ESA’s Solar Orbiter traces the solar wind back to its source

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

Yet, 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 it to specific regions of the Sun’s surface.

As the solar wind passes through the solar system, it interacts with celestial bodies and spacecraft. These interactions range from benign, in the case of the Northern Lights on our planet, to very disruptive, in that solar storms can interfere with, or even damage, electrical systems on the ground or in spacecraft.

This is why understanding the solar wind is a priority for solar physicists. One of the main goals of Solar Orbiter’s mission was to connect the solar wind around the spacecraft to its source regions on the Sun. This new result, using data collected during Solar Orbiter’s first close approach to the Sun, shows that this is possible, fulfilling a key mission objective and opening a new avenue for studying the origin of the solar wind.

Connecting data near and far

Solar Orbiter can make these connections because it has both on the site and remote sensing instruments. THE on the site The instruments measure solar wind plasma and the magnetic field around the spacecraft, while remote sensing instruments take images and other data from the Sun itself. The difficulty is that the cameras show the Sun as it currently appears, while the on the site instruments reveal the state of the solar wind emitted from the surface of the Sun a few days earlier. This is because it takes a while for particles from the solar wind to reach the spacecraft.

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

“You can predict a few days in advance where you think Solar Orbiter will be connected on the solar surface,” says Stephanie Yardley, of Northumbria University, UK, who is lead author of the paper. announcing 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 fly through the solar wind emitted by these surface features. Solar Orbiter’s unique set of instruments, which covers both on the site the measurements and remote sensing, as well as its orbit which brings it closer to the Sun, have been specially designed to make it possible to attempt this type of scientific link.

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

The solar wind moves quickly or slowly

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

While the fast solar wind is known to originate from magnetic configurations called 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 linked to the “active regions” of the Sun, where sunspots appear, but the details are elusive. Sunspots are cooler areas of the solar photosphere where intense magnetic fields twist and concentrate. They indicate active regions of the Sun, often responsible for solar flares.

Prove the team’s ability to connect the measured slow solar wind on the site To its point of origin on the solar surface, the spacecraft had to fly through the magnetic field connected to the edge of either a coronal hole or a sunspot complex. This allowed the team to observe how the solar wind changed speed – from fast to slow or vice versa – as well as other properties, confirming that they were observing the correct region. In the end, they got a perfect combination of both types of features.

“Solar Orbiter flew over the coronal hole and active region, and we saw fast solar wind currents, followed by slow winds. We saw a lot of complexity that we could relate to the source regions,” explains Stéphanie. This included variations in composition and temperature in these particular regions.

A new era of solar energy research

Through their analysis of the different solar wind flows detected by Solar Orbiter, the team clearly showed that the solar wind always exhibits the “fingerprints” transmitted by its different source regions, which will allow solar physicists to more easily trace the flows to their points of origin on the Sun.

Now that the concept has been proven, it opens up a host 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 capable of establishing strong 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 origin of the solar wind in unprecedented detail,” explains Daniel Müller, ESA project scientist for Solar Orbiter.

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

For more information please contact:
ESA Media Relations
Media@esa.int