Observations from NASA’s Parker Solar Probe (PSP), launched in 2018, have revealed that close to the Sun the solar wind is tremendously variable, much more so than further out in the Heliosphere. This variability primarily takes the form of local reversals of the usually radial magnetic field known as magnetic switchbacks. These structures seem to dominate the wind in its early development, and therefore understanding how they are formed may give clues as to how the wind itself is accelerated which is a major unsolved problem in solar physics. Two main theories have developed as to how switchbacks are formed. As the majority of switchbacks are Alfvénic in nature, one suggestion is that switchbacks are formed in-situ by the steepening of Alfvén waves in the expanding plasma of the wind. The other is that interchange reconnection in the low corona launches switchbacks directly into the wind. Testing either scenario requires high resolution, 3D MHD simulations to resolve both the large-scale of the overall wind and the small-scales of the fluctuations, hence Dirac is the only UK facility capable of supporting such investigations.

Figure 1. Top left: Grid blocks (8x8x8 cells) showing the adaptive mesh localised around the current sheet and its outflow into the solar wind. Bottom left: arrows highlight the enhanced plasma density within multiple small-scale flux ropes. Right: field lines within the curtain of Alfvénic waves created by the ejection of the small-scale flux ropes.

In a new study we have addressed the interchange reconnection hypothesis with a 3D adaptive mesh MHD simulation conducted with the ARMS code. For the first time in a 3D interchange reconnection simulation enough resolution was included that the reconnection process entered the high Lundquist number regime where the current sheet fragmented and became dominated by small-scale flux ropes (as occurs in the solar corona; Fig. 1, left). Our work showed that the ejection of these small-scale flux ropes does not launch switchbacks directly into the solar wind but instead creates a propagating curtain of rotating Alfvénic waves (Fig. 1, right). This has led to a new “best of both worlds” hypothesis that bursty interchange reconnection could provide seed Alfvénic fluctuations which then steepen into full magnetic reversals (switchbacks) within the wind itself. This work was published in the Astrophysical Journal Letters (Wyper et al 2022 ApJL 941 L29).

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