Mystery of slow solar wind unveiled by Solar Orbiter mission

Scientists including researchers from the University of Reading have come a step closer to identifying the mysterious origins of the ‘slow’ solar wind, using data collected during the Solar Orbiter spacecraft’s first close journey to the Sun.

Solar wind, which can travel at hundreds of kilometres per second, has fascinated scientists for years, and new research published in Nature Astronomy, is finally shedding light on how it forms.

Solar wind describes the continuous outflow of charged plasma particles from the Sun into space – with wind travelling at over 500km per second known as ‘fast’ and under 500km per second described as ‘slow’.

When this wind hits the Earth’s atmosphere it can result in the stunning aurora we know as the Northern Lights. But when larger quantities of plasma are released, in the form of a coronal mass ejection, it can also be hazardous, causing 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 well understood – particularly the slow solar wind.

In 2020 the European Space Agency (ESA), with support from NASA, launched the Solar Orbiter mission. One of the mission’s main aims is to measure and link the solar wind back to its area of origin on the Sun’s surface.

By combining photographic and instrumental data from Solar Orbiter, scientists have for the first time been able to identify more clearly where the slow solar wind originates. This has helped them to establish how it is able to leave the Sun and begin its journey into the heliosphere – the giant bubble around the Sun and its planets which protect our solar system from interstellar radiation.

The speed difference between fast and slow solar wind is attributed to their origins in the Sun's corona. Fast wind originates from cooler, open magnetic field regions, while slow wind is thought to come from hotter, closed magnetic loops that occasionally break and reconnect, allowing solar material to escape. Solar Orbiter tested this theory by measuring the composition of solar wind streams and comparing them to images of the Sun's surface, confirming that slow wind originates from areas where open and closed corona meet.

Dr Steph Yardley of Northumbria University and formerly of the University of Reading, led the research and explains: “The variability of solar wind streams measured in situ at a spacecraft close to the Sun provide us with a lot of information on their sources, and although past studies have traced the origins of the solar wind, this was done much closer to Earth, by which time this variability is lost.

“Because 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 origins and how this complexity is driven by the changes in different source regions.”

Professor Mathew Owens and Michael Lockwood, of the University of Reading, are co-authors on the study. The professors provided interpretation of the in-situ measurements of the solar wind conditions at the Solar Orbiter spacecraft.

Professor Mathew Owens, Professor of Space Physics said: “Solar Orbiter has provided the clearest evidence yet that the slow solar wind originates near boundaries between open and closed magnetic fields on the Sun's surface. Pinpointing the sources of the solar wind is crucial for predicting how space weather may impact our satellites and infrastructure. These exciting results enabled by this groundbreaking mission bring us closer to unlocking the mysteries of the Sun's influence on the solar system."

Multi-source connectivity as the driver of solar wind variability in the heliosphere, is published in Nature Astronomy