First Observation of Kelvin–Helmholtz Instability in Stream Interaction Regions using Solar Orbiter

Velocity shear in plasmas generates Kelvin–Helmholtz instability (KHI), enabling the transfer of mass, momentum, and energy across the interfaces. While KH waves are well-documented at planetary magnetopauses, their development within large-scale heliospheric flows remains observationally elusive. Here, we report first observations of in-situ signatures of KH waves within a stream interaction region at 0.35 au, observed by Solar Orbiter. The KHI interval exhibits quasi-periodic variations in magnetic field, plasma velocity, and density, with an average period of ∼18 min, alongside boundary normal vectors indicative of an evolving shear layer. Complete linear MHD analysis confirms the onset of the instability, enabled by the strongly oblique magnetic field geometry that reduces the stabilizing effect of magnetic tension. Inferred wavelength and e-folding time indicate an early, weakly nonlinear stage of development. This provides direct evidence that KH waves can be generated in the inner heliosphere, which is a pathway for energy transfer in astrophysical plasmas.