Scientists have moved a step closer to determining the mysterious origin of the “slow” solar wind using data collected during the Solar Orbiter spacecraft’s first close trip to the Sun. The solar wind, which can travel at hundreds of kilometers per second, has fascinated scientists for years, and a new study has been published. Nature Astronomyfinally sheds light on how it was formed.

The solar wind describes the continuous flow of charged plasma particles from the Sun into space; Wind speeds greater than 500 km/h are known as “fast” and speeds less than 500 km/h are known as “slow”.

When this wind touches Earth’s atmosphere, it can produce the stunning aurora borealis we know as the Northern Lights. But when more plasma is ejected as a coronal mass ejection, it can also be dangerous and cause serious damage to satellites and communications systems. Despite decades of observations, the sources and mechanisms that release, accelerate, and transport solar wind plasma from the Sun to our Solar System are not fully understood, especially the slow solar wind.

In 2020, the European Space Agency (ESA) launched the Solar Orbiter mission with support from NASA. One of the mission’s main goals is to obtain the closest and most detailed images of the Sun, as well as to measure the solar wind and correlate it with its origin on the Sun’s surface.

Described as “the most advanced science laboratory ever sent to the Sun,” Solar Orbiter carries ten different science instruments – some of which collect and analyze solar wind samples on site as they pass by the spacecraft – and carries other instruments as well. remote sensing Instruments designed to obtain high-quality images of activity on the Sun’s surface.

By combining photographic and instrumental data, scientists were able to more clearly determine where the slow solar wind was coming from for the first time. This helped them determine how it might leave the sun and begin its journey into the heliosphere, the giant bubble around the sun and its planets that protects our solar system from interstellar radiation.

from Northumbria University in Newcastle-upon-Tyne. Steph Yardley led the study and explained: “The variability of solar wind fluxes measured in situ on a spacecraft close to the Sun gives us a lot of information about their sources; when we trace the origin of the solar wind, this was done much closer to Earth and by then this variability disappeared.

“Because Solar Orbiter moves so close to the sun, we can capture the complex nature of the solar wind, providing a clearer picture of its origin and how this complexity is affected by changes in different source regions.”

The difference between fast and slow speeds of the solar wind is thought to arise from different regions of the solar corona, the outer layer of the atmosphere, where they form. Open corona refers to regions where magnetic field lines exist. Only one end is connected to the sun, while the others are pulled into space, creating a highway for solar energy to exit into space. These areas are cooler and are believed to be the source of the fast solar wind.

Closed corona refers to regions where the Sun’s magnetic field lines are closed, that is, connected to the solar surface at both ends. They can be seen as large, bright rings that form over magnetically active regions.

Sometimes these closed magnetic loops break and, just like open magnetic field lines, provide a brief opportunity for solar materials to escape, then recombine and form a closed loop again. This usually occurs at the meeting points of the open and closed crowns.

One of Solar Orbiter’s goals is to test the theory that the slow solar wind originates in the closed corona and can escape into space by breaking and reconnecting magnetic field lines. One way the research team was able to test this theory was by measuring the “composition” or composition of solar wind currents.

The combination of heavy ions found in solar material differs depending on where it comes from; Hotter closed corona vs colder open corona. Using instruments on Solar Orbiter, the team was able to analyze activity occurring on the Sun’s surface and then match this with solar wind currents collected by the spacecraft.

Using images of the solar surface taken by Solar Orbiter, they were able to detect that slow wind currents were coming from the region where the open and closed corona meet; This supported the theory that the slow wind could be caused by closed magnetic field lines. through a process of disconnection and reconnection.

As Dr Yardley, from Northumbria University’s Solar and Space Physics Research Group, explains: ‘The changing composition of the solar wind measured by Solar Orbiter was consistent with the changing composition of sources in the corona.

“Changes in the composition of heavy ions along with electrons provide convincing evidence that the variability is driven not only by different source regions but also by reconnection processes occurring between closed and open circuits in the corona.”

The ESA Solar Orbiter mission is an international collaboration in which scientists and institutions from around the world work together, providing specialized skills and equipment.

Daniel Müller, ESA’s Solar Orbiter Project Scientist, said: “From the very beginning, the main aim of the Solar Orbiter mission has been to relate dynamic events on the Sun to their effects on the surrounding plasma bubble of the heliosphere.

“To achieve this, we need to combine remote observations of the sun with in situ measurements of the solar wind as the spacecraft passes by. I am extremely proud of the entire team who successfully carried out these challenging measurements.

“This result confirms that Solar Orbiter is able to establish reliable connections between the solar wind and its sources on the sun’s surface. This was one of the key objectives of the mission and paves the way for an unprecedentedly detailed study of the origin of the solar wind.”

Instruments aboard Solar Orbiter include the Heavy Ion Sensor (HIS), developed in part by researchers and engineers at the Space Physics Research Laboratory in the University of Michigan’s Department of Climate and Space Sciences and Engineering. The sensor is designed to measure heavy ions in the solar wind, which can be used to determine where the solar wind is coming from.

“Each region of the Sun may have a unique combination of heavy ions, which determines the chemical composition of the solar wind flux.

“Since the chemical composition of the solar wind remains constant as it propagates throughout the solar system, we can use these ions as fingerprints to determine the origin of a particular solar wind flow in the lower part of the Sun’s atmosphere,” said Susan. Lepree is a professor of climate and space sciences and engineering at the University of Michigan and Deputy Principal Investigator for the Heavy Ion Sensor.

Electrons in the solar wind are measured using the Electron Analysis System (EAS) developed by the UCL Mallard Space Sciences Laboratory, of which Dr Yardley is an Honorary Fellow.

UCL Professor Christopher Owen said: “Instrument teams have spent more than a decade designing, building and preparing their sensors for launch and planning how best to use them in concert. “So it’s great to see the data coming together to discover which parts of the sun are driving the slow solar wind and its variability.”

The Proton-Alpha Sensor (PAS), which measures wind speed, was designed and developed by the Research Institute of Astrophysics and Planetology at Paul Sabatier University in Toulouse, France. Together these devices form the Solar Wind Analyzer sensor suite on Solar Orbiter, of which UCL’s Professor Owen is principal investigator.

Speaking of plans for future research, Dr Yardley said: ‘So far we have only analyzed Solar Orbiter data from this particular interval. “It will be very interesting to look at other use cases for Solar Orbiter and compare it with data sets from other nearby missions, such as NASA’s Parker Solar Probe.”