Sun's atmosphere 8 million km larger than thought

Sun's corona - the vast atmosphere of solar particles that surround our star - is much larger than thought, extending out more than eight million kilometres above the solar surface, scientists say.
The finding has implications for NASA's upcoming Solar Probe Plus mission, due to launch in 2018 and go closer to the Sun than any man-made technology ever has before.
Surrounding the Sun is a vast atmosphere of solar particles, through which magnetic fields swarm, solar flares erupt, and gigantic columns of material rise, fall and jostle each other around.
Now, using NASA's Solar Terrestrial Relations Observatory, scientists have found that this atmosphere, called the corona, is even larger than thought.

These STEREO observations provide the first direct measurements of the inner boundary of the heliosphere - the giant bubble sparsely filled with solar particles that surrounds the Sun and all the planets.
Combined with measurements from Voyager 1 of the outer boundary of the heliosphere, we have now defined the extent of this entire local bubble, researchers said.

"We've tracked sound-like waves through the outer corona and used these to map the atmosphere," said Craig DeForest of the Southwest Research Institute in Boulder, Colorado.
"We can't hear the sounds directly through the vacuum of space, but with careful analysis we can see them rippling through the corona," DeForest said.

The researchers studied waves known as magnetosonic waves, and they are a hybrid of sound waves and magnetic waves called Alfven waves.
Unlike sound waves on Earth, which oscillate several hundred times per second, these waves oscillate about once every four hours - and are about 10 times the length of Earth.
Tracking magnetosonic waves showed DeForest and his team that the material throughout this extended space remained connected to the solar material much further in.
That is to say that even out to 8 million kilometres from the Sun, giant solar storms or coronal mass ejections can create ripple effects felt through the corona.

Beyond that boundary, however, solar material streams away in a steady flow called the solar wind - out there, the material has separated from the star and its movement can't affect the corona.
Realising that the corona extends much further than previously thought has important consequences for NASA's Solar Probe Plus because the mission will travel to within more than 6 million kilometres of the Sun.
The research was published in The Astrophysical Journal.