First measurements of the termination shock
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: July 3, 2008

Voyager 2 crossed the solar wind termination shock last year, making the first direct measurements of the region where the hot solar wind slams into the cold interstellar medium, while the twin solar STEREO spacecraft unexpectedly detected particles emanating from the same locality, from their viewpoint in Earth orbit around the Sun.

The twin STEREO spacecraft were launched in 2006 to obtain stereo pictures of the Sun and to measure magnetic fields and ion fluxes associated with solar explosions. However, between June and October 2007, the spacecraft detected neutral atoms coming from the outer reaches of the Solar System. Image: SSTD/NASA.

The fact that the STEREO spacecraft were able to map this region in particles instead of light "heralds a new kind of astronomy using neutral atoms," says Robert Lin, UC Berkeley professor of physics and lead for the suprathermal electron sensor aboard STEREO. "You can't get a global picture of this region, one of the last unexplored regions of the heliosphere, any other way because it is too tenuous to be seen by normal optical telescopes."

The heliosphere is like a bubble in space within which the effects of the solar wind are contained, stretching from the Sun to more than twice the distance of Pluto. Last year, the Voyager 2 spacecraft crossed the termination shock – the region of the heliosphere where the supersonic solar wind slows to subsonic speed as it merges with the interstellar medium – and entered the surrounding heliosheath.

Don Gurnett, principal investigator for the plasma wave instrument on Voyager 2, and Bill Kurth, University of Iowa research scientist and Voyager co-investigator, said that the shock crossing was marked by an intense burst of plasma wave turbulence. However, there was a noticeable discrepancy between the amount of energy expected to be dumped into space by the decelerating solar wind than what was actually measured. The new measurements by STEREO reveal a population of ions originating from the heliosheath which contain about 70 percent of the energy dissipated in the termination shock, exactly the amount unaccounted for by Voyager 2's instruments.

"This is the first mapping of energetic neutral particles from beyond the heliosphere," says Lin. "These neutral atoms tell us about the hot ions in the heliosheath.” The UC Berkeley physicists concluded that these energetic neutral atoms, which are probably hydrogen, were originally ions heated up in the termination shock front that lost their charge to cold atoms in the interstellar medium and, no longer hindered by magnetic fields, flowed back toward the Sun and into the suprathermal electron sensors on STEREO.

STEREO detected energetic neutral atoms from the edge of the Solar System, where the solar wind meets the interstellar medium. Hot ions in the heliosheath - the region between the termination shock and heliopause - are uniquely traced by these atoms and are more intense (indicated by colour code) around the nose of the heliosphere, with an asymmetric double peak. The twin STEREO A and B spacecraft are shown in the Sun-centred orbit they share with Earth. Last year, the Voyager 2 spacecraft passed into the heliosheath, joining Voyager 1. There, these interstellar explorers continue their journey into the farthest reaches of the heliosphere. Image: Linghua Wang/UC Berkeley .

The termination shock is believed to be responsible for the origin of 'anomalous cosmic rays,' cosmic rays that are less intense than those produced in supernova explosions, but nonetheless are still extremely energetic atomic particles that continually bombard the Earth. The direct Voyager observations combined with the serendipitous STEREO measurements of the termination shock boundary are expected to help physicists understand how cosmic rays are produced and subsequently accelerated by the turbulent fields that exist in both termination shocks and supernova events.

"There is no way for us to make direct measure of a supernova 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," says Gurnett.

Later this year a new NASA mission, the Interstellar Boundary Explorer, will launch into space to map the lower-energy energetic ions in the heliosheath in more detail, to determine the structure of the termination shock and how hydrogen ions are accelerated there.

The Voyager 2 results and the complementary results from STEREO are both reported in the same July 3 issue of Nature.