U.S. scientists say helium gas in the sun may be responsible for setting the minimum speed of the solar wind, the stream of electrically charged gas that can disrupt spacecraft and satellites when it strikes the Earth's magnetic field.

The solar wind is mostly made up of protons and electrons from hydrogen atoms that escape from the sun and travel into space at speeds of at least 260 kilometres per second.

Scientists have never understood what was behind this minimum speed, but now researchers at the Massachusetts Institute of Technology say the presence of helium gas in the solar wind might hold the key.

Using data from NASA's Wind spacecraft taken between 1995 and 2005, the researchers found that the amount of helium increased as the solar wind speed increased. At the minimum speed, there were near-zero amounts of the gas, while at speeds greater than 500 kilometres per second, there were more than four helium atoms for every 100 hydrogen atoms.

"It's still not clear exactly how the helium sets the minimum speed at its particular value of around 260 kilometers per second, or why more helium is found as the solar wind speed increases, but it's a clue that we are missing something fundamental about what makes the solar wind blow," said Justin Kasper, a research scientist at MIT's Kavli Institute for Astrophysics and Space Research.

Hydrogen makes up most of the sun, with helium the second-most abundant element inside the star. But while helium makes up about seven per cent of the atoms in the sun, far less escapes in the form of the solar wind.

The researchers speculate that for the heavier helium atoms to travel with the solar wind, the hydrogen atoms must drag them along. Kasper said they believe the helium atoms slow the hydrogen atoms enough that they cannot escape the sun's corona at speeds lower than 260 kilometres per second.

"This result gives us another clue about how the solar wind is accelerated, which may help us better understand space weather," said Kasper.

The finding may provide new insight into the inner workings of plasma, the electrically charged or ionized gas at the heart of stars and in use on Earth in plasma televisions and some fusion reactors.

A paper on the findings was published in the May 1 issue of the Astrophysical Journal.