A University of Iowa space physicist says that NASA’s Voyager 1
spacecraft — the most distant manmade object at some 90 astronomical
units (AU) or 8.4 billion miles from the sun — has provided a new
estimate of the distance to the heliopause, the boundary between the
sun’s relentless solar wind and interstellar space.

Don Gurnett, professor of physics in the UI College of Liberal Arts
and Sciences and principal investigator for the plasma wave
instrument on Voyager 1, will present his findings in a Dec. 8 talk
during the 2003 Fall Meeting of the American Geophysical Union in San

“Using the Voyager 1 plasma wave instrument, we started detecting a
strong radio emission event in November 2002 that is continuing to
the present time, the first to be observed during the current solar
cycle. The new event is thought to be linked to a period of intense
solar activity in April 2001. The time delay between the solar flare
activity and the onset of the radio emission indicates the heliopause
is located about 153 to 158 AU (14.3 to 14.7 billion miles) from the
sun,” Gurnett says. “From computer simulations that give the ratio of
the radial distance to the termination shock to the radial distance
to the heliopause, the distance to the termination shock can also be
calculated and is estimated to be about 101 to 118 AU.

“Periods of increased solar flare activity occur about every 11 years
and are accompanied by energetic bursts of plasma, or clouds of
electrically charged gas. From the propagation velocity of the plasma
cloud, and the time it takes to reach the heliopause, as indicated by
the onset of the radio emission, the distance to the heliopause can
be computed. This latest finding is very consistent with our 1993
report of a similar radio burst that occurred in 1992,” he says.

In 1993, Gurnett and his colleagues reported the first direct
evidence of the distance to the heliopause, in the form of a powerful
low-frequency radio burst. They theorized that the 10 trillion watt
radio burst detected in August 1992 by Voyagers 1 and 2 occurred when
a plasma cloud, ejected from the sun during a period of intense solar
flare activity about a year earlier, collided with the cold
interstellar medium. The collision created the powerful radio burst
that propagated back toward the sun at the speed of light. From the
speed and travel time of the plasma cloud and its associated shock
wave, they estimated that the heliopause lies between 116 and 177 AU
from the sun.

The location of the heliopause, which marks the outermost edge of the
solar system, is a subject of scientific speculation. In two papers
recently published in the journal Nature, scientists debated whether
Voyager 1 has already reached the termination shock, a sign that the
heliopause may be near. The termination shock is caused by a
reduction in the speed of the solar wind as it slams into cooler
plasma at the edge of the solar system and is similar to the sonic
boom that occurs on Earth when an airplane crosses the sound barrier.

In some ways, the search for the heliopause is a search for a moving
target because its location likely changes over time, depending upon
the strength of the sun’s output of plasma, or solar wind. Just as
the edge of a cloud of smoke surrounding a Fourth of July sparkler
alternately swells and contracts, the heliopause changes shape.

Voyager 1, launched Sept. 5, 1977, completed fly-bys of both Jupiter
and Saturn and currently is escaping the solar system at a speed of
about 3.6 AU per year. A sister spacecraft, Voyager 2 was launched
Aug. 20, 1977 on a flight path that took it to encounters with
Jupiter, Saturn, Uranus and Neptune. At present Voyager 2 is about 72
AU from the sun and traveling at about 3.3 AU per year.

The sounds of the recent radio emission and other sounds of space can
be heard by visiting Gurnett’s Web site at:
http://www-pw.physics.uiowa.edu/space-audio/index.html. A member of
the National Academy of Sciences, Gurnett has seen his 40 years of
collected space sounds serve as the inspiration for the
NASA-commissioned and critically acclaimed music and visual
composition “Sun Rings,” composed by Terry Riley and performed around
the world by the famed Kronos Quartet.

Gurnett whose co-authors on the research paper are William Kurth,
research scientist at the University of Iowa, and Ed Stone, Morrisroe
Professor of Physics at the California Institute of Technology and a
former JPL director, acknowledged the support given by NASA and
NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif. JPL, a
division of Caltech, manages the Voyager mission for NASA’s Office of
Space Science, Washington, D.C.

STORY SOURCE: University of Iowa News Services, 300 Plaza Centre One,
Suite 301, Iowa City, Iowa 52242-2500.