Sun Travels Through a Turbulent Interstellar Cloud, Six-Degree Shift of Interstellar Wind Proves It

September 5, 2013

Sun on its way through the local interstellar cloud and the resulting interstellar wind.

Sun on its way through the local interstellar cloud and the resulting interstellar wind.

As the Sun, together with its planets, travels through the space, it encounters various particles, mostly hydrogen and helium. These particles create an interstellar wind, which streams in from the direction of the Scorpius constellation almost at the right angle to the Sun's path through the galaxy. A new study in the journal Science observed changes in this wind to show that our solar system lives in a complex and dynamic part of the Milky Way galaxy.

Our star creates a stream of charged particles, mostly electrons and protons, which it then emits in all directions, creating something called the heliosphere. The heliosphere creates a shield around the solar system. Outside of this shield is the Local Interstellar Cloud (LIC), which consists of already mentioned hydrogen and helium atoms. It has a diameter of some 30 light-years. The next picture should give you an idea how big it is.

Sun in the middle of the turbulent local interstellar cloud.

Sun in the middle of the turbulent local interstellar cloud.

The LIC on average has only 0.264 atoms per cubic centimeter, most of which is blocked by outward streaming particles – the heliosphere – as the Sun travels through this cloud. However, a small portion, about 0.015 atoms per cubic centimeter, pass through, thus creating the said interstellar wind.

"Right now, the Sun is moving through an interstellar cloud at a relative velocity of 82,800 kilometers per hour," Priscilla Frisch, University of Chicago, USA told "This motion allows neutral atoms from the cloud to flow through the heliosphere — the solar wind bubble — and create an interstellar 'wind.'"

Frisch is the lead author of the study that began after NASA's Interstellar Boundary Explorer (IBEX) discovered that the interstellar wind changed its direction in the past decade. Her team then decided to find out by how much. After studying data from IBEX, Ulysses probe, Mariner 10, Prognoz 6 and several other crafts, they found out the interstellar wind changed its course by 6 degrees over the last 40 years.

This may show that our neighbourhood is not static, but very turbulent. "The shift in the wind is evidence that the sun lives in an evolving galactic environment," Frisch said. "Winds on Earth are turbulent, and other data show that interstellar clouds are also turbulent. We find that the six-degree change is comparable to the turbulent velocity of the surrounding cloud on [the] outside of the heliosphere."

As the interstellar wind interacts with the Sun, it creates a dense cone of particles behind the star. As it is denser than anywhere else, it is also easier to study. "Helium is gravitationally focused to create a trail of helium known as the 'focusing cone' behind the Sun as it moves through space," Frisch said.

She also pointed out that this could also tell us more about the solar and not just the interstellar wind. "When we try to understand the past and present heliosphere, we can no longer assume that the heliosphere changes only because of the solar wind," Frisch says. "Now we have evidence that changes in the interstellar wind may be important."



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