Printer Friendly

Mysterious radio bursts hint at heliopause.

Hurtling through space a few billion kilometers beyond Neptune and Pluto, two aging spacecraft have recorded intense bursts of radio waves that may originate from the very edge of the solar system. If verified by further analysis, the bursts will provide a new way to gauge the size of the solar system, as well as reveal the structure of the heliopause -- the proposed boundary between the solar system and interstellar space.

The radio emissions, detected by Voyagers 1 and 2, peaked in intensity in December. Their discovery marks a new era of exploration for the nearly 16-year-old probes, best known for their close-up portraits of Jupiter, Saturn, Uranus, and Neptune. Voyager 1 is now 52 astronomical units (AU) from the sun (one AU equals about 149.6 million kilometers), while Voyager 2 lags behind at 40 AU.

Since July 1992, the craft have recorded radio emissions from an unknown source whose total power exceeds 10 trillion watts. These bursts have about two to 30 times the intensity of an equally puzzling group of bursts detected by the Voyagers in 1983 and 1984, says Donald A. Gurnett of the University of Iowa in Iowa City, and may represent the most powerful radio source in the solar system. However, the bursts are too low in frequency to penetrate the solar system's sea of charged particles to reach Earth.

Gurnett, William S. Kurth of the University of Iowa, and their colleagues reported the findings in Baltimore this week at an American Geophysical Union meeting. They trace the radio emissions to events that began in May 1991. A series of large solar flares erupted then, accompanied by an explosive release of gas into the solar wind, the stream of charged particles emanating from the sun.

Three months after this explosive release, the two Voyagers and the Pioneer 10 craft recorded a sudden drop in the intensity of cosmic rays -- high-energy particles from outside the solar system. One explanation for the falloff is that the enhanced solar wind deflected the cosmic rays. The time lag between the flare activity and the cosmic-ray decline suggests that the solar wind traveled at a speed of 600 to 800 kilometers per second, compared to the 400 km/sec speed of the average solar wind, John A. Lockwood of the University of New Hampshire in Durham and William R. Webber of New Mexico State University in Las Cruces calculate in the May 1 JOURNAL OF GEOPHYSICAL RESEARCH.

Eventually, Gurnett suggests, the bulkier solar wind reached the heliopause. At this boundary, the solar wind must halt: Pressure from the interstellar wind equals the pressure from the solar wind. This collision created intense bursts of radio waves, some of which headed back into the solar system and were detected by the Voyagers, he speculates.

Gurnett says measurements from different orientations of the Voyagers indicate the bursts come from where the solar wind collides head-on with the interstellar wind. He adds that the emissions have gradually shifted to higher frequencies. This suggests a well-defined heliopause width, with lower-frequency radio bursts created earlier at the boundary's near, lower-density edge, and higher-frequency signals created later, when some of the solar wind penetrates deeper into the heliopause's denser parts.

Researchers a decade ago had proposed that the heliopause would lie closer to the sun, about 70 AU distant. Based on the new radio data, Gurnett places it some 100 to 160 AU from the sun. Ralph L. McNutt Jr., of Johns Hopkins University Applied Physics Laboratory in Laurel, Md., estimates the boundary may lie between 90 and 120 AU away. These new estimates are consistent with recent Hubble Space Telescope data that suggest a distance of 100 AU (SN: 5/22/93, p.326).
COPYRIGHT 1993 Science Service, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:intense radio wave bursts detected by Voyager 1 and
Author:Cowen, Ron
Publication:Science News
Date:May 29, 1993
Previous Article:Midcontinent heat may explain great quakes.
Next Article:Chlorination products linked to cancer.

Related Articles
Uranus' signature in a radio signal.
Listening for hints of the sun's heliopause.
Voyager instruments get new instructions.
Shedding more light on gamma-ray bursts.
One-man band: x-ray source plays two tunes.
Take it to the max: supernova in overdrive.
Gamma-ray bursts reveal distant galaxies.
Cosmic dawn: pinning down the origin and nature of the first stars.
Next stop, interstellar space: Voyager journeys to the edge of the solar system.
Puzzling radio blasts.

Terms of use | Copyright © 2017 Farlex, Inc. | Feedback | For webmasters