The mystery of the ‘slow’ solar wind revealed by the Solar Orbiter mission

Scientists are one step closer to identifying the mysterious origin of the “slow” solar wind using data collected during the Solar Orbiter’s first close pass to the Sun.

The solar wind, which can travel at hundreds of kilometers per second, has fascinated scientists for years, and new research was published in Astronomy of naturefinally sheds light on how they are formed.

The solar wind describes the continuous outflow of charged plasma particles from the Sun into space – the wind travels at speeds over 500 km per second, which is known as “fast”, and below 500 km per second, which is referred to as “slow”.

When this wind hits the Earth’s atmosphere, it can lead to the stunning aurora borealis known as the Aurora Borealis. But when more plasma is released in the form of a coronal mass ejection, it can also be dangerous and cause significant damage to satellites and communication systems.

Despite decades of observations, the sources and mechanisms that release, accelerate, and transport solar wind plasma away from the Sun and into our Solar System are not well understood—especially the slow solar wind.

In 2020, the European Space Agency (ESA) launched the Solar Orbiter mission with the support of NASA. In addition to capturing the closest and most detailed images of the Sun ever taken, one of the main goals of the mission is to measure and link the solar wind back to its region of origin on the Sun’s surface.

Described as “the most comprehensive science laboratory ever sent to the Sun,” Solar Orbiter is home to ten different science instruments—some in situ to collect and analyze samples of the solar wind as the spacecraft passes by, and another remote survey. instruments designed to capture high-quality images of activity on the solar surface.

By combining photographic and instrument data, scientists were for the first time able to more clearly identify where the slow solar wind comes from. This helped them figure out how it is able to leave the Sun and begin its journey into the heliosphere – the giant bubble around the Sun and its planets that shields our Solar System from interstellar radiation.

Dr. Steph Yardley, from Northumbria University, Newcastle upon Tyne, led the research and explains: “The variability of solar wind streams measured in situ on a spacecraft close to the Sun gives us a lot of information about their sources, and although past studies have traced the origin of the solar wind, this has been done much closer to Earth, by which time this variability is lost.

“Because the Solar Orbiter travels so close to the Sun, we can capture the complex nature of the solar wind to get a much clearer picture of its origin and how that complexity is driven by changes in the various source regions.”

The difference between the speed of the fast and slow solar wind is thought to be due to the different regions of the Sun’s corona, the outermost layer of its atmosphere, from which they originate.

An open corona refers to regions where magnetic field lines anchor the Sun at only one end and extend into space at the other, creating a highway for solar material to escape into space. These regions are cooler and are thought to be the source of the fast solar wind.

Meanwhile, the closed corona refers to regions of the Sun where its magnetic field lines are closed – meaning they are connected at both ends to the solar surface. These can be seen as large bright loops that form over magnetically active regions.

Occasionally, these closed magnetic loops break, providing a brief opportunity for solar material to escape, as well as through open magnetic field lines, before rejoining to form a closed loop. This usually takes place in the areas where the open and closed corona meet.

One of Solar Orbiter’s goals is to test the theory that the slow solar wind originates in the closed corona and is able to escape into space through this process of breaking and rejoining magnetic field lines.

One way the science team could test this theory was by measuring the “composition,” or composition, of the solar wind streams.

The combination of heavy ions contained in solar material varies depending on where it comes from; hotter, closed versus cooler, open corona.

Using instruments aboard the Solar Orbiter, the team was able to analyze the activity taking place on the Sun’s surface and then compare it to the solar wind streams collected by the spacecraft.

Using images of the Sun’s surface captured by Solar Orbiter, they were able to determine that the slow wind streams came from the region where the open and closed corona met, proving the theory that the slow wind is able to escape from closed magnetic field lines. through a process of breaking and rejoining.

As explained by Dr. Yardley, from Northumbria University’s Solar and Space Physics research group: “The different compositions of the solar wind measured by Solar Orbiter were consistent with the variation in composition across sources in the corona.

“The changes in the composition of heavy ions along with electrons provide strong evidence that not only is the variability driven by different source regions, but it is also caused by reconnection processes between closed and open loops in the corona.”

The ESA Solar Orbiter mission is an international collaboration with scientists and institutions from around the world collaborating and contributing specialized skills and equipment.

Daniel Müller, ESA’s Project Scientist for Solar Orbiter, said: “From the beginning, the main objective of the Solar Orbiter mission has been to link dynamical events on the Sun with their impact 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 it flows around the spacecraft. I am extremely proud of the entire team for successfully making these complex measurements.”

“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 to study the origin of the solar wind in unprecedented detail.” “

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

“Each region of the sun can have a unique combination of heavy ions that determines the chemical composition of the solar wind stream.

“Because the chemical composition of the solar wind remains constant as it travels through the solar system, we can use these ions as a fingerprint to determine the origin of a specific solar wind stream in the lower part of the solar atmosphere,” said Susan Lepri, professor of climate and space sciences and engineering at the University of Michigan and Deputy Principal Investigator of the Heavy Ion Sensor.

Electrons in the solar wind are measured by the Electron Analyzer System (EAS), developed by UCL’s Mullard Space Science Laboratory, where Dr. Yardley Honorary Member.

UCL’s Professor Christopher Owen said: “Instrument teams have spent more than a decade designing, building and preparing their sensors for launch, as well as planning how best to control them in a coordinated way. So it’s very pleasing that we can now see the data being collected, to reveal which regions of the Sun drive the slow solar wind and its variability.”

The Proton-Alpha Sensor (PAS), which measures wind speed, was designed and developed by the Institut de Recherche en Astrophysique et Planétologie of the Paul Sabatier University in Toulouse, France.

Together, these instruments make up the Solar Wind Analyzer sensor suite on board the Solar Orbiter, for which UCL’s Professor Owen is the principal investigator.

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