Voyager 1 and 2 are now in the 'Heliosheath' - the outermost layer of the heliosphere where the solar wind is slowed by the pressure of interstellar gas.
Credit: NASA/JPL-Caltech
PARIS: Astronomers have discovered that the 'solar wind' seems to come to a gentle stop some 17 billion km from the Sun, contrary to current theories that suggest it should come to a far more abrupt halt in this area of our Solar System.
This finding was the result of new observations from NASA's Voyager 1 spacecraft. Launched in 1977, Voyager 1 and its twin Voyager 2 now find themselves at the edge of our Solar System after having delivered numerous exciting and important details about many of its planets and moons.
These include the discovery of volcanoes on Jupiter's moon Io; evidence for an ocean beneath the icy surface of Europa; and the presence of methane rain on Saturn's moon Titan. The spacecraft are set to head off into the Galaxy very soon.
"Voyager 1 may be close to the heliopause, or the boundary at which the interstellar medium basically stops the outflow of solar wind," said Krimigis, principal investigator for Voyager's Low-Energy Charged Particle instrument and co-author of the paper in the current issue of Nature.
Limits of the Solar System
The limits of our Solar System lie between about 85 and 120 astronomical units away, where one astronomical unit (AU) is about 150 million km (or the distance between the Earth and the Sun).
At these distances the solar wind - which is a supersonic plasma of charged particles emanating from the Sun - collides with the interstellar plasma of outer space to create a 'termination shock'.
But just before we reach the border with the Galaxy - the heliopause - we find the 'heliosheath', a frontier reservoir of energetic ions and electrons that is estimated to be between 30 and 50 astronomical units wide (4.5 to 7.5 billion km).
Current theory predicts that the speed of the solar wind will decrease abruptly as it hits the heliopause, coming up against the immense pressure of our Galaxy's magnetic field.
What is the opposing force?
Tom Krimigis and colleagues at Johns Hopkins University in the U.S. analysed energetic particles such as protons and electrons from 85 AU and beyond using the LECP instrument on Voyager 1. They found the speed of the solar wind travelling away from the Sun has been decreasing almost linearly over the past three years from about 150 000 km/h to zero (as measured in April 2010). It has remained at zero ever since.
These results indicate Voyager 1 may be close to the heliopause, according to the researchers. The data also contradict the current theories because Krimigis' team observed a gradual slow-down of the plasma velocity and no abrupt halt. "The solar wind in this region appears to have come to a stop," Krimigis said. "There is clearly an opposing force that caused this to happen - most likely the galactic magnetic field."
The researchers say they expect magnetic field measurements at Voyager 1 to show an increase in the near future as the spacecraft encounters the magnetic field of the Galaxy. This would back up their hypothesis.
