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

TL;DR

This study analyzes solar wind turbulence near Mercury, revealing stable inertial-range properties but radial evolution at kinetic scales, with implications for plasma processes in the inner heliosphere.

Contribution

It provides the first comprehensive statistical analysis of turbulence evolution from 0.31 to 0.47 au using MESSENGER data, highlighting scale-dependent radial changes.

Findings

Inertial-range spectral slopes remain near -3/2 with no significant radial change.

Kinetic-range spectral slopes become shallower with increasing distance.

The ion-scale spectral break frequency varies with distance and plasma conditions.

Abstract

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 MESSENGER mission. Owing to Mercury's highly elliptical orbit and the spacecraft's repeated, extended residence in the upstream solar wind, the data set provides more than 17,000 hours 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 Alfvenic 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 a single ion scale 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.