ESA’s Solar Orbiter follows the solar wind to its source

The solar wind is an endless shower of electrically charged particles emitted by the Sun. It is highly variable, changing its properties such as speed, density and composition depending on which part of the Sun’s surface it comes from.

However, despite decades of study, some aspects of the origin of the solar wind remain poorly understood. And by the time the wind reaches Earth, much of the detail has been blurred, making it virtually impossible to trace 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 harmless, in the case of sparking auroras on our planet, to highly disruptive, where solar storms can disrupt or even damage electrical systems on Earth or in spacecraft.

Understanding the solar wind is therefore a priority for solar physicists. A key goal of the Solar Orbiter 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 the Solar Orbiter’s first close approach to the Sun, shows that it is possible, fulfilling a key mission goal and opening a new way to study the origin of the solar wind.

Connecting data from nearby and distant surroundings

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 remote sensing instruments take images and other data of the Sun itself. The problem is that the cameras show the Sun as it appears now, while in situ the instruments reveal the state of the solar wind that was released from the Sun’s surface a few days earlier. Solar wind particles take some time to reach the spacecraft.

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

“You can predict where you think the Solar Orbiter will dock on the sun’s surface days in advance,” says Stephanie Yardley, Northumbria University, UK, who is lead author of the paper reporting the results.

The team chose their observation targets on the Sun’s surface and used the Magnetic Connectivity Tool to predict when the spacecraft would pass through the solar wind released from those surface features. A unique Solar Orbiter instrument suite that covers both in situ measurements and remote sensing, as well as its orbit, which brings it close to the Sun, were specially designed to allow this kind of scientific connection to be attempted.

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

The solar wind moves fast or slow

Generally speaking, the solar wind comes in two types: the fast solar wind moving at speeds greater than 500 km/s and the slow solar wind moving at speeds less than 500 km/s.

While the fast solar wind is known to originate from magnetic configurations known as coronal holes that channel the solar wind into space, the origin of the slow solar wind is still poorly understood. It is known to be associated with “active regions” on the Sun where sunspots appear, but details are elusive. Sunspots are cooler regions in the Sun’s photosphere where intense magnetic fields curl and concentrate. They indicate active regions of the Sun, often responsible for solar flares and eruptions.

To demonstrate the team’s ability to link the measured slow solar wind in situ to its point of origin on the solar surface, the spacecraft needed to fly through the magnetic field associated with the edge of either the coronal hole or the sunspot complex. This allowed the team to observe how the solar wind changed its speed – from fast to slow or vice versa – and other properties, confirming that they were looking at the right area. In the end, they got a perfect combination of both types of functions together.

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

A new age of solar wind research

Through their analysis of the various solar wind streams detected by Solar Orbiter, the team clearly showed that the solar wind still exhibits “traces” carried by its various source regions, making it easier for solar physicists to trace the streams back to their starting points on the Sun.

Now that the concept has been proven, it opens up a lot of future possibilities for using data from other spacecraft near 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 establish a robust connection between the solar wind and its source regions on the solar surface. This was a key goal of the mission and opens the way for us to study the origin of the solar wind in unprecedented detail,” says Daniel Müller, ESA project scientist for the Solar Orbiter.

Notes for editors
‘Multisource connectivity as a driver of solar wind variability in the heliosphereStephanie Yardley et al. released today in Astronomy of natureDOI: 10.1038/s41550-024-02278-9

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