Helium in the Sun may set the minimum speed limit for the solar wind, which has never been explained before, a new study says. The new insight could help scientists understand the solar outbursts that can interfere with GPS signals and pose a danger to astronauts.

The solar wind is a constant stream of electrically charged particles from the Sun. It is a mixture composed mainly of hydrogen nuclei--or protons--and electrons. Its speed varies but almost never drops below 260 kilometers per second Ð a phenomenon that has never been explained.

Now, measurements made by NASA’s Wind spacecraft between 1995 and 2005 suggest that helium is responsible for this minimum speed limit.

Justin Kasper of MIT in Cambridge, US, led a team that found a connection between the wind speed and the amount of helium it contained--the faster the wind, the more helium was present.

At higher wind speeds of around 500 kilometers per second, the proportion of helium was about 4%. But the proportion dropped towards zero for speeds near the lower limit of 260 kilometers per second.

Get Up to Speed

This suggests that the Sun’s helium is somehow creating the lower speed limit. Helium, which has two protons and two neutrons and is therefore about four times as massive as hydrogen, is not easily accelerated by the processes that fling hydrogen nuclei from the Sun. So its presence in the solar wind suggests it may be getting dragged along by the hydrogen.

But hauling helium in this way slows down the hydrogen, and Kasper’s team suspects hydrogen moving below 260 kilometers per second simply cannot overcome this drag, explaining the minimum speed.

“It seems to imply that the hydrogen has to get up to a sufficient speed or it’s not able to get past the helium,“ Kasper told New Scientist. “These observations really say we need to do a lot more work on the role helium plays in regulating the solar wind.“

He says helium may also play a role in solar outbursts called coronal mass ejections (CME), which are known to contain about 20% helium, a much higher proportion than in the normal solar wind.

Alternative Explanation

Since helium particles are heavier than hydrogen particles, a 20% helium abundance means half the mass of the material is helium. Understanding how the helium affects the behavior of the material that makes up CMEs could therefore be important for understanding how these eruptions unfold, he says.

But solar physicist David Hathaway of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, US, says other phenomena Ð and not helium Ð might explain the observations.

The changes in helium abundance at different wind speeds might simply indicate that hydrogen and helium respond differently to the mechanisms that accelerate solar wind particles, he says.

“It may have more to do with the mechanisms for accelerating the wind than with helium itself being present and somehow holding back the predominant species,“ he told New Scientist.

PIN/ NEWSCIENTIST.COM