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.

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