Voyager I Cross Termination Shock, Heads for Outer Space

In November 2003, Stamatios Krimigis of Johns Hopkins University in Laurel, Maryland reported that Voyager I had crossed the termination shock sometime in August 2002, when it was 85 Astronomical Units (AU) from the sun. His claim was based on estimates of the solar wind speed. However, many scientists disagreed with this claim, because there was no significant increase in the Sun’s magnetic field, which would have been a sign that solar particles were slowing down and bunching together at the termination shock.

In the middle of December 2004, when Voyager I was about 94 AU from the Sun, Voyager I’s magnetometer spotted a two-and-a-half-fold increase in the magnetic field, and its cosmic ray detector noticed an increase in galactic cosmic rays, ruling out the possibility that the increase in the magnetic field was caused by solar flares. The cosmic rays appeared to be coming from all over, rather than from specific directions. This is exactly what would happen outside the termination shock, with the strong solar magnetic field tossing cosmic rays around. Dr. Ed Stone of Caltech, at an American Geophysical Union Meeting in New Orleans on May 24, 2005, announced that the entire Voyager I team now agrees that Voyager I has crossed termination shock.

The only inexplicable piece of data left is the scarcity of anomalous cosmic rays (ACRs). ACRs enter the solar system as neutral particles from interstellar space, but get ionized by photons from the Sun or by solar wind and are accelerated at the termination shock. It was predicted that Voyager I would see dramatically higher numbers of ACRs as it crossed the termination shock. This did not happen. Scientists now speculate that ACRs are still coming through the termination shock, but not through the place traversed by Voyager.