Ever since the 1950s discovery of the solar wind – the constant flow of charged particles from the Sun - there has been a disconnect between this outpouring and the Sun itself.
As it approaches Earth, the solar wind is gusty and turbulent. But near the Sun where it originates, this wind is structured in distinct rays, much like a child's simple drawing of the Sun.
The details of the transition from defined rays in the corona, the Sun's upper atmosphere, to the solar wind have been, until now, a mystery. Now, using NASA's Solar Terrestrial Relations Observatory, or STEREO, scientists have for the first time imaged the edge of the Sun and described that transition, where the solar wind starts.
Defining the details of this boundary helps us learn more about our solar neighbourhood, which is bathed throughout by solar material - a space environment that we must understand to safely explore beyond our planet, researchers said.
"Now we have a global picture of solar wind evolution," said Nicholeen Viall, scientist at NASA's Goddard Space Flight Centre in the US.
"This is really going to change our understanding of how the space environment develops," said Viall.
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.
However, 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, 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," said DeForest.
Twenty million miles 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.