On the Importance of the Kelvin‐Helmholtz Instability on Magnetospheric and Solar Wind Dynamics During High Magnetic Shear

The secondary processes driven by the velocity shear driven Kelvin‐Helmholtz Instability (KHI) in the magnetized plasmas have been shown to be important in producing plasma transport and heating from the shocked solar wind into the Earth's magnetosphere (MSP). The plasma transport into the MSP due to KHI has been shown to be strongest during northward interplanetary magnetic field (IMF) via KHI driven low‐ and mid‐latitude reconnection process. In a recent article, Li et al. (2023), https://doi.org/10.1029/2023gl105539 show Magnetosphere Multi‐Scale (MMS) spacecraft observations of multiple, Alfvénic reconnection jets during southward IMF at the dawn‐side MSP flank. The quasi‐periodic oscillations in plasma parameters and compressed, ion‐scale current sheets were strongly indicative of the MMS crossing regions of MSP‐like and magnetosheath‐like plasma within Kelvin‐Helmholtz waves. In this brief commentary, the importance of this new discovery for magnetospheric and solar wind dynamics is discussed.


Introduction
As the solar wind gets processed by the bow shock, it heats and first slows down at the sub-solar MSH, then gradually increasing in speed, finally reaching the solar wind speed at the vicinity of the dawn-dusk terminator (see e.g., Dimmock and Nykyri (2013)).This fast flowing plasma close to the magnetopause and relatively stagnant plasma inside the magnetosphere (MSP) leads to a strong gradient in the plasma velocity field.In the absence of a strong magnetic field along the plasma velocity and without strong plasma compressibility (which occurs if plasma velocity and pressure are too high), this velocity shear gives free energy for the Kelvin-Helmholtz Instability (KHI) to grow.
As an ideal instability KHI was traditionally not considered to be important for mass transport.However, 2-D MHD simulations revealed that in its non-linear stage KHI can produce significant plasma transport (with rate of 10 9 m 2 /s) via magnetic reconnection and can detach high-density islands of magnetosheath origin even when the magnetic fields at the MSH and MSP sides of the boundary are initially nearly aligned (low magnetic shear perpendicular to the shear flow plane) during strongly northward IMF (Nykyri & Otto, 2001, 2004).The 3-D MHD simulations showed that the mid-latitude reconnection driven by KHI during strongly northward IMF, and occurring both above and below the main shear flow plane, can produce even higher mass transport rates, on the order of 10 10 m 2 /s (Ma et al., 2017).Furthermore, large-scale 3-D kinetic simulations allow development of highly wrapped thin boundaries resulting in reconnection and formation of ion-scale jets that rapidly decay through self-generated turbulence and yield mass transfer rates up to 10 11 m 2 /s (Nakamura et al., 2017).In the real magnetosphere the extent of the boundary roll-up and final transport rate is likely restricted by the 3-D field line curvature and line-tying into the ionosphere.
The ideal onset condition (Chandrasekhar, 1961) for the KHI, which assumes infinitely thin boundary, is equally satisfied for purely northward (B z > 0) and purely southward IMF (B z < 0).However, as the strongly southward IMF leads to a dayside reconnection close to the sub-solar point, and to flux transfer events (FTEs) (even with a strong B y ) the clear identification of KHI events during southward IMF (Blasl et al., 2022;Hwang et al., 2011;Nykyri et al., 2006) has been more challenging than during strongly northward IMF (Eriksson et al., 2016;Hasegawa et al., 2004;Henry et al., 2017;Kavosi & Raeder, 2015;Li et al., 2016;Lin et al., 2014;Taylor & Lavraud, 2008).This is partly because FTEs and KHI can both produce similar signatures in the spacecraft data (Nykyri et al., 2003) (e.g., bipolar variation of the normal component of the magnetic field and reconnecting current sheets that satisfy Walén relation.) Furthermore, 3-D MHD and Hall-MHD simulations of the interaction of the reconnection and KHI during southward IMF (Ma et al., 2014b) (strong magnetic shear perpendicular to shear flow plane) have shown that when reconnection starts as a primary mechanism, it makes the velocity shear layer thinner, generating KH waves with shorter wavelengths.This can produce very messy observational signatures when compared to northward IMF conditions.On the other hand, when the KHI starts as a primary mechanism, "the growing KH modes modify the current layer width, which modulate the diffusion regions, increase the local reconnection rates, and generate field-aligned currents" (Ma et al., 2014a).These studies also demonstrated that the KHI driven reconnection rates were comparable to Petschek reconnection rate even without the inclusion of Hall physics, strongly indicating that KHI can drive fast reconnection during southward IMF, high-magnetic shear conditions.
On the 26th of June 2020 at 00:10-01:20 UT, the four MMS spacecraft were located at the dawn-side tail flank magnetopause (at x ≈ 16 R E ) where they observed for 70 min, very regular, periodic variations of the magnetic field, densities, velocities, and temperatures.The ion energies varied between cold, MSH-like and hot MSP-like plasma.The IMF was strongly southward during this interval.In Section 2, an example of a key-observation of this event is discussed in detail, demonstrating that MMS encountered reconnection jets, embedded within KH waves.In Section 3, the importance of these results for magnetospheric and solar wind dynamics is discussed.

