Mystery of ‘slow’ solar wind revealed by Solar Orbiter mission

Scientists have moved one step closer to identifying the mysterious origins of the ‘slow’ solar wind, using data collected during the Solar Orbiter spacecraft’s first short trip to the sun.

Solar wind, which can travel at speeds of hundreds of kilometers per second, has fascinated scientists for years, and new research has been published in Nature Astronomyfinally sheds light on how it arises.

Solar wind describes the continuous outflow of charged plasma particles from the sun into space – with the wind traveling at speeds above 500 km per second, known as ‘fast’ and below 500 km per second, described as ‘slow’.

When these winds hit Earth’s atmosphere, it can result in the stunning aurora we know as the Northern Lights. But when larger amounts of plasma are released, in the form of a coronal mass ejection, it can also be dangerous and cause significant damage to satellites and communications 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 yet well understood – especially the slow solar wind.

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

Described as ‘the most complex scientific laboratory ever sent to the Sun’, there are ten different scientific instruments on board the Solar Orbiter – some on site to collect and analyze samples of the solar wind as it passes the spacecraft, and other remote sensing systems. instruments designed to capture high-quality images of activity on the Sun’s surface.

By combining photographic and instrumental data, scientists have for the first time been able to more clearly identify where the slow solar wind comes from. This helped them determine how it can leave the sun and begin its journey to the heliosphere – the giant bubble surrounding the sun and its planets that protect 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 flows, measured in situ on a spacecraft close to the Sun, gives us a lot of information about their sources, and although previous studies have not explored the origins of traced the solar wind, this happened much closer to Earth, by which time this variability has been lost.

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

It is believed that the difference between the speed of the fast and slow solar winds is due to the different parts of the Sun’s corona, the outermost layer of the atmosphere, from which they come.

The open corona refers to regions where magnetic field lines anchor to the Sun on one side and extend into space on the other, creating a highway for solar material to escape into space. These areas are cooler and are believed to be the source of the fast solar wind.

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

Occasionally these closed magnetic loops will break, giving solar material a brief opportunity to escape, in the same way that open magnetic field lines do, before reconnecting to form a closed loop again. This usually takes place in areas where the open and closed corona meet.

One of the goals of Solar Orbiter is to test a theory that the slow solar wind originates from the closed corona and can escape into space through this process of magnetic field lines breaking and reconnecting.

One way the scientific team was able to test this theory was by measuring the ‘composition’ or composition of solar wind streams.

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

Using the instruments on board 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 the images of the Sun’s surface captured by Solar Orbiter, they were able to determine that the slow wind currents originated from a region where the open and closed corona met, proving the theory that the slow wind is capable of to escape from closed magnetic field lines. through the process of disconnecting and reconnecting.

Like Dr. Yardley, from Northumbria University’s Solar and Space Physics research group, explains: “The varying composition of the solar wind measured at Solar Orbiter was consistent with the change in composition between the sources in the corona.

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

The ESA Solar Orbiter mission is an international collaboration, with scientists and institutions from around the world working together and contributing specialist skills and equipment.

Daniel Müller, ESA Project Scientist for Solar Orbiter, said: “From the beginning, a central goal of the Solar Orbiter mission has been to link dynamical events on the Sun to 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 past the spacecraft. I am extremely proud of the entire team who successfully performed these complex measurements.

“This result confirms that Solar Orbiter is capable of making robust connections between the solar wind and source regions on the solar surface. This was a key goal of the mission and opens the way for us to study the origins of the solar wind in unprecedented detail. “

One of the instruments aboard 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 Division of Climate and Space Sciences and Engineering. The sensor is designed to measure heavy ions in the solar wind, which can help determine where the solar wind comes from.

‘Each region of the Sun may have a unique combination of heavy ions, which determine the chemical composition of a stream of solar wind.

“Because the chemical composition of the solar wind remains constant as it enters the Solar System, we can use these ions as a fingerprint to determine the origin of a specific solar wind current in the lower part of the Sun’s atmosphere,” said Susan Lepri, professor of climate and space science and engineering at the University of Michigan and deputy principal investigator of the Heavy Ion Sensor.

The electrons in the solar wind are measured by an Electron Analyzer System (EAS), developed by UCL’s Mullard Space Science Laboratory, where Dr. Yardley is an Honorary Fellow.

Professor Christopher Owen from UCL said: “The instrument teams have spent more than a decade designing, building and preparing their sensors for launch, and planning the best way to operate them in a coordinated manner. So it is very gratifying to now see the data being pieced together to reveal 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 Institut de Recherche en Astrophysique et Planétologie of Paul Sabatier University in Toulouse, France.

Together these instruments form the Solar Wind Analyzer sensor suite on board Solar Orbiter, for which Professor Owen from UCL is the principal investigator.

Speaking about future research plans, Dr. Yardley: “So far we have only analyzed Solar Orbiter data in this way for this specific interval. It will be very interesting to look at other cases using Solar Orbiter and also make a comparison with data sets from other close -in missions such as NASA’s Parker Solar Probe.”