Mapping the Edge of the Solar System in a New Way

Author:  Jessica Kloss
Institution:  Princeton University
Date:  July 2008

Thanks to the unexpected discovery of particles from the edge of the solar system by NASA's STEREO spacecrafts and some innovative new astronomical methods from the University of California, Berkeley, for the first time scientists are able to map the frontier of our solar system. By looking at neutral atoms, instead of light, scientists can explore the mysterious region where hot solar winds meet the cold interstellar medium.

"You can't get a global picture of this region, one of the last unexplored regions of the heliopshere, any other way because it is too tenuous to be seen by normal optical telescopes," explained Robert Lin, a physics professor at UC Berkeley.

Between the billions of stars found in a galaxy like the Milky Way, there are cold clouds of gas and dust called the "interstellar medium" -- this is the material that may eventually collapse, heat up, and form newborn stars. Once stars are formed, however, the solar winds they generate make "bubbles" in the interstellar medium. This bubble, where the solar winds "blow" away the gas and dust of the interstellar medium, is called the heliosphere. The region is quite large; in fact, the heliosphere is believed to extend all the way out to more than twice the distance from the Sun to Pluto.

Solar wind travels quickly; for the first ten billion kilometers, it travels at a speed of more than a million kilometers per hour. As it begins to collide with the interstellar medium, it starts slowing down, until the solar wind reaches a point where it starts moving slower than the speed of sound. This point -- the crossing of the sound barrier -- is called the "termination shock". The region of the heliosphere beyond the termination shock is called the heliosheath, which has proved difficult to observe. The edge of the heliosphere is known as the heliopause (see diagram).

The twin STEREO spacecrafts were launched in 2006 to get pictures of the surface of the Sun and measure the magnetic fields and ion fluxes generated by explosions on the Sun's surface. However, when exploring the particles detected by the sensors on the spacecraft, lead author Linghua Wang said, "We were surprised that the particle intensities didn't depend on the magnetic field, which meant they must be neutral atoms."

Based on this, the UC Berkeley team concluded that the neutral atoms detected were ions that were heated up in the termination shock, which lost their charge while interacting with the cold atoms in the interstellar medium. After losing their magnetic field, the neutral atoms were no longer repelled by the solar wind and were able to make their way back in to the inner solar system, where they were detected by STEREO.

"This is the first mapping of energetic neutral particles from beyond the heliosphere," Lin said. It provides valuable insight on the amount of energy dumped into space by the solar wind, a point previously not fully understood. This method -- observing based on the detection of neutral particles -- could pave the way to new discoveries.

Written by Jess Kloss

Reviewed by Jeff Kost, Pooja Ghatalia

Published by Pooja Ghatalia.