The origins of the constant flow of charged particles from the Sun, known as the solar wind, and the details of the transition from defined rays in the Sun's upper atmosphere to the solar wind have been a mystery ever since the discovery of solar wind in 1950s. Now, a new study conducted by scientists using NASA's Solar Terrestrial Relations Observatory (STEREO) reveals images of the edge of the Sun and describes that transition, where the solar wind starts.
We now have a global picture of solar wind evolution, said Nicholeen Viall, a co-author of the paper published this week in The Astrophysical Journal 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.”
Both near Earth and far past Pluto, our space environment is dominated by activity on the Sun. The Sun and its atmosphere are made of plasma – a mix of positively and negatively charged particles which have separated at extremely high temperatures, that both carries and travels along magnetic field lines. Material from the corona streams out into space, filling the Solar System with the solar wind.
GIF showing the before (left) and after (right) video of the solar wind, as seen by NASA's STEREO spacecraft. Scientists used an algorithm to dim the appearance of bright stars and dust in images of the faint solar wind. This innovation enabled them to see the transition from the corona to the solar wind. It also gives us the first video of the solar wind itself in a previously unmapped region. Credit: data from Craig DeForest, SwRI
But scientists found that as the plasma travels further away from the Sun, things change: The Sun begins to lose magnetic control, forming the boundary that defines the outer corona – the very edge of the Sun.
“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. “Eventually, the material starts to act more like a gas, and less like a magnetically structured plasma.”
The breakup of the rays is similar to the way water shoots out from a squirt gun. First, the water is a smooth and unified stream, but it eventually breaks up into droplets, then smaller drops and eventually a fine, misty spray. The images in this study capture the plasma at the same stage where a stream of water gradually disintegrates into droplets.
In order to resolve the transition zone, scientists had to separate the faint features of the solar wind from the background noise and light sources over 100 times brighter: the background stars, stray light from the Sun itself and even dust in the inner solar system. In a way, these images were hiding in plain sight.
Images of the corona fading into the solar wind are crucial pieces of the puzzle to understanding the whole Sun, from its core to the edge of the heliosphere, the region of the Sun’s vast influence. With a global perspective, scientists can better understand the large-scale physics at this critical region, which affect not only our planet, but also the entire Solar System.
GIF excerpt from processed STEREO data of the solar wind. Data credit: Craig DeForest, SwRI
Such observations from the STEREO mission also help inform the next generation of Sun-watchers. In 2018, NASA is scheduled to launch the Solar Probe Plus mission, which will fly into the Sun’s corona, collecting more valuable information on the origin and evolution of the solar wind.
- "Fading coronal structure and the onset of turbulence in the young solar wind" - C. E. DeForest, W. H. Matthaeus, N. M. Viall, and S. R. Cranmer - Published September 1, 2016 -The American Astronomical Society - DOI: 10.3847/0004-637X/828/2/66
Featured image: processed STEREO data of the solar wind. Data credit: Craig DeForest, SwRI