"Evolution of Turbulence at an Interplanetary Shock Observed by the Solar Orbiter and Wind Spacecraft"

Presented by Sujan Gautman, Graduate Research Assistant, from the University of Alabama, Huntsville

This work presents a case study of the evolution of turbulence across an interplanetary shock observed by Solar Orbiter at 0.83 au and Wind at 1 au on 30 November 2023. Turbulence properties, including total turbulent energy, correlation length, cascade rate, normalized cross helicity, residual energy, magnetic helicity, magnetic compressibility, and spectral characteristics, are estimated upstream and downstream of the shock at both locations using in situ magnetic field and plasma measurements. These quantities exhibit substantial changes across the shock and evolve with heliocentric distance, reflecting both shock-driven amplification and radial transport effects. The Linear Mode Decomposition technique is applied to separate the fluctuations into individual magnetohydrodynamic wave modes and to investigate the modal composition of the turbulence. Specifically, six propagating wave modes (forward and backward fast, slow, and Alfvén modes) and two non-propagating advected structures (entropy and magnetic island modes) are identified. All decomposed modes show significant downstream enhancement, although the amplification rates vary among components, and the overall fluctuation level decreases from Solar Orbiter to Wind. The cascade rates contributed by different propagating wave modes are also estimated. The turbulent cascade rate is dominated by fast-mode fluctuations upstream, while Alfvénic contributions become more important downstream. The turbulence also exhibits enhanced two-dimensional (2D) dominance in the downstream region of both spacecraft. These results provide new observational insight into the radial evolution of shock-driven turbulence and demonstrate that interplanetary shocks not only amplify turbulence but also systematically reorganize its modal composition, anisotropy, and cascade properties as they propagate through the heliosphere.