The evolution of CME sheath turbulence from L1 to Earth: Wind and MMS observations of the 2023-04-23 CME

An interplanetary shock driven by a coronal mass ejection (CME) containing an interval of sub-Alfvénic flow impacted Earth on April 23, 2024. In this article, we analyze the turbulence in the sheath region between the shock and CME to determine how it evolves from L1 (as observed by Wind) to Earth (as observed by MMS, upstream of the bow shock). Wind and MMS were separated by $55\,\mathrm{R_{E}}$ in the dawn-dusk direction, but the shock normals differ by only $2.8^{\circ}$ and the Pearson correlation coefficient between time-shifted magnetic field components is $ρ=0.93$. We observe a shift in the break point of the magnetic power spectral density between inertial and ion kinetic scales toward the ion inertial length and a steepening of the spectral slope, indicating more active energy cascade closer to Earth. The distribution of increments becomes more non-Gaussian near Earth, particularly at ion kinetic scales, indicating the turbulence becomes more intermittent. Finally, the correlation length at Earth is 25\% longer than at L1, indicating that the turbulence is smoothing out the magnetic field. The results present an example of substantial evolution of CME sheath turbulence from L1 to Earth.