In search of ever-higher velocities leaving the Solar System, we need to keep in mind the options offered by the solar wind. This stream of charged plasma particles flowing outward from the Sun carves out the protective bubble of the heliosphere, and in doing so can generate ‘winds’ of more than 500 kilometers per second. Not bad if we’re thinking in terms of harnessing the effect, perhaps by a magnetic sail that can create the field needed to interact with the wind, or an electric sail whose myriad tethers, held taut by rotation, create an electric field that repels protons and produces thrust.

But like the winds that drove the great age of sail on Earth, the solar version is treacherous, as likely to becalm the ship as to cause its sails to billow. It’s a gusty, turbulent medium, one where those velocities of 500 kilometers and more per second can as likely fall well below that figure. Exactly how it produces squalls in the form of coronal mass ejections or calmer flows is a topic under active study, which is where missions like Solar Orbiter come into play. Studying the solar surface to pin down the origin of the wind and the mechanism that drives it is at the heart of the mission.

Launched in 2020, Solar Orbiter carries a panoply of instruments, ten in all, for the analysis, including an Electron Analyzer System (EAS), a Proton-Alpha Sensor (PAS) for measuring the speed of the wind, and a Heavy Ion Sensor (HIS) designed to measure the heavy ion flow. Critical to the analysis of this paper is the Spectral Imaging of the Coronal Environment (SPICE) instrument, as we’ll see below. Steph Yardley (Northumbria University) is lead author of the paper on this work, which has just appeared on Nature Astronomy:

“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.”

What the work is analyzing is a theory that the process of magnetic field lines breaking and reconnecting is critical to producing the slower solar wind. Different areas of the Sun’s corona are implicated in the origin of both the fast and slow winds, with the ‘open corona’ being those regions where magnetic field lines extend from the Sun into space, tethered to it at one end only and creating the pathway for solar material to flow out in the form of the fast solar wind. Closed coronal regions, on the other hand, are those where the magnetic field lines connect to the surface at both ends, forming loops.

As you would imagine, the process is wildly turbulent and marked by the frequent breakage of these closed magnetic loops and their subsequent reconnection. The researchers have probed the theory that the slow solar wind originates in the closed corona during these periods of breakage and reconnection by studying the composition of solar wind streams, for the heavy ions emitted vary depending on their origins in either the closed or open corona. Solar Orbiter’s Heavy Ion Sensor (HIS) is able to take the needed measurements to relate the effects of this activity on the surrounding plasma.

The image below is from the Solar Dynamics Observatory spacecraft rather than Solar Orbiter, reminding us of the different views we are gaining by our various missions to our star. The comparison of key datasets tells the story.

Image: This is part of Figure 1 from the paper. The caption reads: SDO/AIA [Solar Dynamics Observatory data using its Atmospheric Imaging Assembly] 193 Å image showing the source region from the perspective of an Earth observer. Open magnetic field lines that are constructed from the coronal potential field model are overplotted, coloured by their associated expansion factor F. The large equatorial CH [Coronal Hole] and AR [Active Region] complex are labeled in white. The FOVs [fields of view] of SO EUI/HRI and PHI/HRT [references to instruments aboard Solar Orbiter] are shown in cyan and pink, respectively. The back-projected trajectory of SO [Solar Orbiter] from 1 March 2022 until 9 March 2022 is shown by the olive dotted line (from right to left).

So because we have Solar Orbiter, we can now combine observations of the Sun from various sources including other space missions, like the Solar Dynamics Observatory, with the measurements of the solar wind actually flowing past the spacecraft. Susan Lepri (University of Michigan) is deputy principal investigator on the HIS system:

“Each region of the Sun can have a unique combination of heavy ions, which determines the chemical composition of a stream of solar wind. Because the chemical composition of the solar wind remains constant as it travels out into the solar system, we can use these ions as a fingerprint to determine the origin of a specific stream of the solar wind in the lower part of the Sun’s atmosphere.”

The results have been productive. The analysis gives us a precise breakdown of just what Solar Orbiter has encountered during the period studied. This is a thorny quotation but it includes a key finding. From the paper:

Combining the SO [Solar Orbiter] trajectory, coronal field model, magnetic connectivity tool, the SPICE composition analysis of the AR [Active Region] complex, and the in situ plasma and magnetic field parameters, we suggest that SO was immersed in three fast wind streams… originating from the three linked sections of the large equatorial CH [Coronal Hole]… These were followed by two slower streams associated with the negative polarities of the AR complex… The decrease of the solar wind speed can be explained by the expansion of the open magnetic field associated with the CH-AR complex, as the connectivity of SO transitioned across these regions. Credit: Yardley et al.

The findings described here are significant. We learn from this work just how complex the solar wind flow is, in this case involving three fast streams and two slower ones, all involving changes in magnetic field connectivity. Matching the composition of the solar wind streams to different areas on the corona gives us new insights into the turbulent mix found where the open and closed corona meet. The slow solar wind’s ‘breakout’ from closed magnetic field lines is demonstrated. The phenomenon of magnetic reconnection proves critical to the wind’s variability.

Demonstrating these linkages means that we can now use the findings to probe further into the origins of the solar wind. But this is a variability that is in no way predictable, making the prospect of riding the solar wind via electric or magnetic sail a daunting one. We’ll continue to learn more, though, as we bring in data from missions like the Parker Solar Probe. It will be fascinating to see one day how we use the solar wind to test out possible spacecraft designs in search of a faster route to the outer Solar System.

Addendum: In an earlier draft, I mistakenly criticized the authors for not initially clarifying some of the acronyms in this paper. I’ve removed that comment because a later reading showed I was mistaken about the two examples I cited.

The paper is Yardley et al., “Multi-source connectivity as the driver of solar wind variability in the heliosphere,” Nature Astronomy 28 May 2024 (full text).