The Sun is constantly emitting a stream of charged particles known as the solar wind, which affects the atmosphere of planets and is responsible for the auroras.

How the solar wind forms has not been exactly clear, though. Near the Sun, in the solar corona, the wind is structured in distinct rays. But when it reaches the planets it is a turbulent flow, just like air wind. Astronomers were finally able to image the edge of the Sun's atmosphere and look at how and why the solar wind changes.

The research, presented in the Astrophysical Journal, was possible thanks to NASA’s Solar Terrestrial Relations Observatory (STEREO). In the Sun's upper atmosphere, the magnetic field of the Sun is the dominating force that structures the solar wind, but as it moves away from our star, internal forces become more important.

"As you go farther from the Sun, the magnetic field strength drops faster than the pressure of the material does," said Craig DeForest, lead author of the paper and a solar physicist at the Southwest Research Institute in Boulder, Colorado in a statement.

"Eventually, the material starts to act more like a gas, and less like a magnetically structured plasma."

The researchers liken the break-up of the charged particles rays to how water shoots out from a hose. The flow of water is first smooth and unified in a stream, but it then breaks into smaller and smaller droplets until it turns into a mist.

^{This gif shows the computer-processed data of the solar wind as it leaves the Corona. Craig DeForest et al./SwRI}

Scientists have long suspected that the magnetic forces were responsible for shaping the edge of the solar corona, but actually observing it was incredibly challenging. To photograph this region, the researchers used sunlight scattered on the electrons in the solar wind. This was incredibly difficult because this light is 100 times less bright than background sources.

This newfound understanding is very important. The solar wind shapes the space environment from the Sun all the way beyond Pluto. If we hope to explore the Solar System, we need to understand it well.

"Now, we have a global picture of solar wind evolution," said Nicholeen Viall, a co-author of the paper and a solar scientist at NASA's Goddard Space Flight Center. "This is really going to change our understanding of how the space environment develops."

STEREO’s work is also informing the next generation of Sun observatories, especially Solar Probe Plus, which is going to fly directly into the solar corona in 2018.