The details of the transition from defined rays in the upper atmosphere of the Sun to the solar wind have always been a mystery. Using NASA’s STEREO (Solar Terrestrial Relations Observatory) spacecraft, solar astronomers have for the first time imaged the edge of the Sun and described that transition, where the solar wind starts. A paper on the findings appears in the Astrophysical Journal (arXiv.org preprint).
The Sun’s corona and solar wind. Image credit: NASA’s Goddard Space Flight Center / Lisa Poje.
Both near Earth and far past Pluto, our space environment is dominated by solar activity.
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 solar corona streams out into space, filling our Solar System with the solar wind.
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,” explained lead author Dr. Craig DeForest, of the Southwest Research Institute.
“Eventually, the material starts to act more like a gas, and less like 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 new images from NASA’s STEREO spacecraft capture the plasma at the same stage where a stream of water gradually disintegrates into droplets.
Before this study, scientists hypothesized that magnetic forces were instrumental to shaping the edge of the solar corona. However, the effect has never previously been observed because the images are so challenging to process.
20 million miles (32 million km) from the Sun, the solar wind plasma is tenuous, and contains free-floating electrons which scatter sunlight. This means they can be seen, but they are very faint and require careful processing.
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.
“Now we have a global picture of solar wind evolution,” said study co-author Dr. Nicholeen Viall, of NASA’s Goddard Space Flight Center.
“This is really going to change our understanding of how the space environment develops.”
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C. E. DeForest et al. 2016. Fading Coronal Structure and the Onset of Turbulence in the Young Solar Wind. ApJ 828, 66; doi: 10.3847/0004-637X/828/2/66
