An X-ray image of the Sun made with the Hinode satellite on 20 February 2007. The insets show the flow of gas away from the bright region marked on the left. The blue image indicates material flowing towards us that will eventually make up the solar wind and the red image shows material flowing away from us back towards the surface of the Sun. Credit: L. Harra/JAXA/NASA/ESA
Astronomers have finally tracked down the missing starting point of one of the two types of solar wind.
The solar wind is a stream of electrically charged particles that flows constantly out from the sun in all directions. The particles can make the journey from the sun to the Earth in fewer than 10 days and, when the wind turns into a storm, create the magnificent auroras that dance across polar skies when they interact with the Earth's magnetic field.
The parts of the solar wind that emanate from the sun's equatorial region originate at the edges of bright regions in the sun's atmosphere and are released when the magnetic fields of two bright regions link up, scientists announced last week at the Royal Astronomical Society's National Astronomy Meeting in Belfast, Northern Ireland.
"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," said study leader Louise Harra of University College London.
Fast and slow
The sun emits radiation, which is pure energy, along with the solar wind, which is fast-moving matter. The wind's particles are accelerated by the sun's magnetic fields, and the configuration of the magnetic fields can influence how fast the solar wind is going when it rushes out into space.
Astronomers recognize two types of solar wind, distinguished by their speed. The fast one is known to originate from coronal hole regions near the sun's poles and travel at about 1.8 million miles per hour (2.9 million kilometers per hour). The slow one flows from the equatorial region of the sun at about 432,000 mph to 1.8 million mph (720,000 kph to 1.8 million kph).
The fast solar wind is so much faster because the magnetic fields that loop out from the polar regions are always "open," meaning they don't loop back toward the sun's surface. So "all the gas can keep streaming out, there's nothing to stop it," Harra said.
At the equator, on the other hand, there are both closed and open magnetic fields, and the closed fields hold the solar plasma back. Only when the fields open, can the solar wind stream out from the region.
As a result, the solar wind coming from the equatorial region is slower and "very, very variable," Harra told SPACE.com.
Big and 'baby' regions
Using the Hinode space observatory, Harra and her colleagues found for the first time that hot gas spurts out at high speeds from the edges of bright, active regions along the equator when the magnetic fields from two regions meet up.
Hinode witnessed such a link-up when the field lines from a big active region and a "baby" region connected and opened up.
"We now know that interacting with smaller regions can open up the field lines," Harra said.
These regions can connect even when they are 500,000 kilometers apart (a distance equivalent to placing 40 Earths side-by-side), Harra said.
For the two regions to connect, the field lines from the two regions have to be in the right direction and of the right strength.
The big region "needs to find its partner to interact," Harra said.
Understanding the solar wind and how it is formed could help scientists better predict how it will affect the Earth and help protect the satellites in orbit around our planet.
