Physicists make first measurements of the solar wind termination shock

Two University of Iowa space physicists report that the Voyager 2 spacecraft, which has been travelling outward from the Sun for 31 years, has made the first direct observations of the solar wind termination shock, according to a paper published in the 3 July issue of the journal Nature.

At the termination shock the solar wind, which continuously expands outward from the sun at over a million miles per hour, is abruptly slowed to a subsonic speed by the interstellar gas. Don Gurnett, professor of physics in the College of Liberal Arts and Sciences and principal investigator for the plasma wave instrument on Voyager 2, and Bill Kurth, UI research scientist and Voyager co-investigator, said that the shock crossing was marked by an intense burst of plasma wave turbulence detected by the UI instrument, as well as by various effects detected by other instruments on the spacecraft.

At the time of the shock crossing, 31 August 2007, Voyager 2 was at a distance of 83.7 astronomical units (AU), roughly twice the distance between the Sun and Pluto. At this great distance, it took 11.2 hours for the radio signal from the spacecraft to reach Earth.

Shock waves in the thin, ionised gas - called plasma - that exists in space are similar in some respects to the shock waves produced by an airplane in supersonic flight. Shock waves in space are believed to play an important role in the acceleration of cosmic rays, which are very energetic atomic particles that continually bombard Earth. The most energetic cosmic rays, which are potentially hazardous to astronauts, are believed to be produced in intense shock waves caused by supernova explosions - immense stellar explosions that occur in massive stars toward the end of their lives.

The termination shock is believed to be responsible for the origin of less energetic cosmic rays called 'anomalous cosmic rays.' The recent observations at the termination shock are expected to help physicists understand how cosmic rays are produced by the turbulent fields that exist in such shocks. Gurnett said, 'There is no way for us to make direct measure of a super nova shock, so the Voyager 2 measurements at the termination shock provide us the best opportunity in the foreseeable future to understand how cosmic rays are produced by supernova cosmic shocks.'

Kurth noted that while some aspects of the termination shock matched scientists' expectations, a number of the observations made by Voyager were surprising and will cause a number of theories to be revised.

Gurnett noted that Voyager 2, launched in 1977, is moving at a speed of 38,000 miles an hour. Even at this considerable speed, the spacecraft will still take 30,000 years to reach a distance equal to that of the nearest star.