BIG BROADCAST
Solar radio bursts put a new wrinkle in space weather
BY RON COWEN
When twisted magnetic fields on the sun snap
and unleash their energy, it’s the most osten-
tatious fireworks that grab the headlines.
Brilliant explosions in the sun’s outer atmos-
phere can send billion-ton clouds of charged
particles speeding towards Earth, where they can short-
circuit electrical power grids and cause large-scale
blackouts. The sudden release of magnetic energy can
also generate solar flares, which pour a torrent of ultra-
violet light and X rays into space. Solar flares can dis-
able satellites and harm space-walking astronauts.
But a subtler kind of solar explosion has often gone under the
radar. It involves powerful bursts of radio waves that often accompany solar flares. At 2:30 p.m. EST on Dec. 6, 2006, about an hour
after a moderately energetic flare erupted, the sun emitted the
most powerful burst of radio waves ever recorded. During a high-intensity blitz that lasted more than 10 minutes, the storm swamped
the entire sunlit side of Earth with radio noise. Across North and
South America and parts of the
Pacific, it overwhelmed dozens of
radio receivers linked to the Global
Positioning System (GPS). The network of GPS satellites provides critical distance and time information
for everything from airplane navigation to maintaining the critical
alignment of oil rigs as they drill into
the seafloor.
The U.S. military reported a
“widespread” loss of GPS signals in
New Mexico and Colorado.
Although the storm caused no
casualties, “the effect on GPS
receivers was more profound and
widespread than expected,” says
Paul Kintner, an electrical engineer
and computer specialist at Cornell
University. The storm was especially
surprising because it happened
when the sun, near the minimum of
its 11-year activity cycle, was relatively calm.
“Now, we’re concerned more-severe consequences will occur
during the next solar maximum,” Kintner adds.
Radio astronomer Dale Gary of the New Jersey Institute of
Technology in Newark says that the Dec. 6 event challenges scientists’ assumptions about how often, and when, the sun can
interfere with GPS and other wireless communications.
Gary, Kintner, and their colleagues described their findings during an April conference on space weather held in Washington, D.C.
Says space physicist Anthea Coster of the Massachusetts Institute of Technology’s Haystack Observatory in Westford, Mass.,
“It’s obvious that society is increasingly dependent on space-based technology, so it’s important to understand why this burst
occurred and to be able to quantify the power and potential
impact of future radio bursts.”
RADIO PUZZLES Solar flares and radio bursts usually occur in
tandem, emerging from the same region on the sun. Compared with
flares, however, radio bursts are much more difficult to predict
and track, notes radio astronomer Don Gurnett of the University
of Iowa in Iowa City.
Both kinds of outbursts arise within sunspots, dark areas
threaded by strong magnetic fields. The spots look dark because
the powerful magnetism acts as a lid, preventing heat and light from
rising to the sun’s surface.
Magnetic fields twist and tangle in response to the sun’s rotation. When they break apart and reconnect, they release vast
amounts of heat and radiation. Flares are direct products of that
energy discharge. One of the most common types of radio burst,
however, results from a more complex process.
First, the strong electric fields associated with a flare accelerate
electrons that freely circulate in layers of hot, ionized gas, or plasma,
above the sun’s surface. Beams of
these accelerated electrons, speeding away from the surface, then
slam into the plasma that forms the
sun’s atmosphere, or corona. Just
as the stream of air in a flute produces vibrations at specific frequencies, the electron beams striking the
background plasma set up oscillations known as plasma waves.
Next, the plasma waves generate
bursts of radio waves both at the frequency of the vibrating plasma and
at twice that frequency. The bursts
gradually shift to lower frequencies
as the electron beams travel higher
in the sun’s atmosphere, where they
encounter plasma of progressively
lower density.
The Dec. 6 event appears to have
followed this general pattern, but data show little correlation
between the strength of a flare and the severity of the subsequent
burst. The radio storm caught everyone by surprise.
“We don’t know the physics well enough to be able to predict
when bursts are going to happen and how large they can be,” says
Gary. But he points out one clue to the strength and duration of
the burst: The so-called active region from which the burst
emerged appeared to have contained several adjoining bundles
of magnetic fields. Gary speculates that the north and south poles
GARY, OVSA
RADIO NOISE — Six months ago, these radio telescopes,
part of a network at the Owens Valley Solar Array in
California, recorded the most powerful known radio
storm generated by the sun.
