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 Francisco.

"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.

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