atmospheric and oceanic sciences, and Nigel P. Meredith at the British
Antarctic Survey, Cambridge, U.K.

UCLA is California's largest university, with an enrollment of nearly
37,000 undergraduate and graduate students. The UCLA College of Letters
and Science and the university's 11 professional schools feature renowned
faculty and offer more than 300 degree programs and majors. UCLA is a
national and international leader in the breadth and quality of its
academic, research, health care, cultural, continuing education and
athletic programs. Four alumni and five faculty have been awarded the
Nobel Prize.


 




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