ESA’s Solar Orbiter traces solar wind to its source

The solar wind is the never-ending sleet of electrically charged particles given out by the Sun. It is highly variable, changing its characteristics such as speed, density and composition, depending on what part of the Sun’s surface it is coming from.

Yet, despite decades of study, certain aspects of the solar wind’s origin remain poorly understood. And by the time the wind reaches the Earth, much of the detail has been smeared out, making it virtually impossible to trace it back to specific regions on the Sun’s surface.

As the solar wind travels through the Solar System, it interacts with celestial bodies and spacecraft. These interactions range from the benign, in the case of sparking the auroras on our planet, to highly disruptive, in that solar storms can interfere with or even damage electrical systems on the ground or in spacecraft.

As such, understanding the solar wind is a priority for solar physicists. A key goal of Solar Orbiter’s mission was to link the solar wind around the spacecraft back to its source regions on the Sun. This new result, using data taken during Solar Orbiter’s first close approach to the Sun, shows that it is possible, fulfilling a key objective of the mission, and opening a new way to study the origin of the solar wind.

Linking data from near and far

Solar Orbiter can make these connections because it has both in situ and remote sensing instruments. The in situ instruments measure the solar wind plasma and magnetic field around the spacecraft, while the remote sensing instruments take images and other data of the Sun itself. The difficulty is that the cameras show the Sun as it appears now, while the in situ instruments reveal the state of the solar wind that was released from the Sun’s surface a few days earlier. This is because it takes some time for the solar wind particles to reach the spacecraft.

To link the two datasets, astronomers use online software called the Magnetic Connectivity Tool, which was developed to support the Solar Orbiter mission. The raw data for the connectivity tool comes from the Global Oscillation Network Group, a series of six solar telescopes spread around the world that continually monitor oscillations on the surface of the Sun. From these observations, the computer model calculates how the solar wind propagates through the Solar System.

“You can predict where you think Solar Orbiter will be connected to on the solar surface a few days in advance,” says Stephanie Yardley, Northumbria University, UK, who is the lead author on the paper announcing the results.

The team chose their observational targets on the Sun’s surface, and used the Magnetic Connectivity Tool to predict when the spacecraft would be flying through the solar wind that was released from those surface features. Solar Orbiter’s unique set of instruments, that cover both in situ measurements and remote sensing, as well as its orbit that takes it close to the Sun, were specifically designed to allow this kind of scientific linkage to be attempted.

The data were collected between 1 and 9 March 2022, when Solar Orbiter was roughly 75 million km from the Sun, or about half Earth’s distance from the Sun.

Solar wind moves fast or slow

Broadly speaking, the solar wind comes in two types: a fast solar wind travelling at more than 500 km/s, and a slow solar wind travelling at less than 500 km/s.

While the fast solar wind is known to come from magnetic configurations known as coronal holes which channel the solar wind out into space, the origin of the slow solar wind is still poorly understood. It is known to be connected to ‘active regions’ on the Sun, where sunspots appear, but the details are elusive. Sunspots are cooler areas in the Sun’s photosphere where intense magnetic fields become twisted and concentrated. They indicate active regions of the Sun, often responsible for solar flares and eruptions.

To prove the team’s ability to connect the slow solar wind measured in situ to its place of origin on the solar surface, the spacecraft needed to fly through the magnetic field connected to the edge of either a coronal hole or a sunspot complex. This let the team watch the way the solar wind changed its speed – from fast to slow or vice versa – and other properties, confirming that they were looking at the correct region. In the end, they got a perfect combination of both types of features together.

“Solar Orbiter flew past the coronal hole and the active region, and we saw fast solar wind streams, followed by slow ones. We saw a lot of complexity that we could tie back to the source regions,” says Stephanie. This included variations in composition and temperature across these particular regions.

A new age of solar wind research

Through their analysis of the different solar wind streams that Solar Orbiter detected, the team has shown clearly that the solar wind still exhibits the ‘footprints’ imparted by its different source regions, which will make it easier for solar physicists to trace the streams back to their points of origin on the Sun.

Now that the concept has been proven, it opens a wealth of future possibilities for using data from other spacecraft close to the Sun, such as NASA’s Parker Solar Probe and ESA’s BepiColombo, to study the solar wind.

“This result confirms that Solar Orbiter is able to make robust connections between the solar wind and its source regions on the solar surface. This was a key objective of the mission and opens the way for us to study the solar wind’s origin in unprecedented detail,” says Daniel Müller, ESA Project Scientist for Solar Orbiter.

Notes for editors
‘Multi-source connectivity as the driver of solar wind variability in the heliosphere‘, by Stephanie Yardley et al. is published today in Nature Astronomy, DOI: 10.1038/s41550-024-02278-9

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