This summer, humanity begins its first mission to touch the sun: a spaceship is launched into the outer atmosphere of the sun.
Given the temperatures of several million degrees Fahrenheit, the NASA Parker Solar Probe ̵
The solar wind fills our entire solar system. When gusts of solar wind arrive on Earth, they can trigger dazzling Northern Lights – but also expose astronauts to radiation, disrupt satellite electronics, and disrupt communications signals such as GPS and radio waves. The more we understand the fundamental processes that drive the solar wind, the more we can mitigate some of these effects.
In 1958, Parker developed a theory that shows how the hot corona of the sun – then known as millions of degrees Fahrenheit – is so hot that it overcomes the gravitational pull of the sun. According to theory, the material in the corona continuously expands outward and forms a solar wind. A year later, the Soviet space probe Luna discovered 1 solar wind particles in space and three years later the observations of NASA's spacecraft Mariner 2 were confirmed.
All these years ago, Mariner 2 discovered two different solar wind currents: a slow current that travels at around 215 miles per second and a fast current that travels at twice the speed through space. Then, in 1973, the origins of the fast solar wind were identified. X-rays of the corona from Skylab – the first manned space station in the US – showed that the fast wind spits out of coronal holes, which are dark, relatively cold regions on the Sun.
"The slow solar wind is in many ways a bigger puzzle," said Jim Klimchuk, solar physicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "It's promising to reveal a fundamental new understanding."
The origins and acceleration mechanisms of the slow solar wind remain mysterious. It is a decade-long debate between scientists.
But we are not without hints. NASA's Ulysses mission, launched in 1990 to fly the sun's poles, observed that the slow solar wind is confined to the Sun's equator during periods of minimal solar activity – exactly where Parker Solar Probe will fly. As the solar cycle progresses to its maximum, the structure of the solar wind changes from two different regimes – fast at the poles and slow at the equator – to a mixed, inhomogeneous flux.
The debate about the origins of the slow solar wind depends on a distinction between the so-called closed and open corona. The closed corona refers to areas of the sun where their magnetic field lines are closed – that is, connected to the solar surface at both ends. Bright helmet streamer – large loops that form over magnetically active areas in the form of a pointed helmet of the knight – are an example of this. The plasma or ionized gas that moves along the closed loops of a helmet streamer is mostly confined to the area near the sun.
On the other hand, the open corona refers to areas in which the magnetic field lines are anchored only at one end to the sun, on the other hand reaching out into space, thus creating a way in which solar material can escape into space. Coronal holes – the cooler regions at the source of the fast solar wind – are the habitat of open field lines.
When the slow solar wind leaves the solar corona, it also flows on open magnetic field lines the only way to be so far from the sun. But the theories differ in whether they started there or instead grew on closed field lines to switch to open field lines somewhere in the field.
For example, the expansion factor theory claims that the slow solar wind creates open lines in the open field, just like the fast wind. Its (comparatively) slow speed results from the expanding path it takes on its way out of the corona as magnetic field lines pass the helmsteam boundaries. Just as water flowing through a pipe slows to a trickle as the pipe expands, the plasma that travels along these widening maglevs slows down.
Other theories claim that the slow solar wind emanates from closed field lines and later changes to open field lines. Accordingly, the slow wind forms when the open field lines of coronal holes in the closed field lines collide at the edges of the helmet streamer and rewire in an event called magnetic reconnection. Like a train changing rails after the operator has flipped a switch, the plasma is previously located on the streamer's closed field lines abruptly on an open field line where it can escape into space.
The idea that slow solar wind plasma was once on closed field lines is supported by the evidence that it was once faced with the kind of extreme warming that we know happened there.
"It's not about the temperature of the plasma when we measure it, it's the temperature history of that plasma," said Aleida Higginson, a University of Michigan researcher at Goddard. "We can say that the slow solar wind has been much hotter in the past." Moreover, the particular mixture of elements that make up the slow solar wind is in good agreement with those of the closed corona – but not with plasma, which we know to always be on open field lines.
Recent Attempts to Test These Theories Spaceships near Earth are hampered by the large distance between their measurements and the origin of the solar wind (much can happen in 93 million miles). The key is close, tracking the solar wind back to its source – and Parker Solar Probe will do just that.
"If we can measure the slow solar wind, and find it comes from the boundary between open and closed magnetic fields, then that supports the idea that the magnetic reconnection causes the slow solar wind," said Klimchuk.
Parker Solar Probe's instruments will collect downstream evidence of magnetic reconnection – a telltale sign that the closed field – open-field theory is at stake. Certain types of reconnection will twist the resulting magnetic field in various ways, and Parker's instruments will measure the distortions in these fields early, before they have much time to be distorted. In addition, close-ups of the nascent solar wind will show us how coronal structures develop as they spread outward. This will help us answer a long-standing question as to whether the solar wind is a continuous or intermittent stream.
For scientists who ask for data to test their theories, accurate measurements of the magnetic fields of the solar corona will be invaluable. "That's why Parker's mission is so important," said Higginson. "It all goes back to understanding the detailed magnetic structure of the sun."
Send your name to the sun on board the NASA Parker Solar Probe