New MMS Observations of Magnetic Reconnection Driven by the KHI Under Southward IMF
Figure 1 shows the MMS location at the dawn-flank magnetopause (panel a) and magnetic field observations during a 70 min interval at 00:10-01:20 UT (panel b), a schematic of the MMS trajectory across current sheet with the observed ion jet in the l -direction (panel c), and detailed plasma and field observations during a sub-interval 00:53:00-00:54:10 UT (d-g).
As MMS travels from MSP (see the positive B l -component (d), higher energy ions (e), and lower ion number density (f)) to the MSH (negative B l as expected during southward IMF, lower ion energy, and higher ion number density), it detects a strong deflection of the ion flow along the l direction.As the Walén relation and correlation coefficient are well satisfied, this northward ion flow deflection is consistent with an Alfvénic jet, which is observed at the trailing edge current sheet of the KH wave (Li et al., 2023).Also the observed pitch-angle distribution of the energetic electrons (not shown here) were consistent with northward ion jet crossing.
In total, authors performed analysis of 36 current sheets within KH waves, where 19 showed clear signatures of magnetic reconnection, characterized by Alfvénic ion flows that satisfy the Walén relation and open magnetic field lines with escaping magnetospheric electrons.The current sheets of both the leading and trailing edges were compressed down to ion scale due to KHI which likely led to triggering of magnetic reconnection.

Summary and Future Outlook
The presented results clearly demonstrate that during southward IMF, the KHI can drive magnetic reconnection effectively also at the flanks of the magnetosphere.The inertia in KH vortex motion can strongly compress the current sheets, so fields may not initially even need to be closely anti-parallel at the both sides of the MSP and MSH.The estimated period and inferred KH wavelength were 4.8 min and 8.8 R E , respectively.These results also show how KHI driven reconnection leads to an open magnetic field geometry and escape of the magnetospheric electrons into the solar wind.This may be an important mechanism for the energetic particle escape from the magnetotail, for example, those accelerated by substorm related processes in the magnetotail (Turner et al., 2017).
This same mechanism may also work at solar wind magnetic field structures, for example, at the sheath-ejecta interface of the coronal mass ejections (CMEs), where KH waves have been observed (Foullon et al., 2011;Nykyri & Foullon, 2013).Understanding and predicting the energetic particle environment at the lunar orbit both in the solar wind and Earth's magnetotail is crucial for the safe extra vehicular activities of the nearfuture, manned, Artemis space mission.Recently, the ARTEMIS spacecraft detected a CME both with trapped and escaping energetic particles in the lunar orbit with a similar magnetic field geometry as for this MMS event in the magnetosphere, but with a southward B z in the CME core, and northward B z in the CME sheath region (see figure 2 in Nykyri et al. (2023)).Future, multi-spacecraft solar wind missions are required to determine in detail the physics driving the energetic particle dynamics and escape within CMEs (see e.g., Nykyri et al. (2023)).
Both trapped and escaping 100s of keV energetic ions and electrons within KH vortices have been detected by MMS also at the dawn-sector, southern-hemispheric high-altitude cusp region (Nykyri et al., 2021), but the origin of these particles (nearby diamagnetic cavities (Burkholder et al., 2020;Nykyri et al., 2012) or local acceleration within KH vortices) is still not clear.
In future works it would be important to quantify the reconnection rate in these KHI driven current sheets under mostly southward IMF and how it compares to the reconnection rate closer to the sub-solar point, where the flow shear is less and when the KHI is not operating.Finally, it would be important to quantify the importance of the KHI driven reconnection during southward IMF on magnetotail reconnection and substorm onset.Assuming the KHI driven flank perturbations can propagate deeper into the magnetotail, this mechanism may help in triggering of the magnetotail reconnection.Such a "critical flux enhancement" scenario for substorm onset has been proposed for magnetosheath jets under strongly radial and northward IMF (Nykyri et al., 2019), so it may also be at play under KHI driven flank dynamics.
Resolving these outstanding questions will require simultaneous, multi-point measurements at the magnetotail, tail flanks, dayside magnetopause and high-latitude boundary layer (see e.g., Hwang et al. (2023)).
The KHI has also been observed at the magnetopause of Mercury (Boardsen et al., 2010;Sundberg et al., 2012), Jupiter (Ma et al., 2022;Montgomery et al., 2023) and Saturn (Masters et al., 2010), at the ionopause of Mars (Poh et al., 2021), and it also plays an important role in astrophysical and cosmological dynamics, for example, at the heliospheric boundaries (Opher et al., 2004) and in galaxy formation (Mandelker et al., 2019).This highlights the universal importance of this instability.

Figure 1 .
Figure 1. Figure is re-produced from Figures in Li et al. (2023).Panel (a) shows the schematic of Magnetosphere Multi-Scale (MMS) location with respect to dawn-side, flank magnetopause modulated by a KH wave.Panel (b) shows magnetic field fluctuations (in GSE coordinate system) as measured by the MMS during the 70 min interval of southward interplanetary magnetic field.Panel c depicts MMS trajectory from the Magnetosphere to the MSH relative to the observed ion jets.Panels (d) to (g) show magnetic field in the l, m, n -coordinate system (d), ion energies (e), ion number densities (f) and change of the l-component of the ion velocity (g), respectively.