It is well known that magnetic fields stabilize the KH instability from the perspective of theoretical and idealized linear calculations such as those presented in Chandrasekhar (1961), which imply that the velocity shear must be super-Alfvénic to lead to the generation of the KH instability. However, it is by no means clear that more complex and typical environments such as the solar corona resemble such idealized conditions. The argument for suppression of the KH instability assumes that the magnetic fields and the flow across a boundary are highly aligned. If the magnetic field is more azimuthal, the restoring tension is reduced and the flow can be more susceptible to KH instability. In the extreme situation of a magnetic field orthogonal to two tangential streams, the sheared flow is always unstable to the KH instability, just as in the hydrodynamic case, even for subsonic flows, and the magnetic field basically plays no role. If such nontangential magnetic field conditions occur in the low solar corona, as might be expected, various forms of interchange reconnection can occur along the boundaries of nominally fast and slow wind where loops and open field collide, leading to current sheet formation and magnetic fields that cross from one side of what was a tangential flow to the other. For instance, Schwadron & McComas (2021) argue that nontangential magnetic fields crossing shearing flows can be the origin of switchbacks. So the existence of switchbacks may, in their interpretation, be a manifestation of nontangential magnetic fields associated with tangential flows as well. In these regions, very complicated structures with a weak and probably quite disordered magnetic field can arise. The simple idealized KH instability of Chandrasekhar (1961) is certainly unlikely to occur, and the KH instability may resemble its gasdynamic form instead. Hence, despite previous studies that investigated the conditions for the onset of KH instability in the solar wind and in coronal plumes (e.g., Parhi et al. 1999; Velli et al. 2011) and concluded that the KH instability may arise only in super-Alfvénic regions, at distances larger than 10 R
⊙ (Velli et al. 2011 found that the KH instability could become important at 0.2–0.3 au), the KH instability has been widely imaged in the sub-Alfvénic low solar corona, at distances well below 10 R
⊙, in strong magnetic field environments associated with active regions (Yuan et al. 2019), at the flanks of coronal mass ejections (Foullon et al. 2011; Ofman & Thompson 2011; Foullon et al. 2013; Möstl et al. 2013; Nykyri & Foullon 2013), along solar prominence/corona discontinuity layers (Berger et al. 2010; Ryutova et al. 2010; Hillier & Polito 2018; Yang et al. 2018), in solar jets (Kuridze et al. 2016; Li et al. 2018), associated with streamer wave phenomena (Feng et al. 2013), and in white-light eclipse observations (Druckmüller et al. 2014). This is because the realistic solar corona is quite unlike a nice cartoon showing well-defined large-scale magnetic field lines; rather, at least locally, it is characterized by a very complex magnetic topology. The onset of the KH instability depends on the orientation of the magnetic field relative to the tangential flows. It follows that even the sub-Alfvénic velocity shear along the fast/slow boundary in the solar corona can lead to KH instability growth, provided that, as expected, the streamer belt region does not have a nicely ordered magnetic field and instead possesses locally a very complicated structure with a nontangential magnetic field configuration. Although theoretically predicted to play an important role in the plasma dynamics between high- and low-speed coronal streams flowing alongside each other (Ismayilli et al. 2018), the KH instability has not yet been detected at the edges of coronal holes adjacent to equatorial streamers, where shear due to the outflow of fluids with different velocities can initiate KH vortices. This lack of evidence motivates the present work, which indeed aims at providing, for the first time, possible observational evidence for manifestations of the KH instability in the solar corona in the interaction region between fast and slow solar wind.
Citation: Parhi et al. 1999
Citation function: Future Work