Properties and Radial Evolution of Solar Wind Turbulence Near Mercury’s Orbit

We present a comprehensive statistical study of the radial evolution of solar wind turbulence near Mercury’s orbit using long-term magnetic field measurements from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission. Owing to Mercury’s highly elliptical orbit and the spacecraft’s repeated, extended residence in the upstream solar wind, the dataset provides more than 17,000 hr of observations, enabling robust statistics across well-defined heliocentric distance intervals (0.31–0.47 au). We find that inertial-range spectral slopes remain close to −3/2 throughout Mercury’s orbit, showing no significant radial evolution. Combined with low magnetic compressibility, this result indicates a stable, predominantly Alfvénic inertial-range cascade already established here. In contrast, kinetic-range spectral slopes exhibit clear radial evolution, becoming progressively shallower with increasing heliocentric distance, highlighting the greater sensitivity of kinetic-scale turbulence to heliocentric conditions. The ion-scale spectral break frequency decreases with distance in the spacecraft frame, while its normalized form increases relative to the local proton cyclotron frequency, demonstrating that the break is not tied to proton cyclotron frequency but reflects evolving local plasma conditions. Magnetic compressibility shows a similar frequency dependence at all distances, with a subtle radial enhancement of compressive fluctuations at kinetic scales. Autocorrelation analysis reveals strong anisotropy, with the correlation times of field-aligned magnetic fluctuations increasing with heliocentric distance, while those of perpendicular fluctuations remain shorter and nearly invariant. Together, these results demonstrate a clear scale-dependent radial evolution of solar wind turbulence near Mercury’s orbit, providing new constraints on the development of kinetic processes in the inner heliosphere.