An international team of astronomers claims to have pinpointed the source of the “slow solar wind” — a stream of charged particles that is expelled by the Sun. The team believes the particles come from areas on the surface of the star where strong magnetic fields collide.

The discovery was made using the Japanese Space Agency’s Hinode solar observatory and could help scientists gain a better understanding of the solar wind, which can sometimes disrupt satellite communications.

The solar wind consists of mostly electrons and protons that are ejected from the Sun’s upper atmosphere in all directions. It has two components: a "fast" wind moving at about 800 km/s and a "slow" wind at about 400 km/s.

When these particles interact with Earth’s magnetic field they create the beautiful auroras seen at higher latitudes in the northern and southern hemispheres. However, the wind can also interfere with electronic systems on orbiting satellites and even overload electrical power grids on Earth.

Matter of debate

While the fast wind has been studied extensively and is well understood, the origin of the slow wind is a matter of some debate. However, Earth is often immersed in the slow solar wind, so understanding its origins could help mitigate its interference with communications and other technologies.

The Sun’s surface (photosphere) is a plasma of charged particles that is shaped by magnetic fields, which create “active regions” — areas of enhanced brightness and magnetic field strength that are often associated with sunspots. Previously, it had been difficult to determine whether significant amounts of plasma flowed from these regions into the corona — the region just beyond the surface of the sun — a process that was thought to be related to the slow solar wind.

Now Peter Young of the Rutherford Appleton Laboratory in the UK and other members of the Hinode team have used the observatory to study active regions and understand how the plasma escapes the surface to become the slow solar wind.

The team used Hinode’s Extreme Ultraviolet Imaging Spectrometer (EIS) to measure the speed at which material flows out from the Sun. They discovered that plasma is ejected at high speeds at the edges of these active regions, where strong magnetic fields are believed to collide.

"It is fantastic to finally be able to pinpoint the source of the solar wind – it has been debated for many years and now we have the final piece of the jigsaw,” says Louise Harra, another team member at the UCL-Mullard Space Science Laboratory in London. “All the planets in the solar system sit in the solar wind, so understanding it is important for us.”

Fundamental process

Young agrees. “We have identified a fundamental process operating on the Sun that is responsible for accelerating material out into space,” he adds. “Winds are seen in a wide range of astronomical bodies, including many different types of stars, black holes and galaxies, and our result will influence studies of all these other objects.”

The team is now working to confirm their findings and understand the mysterious phenomenon of solar flares — massive explosions on the Sun's surface that send huge amounts of high energy radiation and particles into space and which are a major threat to space exploration and satellite technology.