The solar wind is a 1,610,000 km/h stream of charged particles constantly emitted by the Sun in all directions. However, solar wind dynamics are maddeningly complex as the original contributors to the Solar-Terrestrial Predictions Workshop held in Meudon, France (1986) learned.
For example, fast solar wind can attain speeds in
excess of 500 km/s and emerges from coronal holes. These latter
comprise dark holes visible in coronal imagery indicating where the
Sun's magnetic field lines open up and extend into space thereby providing
an escape channel for hot solar plasma. In the coronal image below these holes,
as well as magnetic field arches, can clearly be seen:
This fast solar wind is differentiated from the much slower moving stream of solar wind that floods the solar system continuously, and defines the "slow solar wind". It defines what we call the heliosphere which is essentially the solar wind's" bubble of influence.
The latest research breakthrough has come thanks to the Solar Orbiter, a joint mission of NASA and the European Space Agency. For the past two years the craft has been in orbit around the Sun and actually getting close enough to study the solar wind in extraordinary detail.
Enter Stephanie Yardley of Northumbria University and her UK colleagues who have used multiple Solar Orbiter instruments to explore the origins of the slow solar wind. As it was positioned about 0.5 AU from the Sun in March, 2022, the Orbiter took high resolution images of the Sun's active regions. More data was collected as the solar wind particles passed the spacecraft a few days later. The team then used spectroscopic techniques to measure the composition of the wind, i.e. the iron-to-oxygen line ratio. Some of the theory behind these spectroscopic methods was examined in previous posts, e.g.
Stellar Absorption and Emission Processes Revisited (2)
Fast solar wind has already been linked to coronal holes, see e.g.
CH_HSS_KM_REDI2016-3.pdf (nasa.gov)
Yardley and her team, meanwhile, traced the slow solar wind to an active region complex consisting of two regions with both open and closed magnetic field lines. This was adjacent to a coronal hole with many open magnetic lines. A graphic showing the difference between the two can be seen below:
Clearly, the proximity between a coronal hole and an active region complex provides a favorable configuration for the solar wind to be expelled. Interestingly, this was proposed as early as June, 1984, in a paper presented at the then Baltimore meeting of the Solar Physics Division of the American Astronomical Society the first one I attended. (Using a Studentship award from the Division)
We already know that closed magnetic field loops in coronal regions have plasma flowing along them but nothing escapes because the loop closure topologically keeps the configuration stable. In order to enable plasma to escape from closed loops we need it to interact with adjacent open loops. It is quite possible that in this process an exchange of magnetic helicity occurs facilitating the plasma release.
Coronal loop scenario for which helicity H(r,r’) may be exchanged.
The instruments on the Orbiter that measure solar wind plasma and magnetic fields were actually able to gather evidence showing that the interaction between the two loop types (in neighboring ARs) is what gives rise to the slow solar wind. Yardley and her colleagues haven't factored in the transfer of magnetic helicity between loops yet but in many ways that may be premature while data is still being collected from the Solar Orbiter.
As reported in the latest (August) issue of Physics Today, Yardley's team is now working on a more complex analysis for subsequent Orbiter approaches to our nearest star. This will also include incorporating data from other missions including the Parker Solar Probe, e.g.
Parker Solar Probe Provides Hitherto Unknown Insights Into The Corona
Certainly future Orbiter observations and data will enhance our understanding of what causes solar wind variability, apart from the origins. In so doing it will certainly improve the quality of space weather forecasts.
See Also:
Nat. Astron., 2024, doi:10.1038/s41550-024-02278-9
And:
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