Watch Solar Wind Pour Off the Sun

In 1958, the astrophysicist Eugene Parker discovered something incredible: our Sun was constantly producing a strong stream of ionized gas, known more colloquially as the solar wind.

Parker's discovery forever changed the way physicists thought about stars and was used to explain everything from widespread power-outages as a result of geomagnetic storms to cycles of human violence, but ultimately provided as many questions as it did answers. Chief among them: why did these streams of ionized gas evolve from neat, orderly rays near the Sun's surface to the turbulent solar winds experienced on Earth and elsewhere in the solar system.

For decades this question remained unresolved, but thanks to new images of the edge of the Sun taken by NASA's Solar Terrestrial Relations Observatory (STEREO) spacecraft, we may finally have some answers.

As detailed in a paper published in The Astrophysical Journallate last week, NASA researchers were able to observe the boundary between the Sun's atmosphere, or corona, and the solar wind flowing away from the corona. By observing the 'edge' of the Sun, the researchers noticed that the further the corona's plasma traveled from its origin, the less magnetic influence could be exerted on the plasma.

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

Prior to the images captured by STEREO, scientists had a hunch that this is what was happening. The problem was capturing the transition in action—the STEREO spacecraft is about 20 million miles from the Sun where the solar wind also contains free-floating electrons which scatter sunlight. This means that the movement of the solar wind can be seen via the electrons, but the light reflected by the particles is incredibly faint and requires some serious post-processing to see against a bright, star-studded background.

So the original image provided STEREO looked like the image on the left, where the true flow of the solar wind is concealed by dust and light from stars in the background. After processing the image to remove the interfering elements, it's possible to see the solar wind hiding in plain sight in the image on the left.

After the researchers were able to remove the background noise from the photos, they got an unprecedented look at the transition from corona to solar wind. With this new information, the researchers hope to shed more light on the way the Sun exerts influence at a distance and the origin and evolution of solar wind was it propagates through the solar system. This will be important not only for increasing our understanding of how stars work, but also for protecting the astronauts and robotic space assets which are exposed to these particles during their missions.

"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."