According to a new study published today online in the Geophysical Research Letters, NASA’s Voyager 1 spacecraft appears to have exited the heliosphere.
The heliosphere is a region of space dominated by the Sun and its wind of energetic particles, and which is thought to be enclosed, bubble-like, in the surrounding interstellar medium of gas and dust that pervades the Milky Way Galaxy.
In 2012, Voyager 1 measured drastic changes in radiation levels, more than 11 billion miles from the Sun. Anomalous cosmic rays, which are cosmic rays trapped in the outer heliosphere, all but vanished, dropping to less than 1 percent of previous amounts.
At the same time, galactic cosmic rays – cosmic radiation from outside of the Solar System – spiked to levels not seen since Voyager’s launch, with intensities as much as twice previous levels.
“Within just a few days, the heliospheric intensity of trapped radiation decreased, and the cosmic ray intensity went up as you would expect if it exited the heliosphere,” said lead author Prof Bill Webber from the New Mexico State University in Las Cruces. He calls this transition boundary the ‘heliocliff.’
“It appears that Voyager 1 has exited the main solar modulation region, revealing hydrogen and helium spectra characteristic of those to be expected in the local interstellar medium.”
“However, scientists are continuing to debate whether Voyager 1 has reached interstellar space or entered a separate, undefined region beyond the Solar System,” Prof Webber said.
“It’s outside the normal heliosphere, I would say that.”
“We’re in a new region. And everything we’re measuring is different and exciting,” Prof Webber concluded.
Bibliographic information: W.R. Webber, F.B. McDonald. 2013. Recent Voyager 1 data indicate that on August 25, 2012 at a distance of 121.7 AU from the Sun, sudden and unprecedented intensity changes were observed in anomalous and galactic cosmic rays. Geophysical Research Letters, in press; doi: 10.1002/grl.50383