Washington, July 3 : The Voyager 2 spacecraft, which has been traveling outward from the Sun for 31 years, has made the first direct observations of the solar wind termination shock.
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.
According to Don Gurnett and Bill Kurth from the University of Iowa (UI), 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, August 31, 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, ionized 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 the turbulent fields that exist in such shocks produce cosmic rays.
According to Gurnett, "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."