Solar Orbiter Observations of the Kelvin-Helmholtz Instability in the Solar Wind

Abstract

The Kelvin-Helmholtz instability (KHI) is a nonlinear shear-driven instability that develops at the interface between shear flows in plasmas. KHI has been inferred in various astrophysical plasmas and has been observed in situ at the magnetospheric boundaries of solar-system planets and through remote sensing at the boundaries of coronal mass ejections. While it was hypothesized to play an important role in the mixing of plasmas and in triggering solar wind fluctuations, its direct and unambiguous observation in the solar wind was still lacking. We report in-situ observations of ongoing KHI in the solar wind using Solar Orbiter during its cruise phase. The KHI is found in a shear layer in the slow solar wind in the close vicinity of the Heliospheric Current Sheet, with properties satisfying linear theory for its development. An analysis is performed to derive the local configuration of the KHI. A 2-D MHD simulation is also set up with empirical values to test the stability of the shear layer. In addition, magnetic spectra of the KHI event are analyzed. We find that the observed conditions satisfy the KHI onset criterion from the linear theory analysis, and its development is further confirmed by the simulation. The current sheet geometry analyses are found to be consistent with KHI development. Additionally, we report observations of an ion jet consistent with magnetic reconnection at a compressed current sheet within the KHI interval. The KHI is found to excite magnetic and velocity fluctuations with power-law scalings that approximately follow $k^{-5/3}$ and $k^{-2.8}$ in the inertial and dissipation ranges, respectively. These observations provide robust evidence of KHI development in the solar wind. This sheds new light on the process of shear-driven turbulence as mediated by the KHI with implications for the driving of solar wind fluctuations.

References in corpus (9)

• The Solar Orbiter mission -- Science overview
• Advancements in the ADAPT Photospheric Flux Transport Model
• Shear-Driven Transition to Isotropically Turbulent Solar Wind Outside the Alfven Critical Zone
• Spectral Slope Variation at Proton Scales from Fast to Slow Solar Wind
• Fading Coronal Structure and the Onset of Turbulence in the Young Solar Wind
• The Kelvin-Helmholtz Instability at CME-Boundaries in the Solar Corona: Observations and 2.5D MHD Simulations
• Solar wind magnetic field background spectrum from fluid to kinetic scales
• Solar wind rotation rate and shear at coronal hole boundaries, possible consequences for magnetic field inversions
• Kinetic features for the identification of Kelvin-Helmholtz vortices in \textit{in situ} observations

Cited by in corpus (5)

• Reconnection-Driven Energy Cascade in Magnetohydrodynamic Turbulence
• Helios 2 observations of solar wind turbulence decay in the inner heliosphere
• Magnetic reconnection as an erosion mechanism for magnetic switchbacks
• Observation of Large-Scale Kelvin-Helmholtz Instability Wave Driven by a Coronal Mass Ejection
• Two-Dimensional Kelvin-Helmholtz Instability with Anisotropic Pressure