The Role of Proton-Cyclotron Resonance as a Dissipation Mechanism in Solar Wind Turbulence: A Statistical Study at Ion-Kinetic Scales

TL;DR

This study investigates how proton-cyclotron resonance contributes to energy dissipation in solar wind turbulence at ion-kinetic scales, using spacecraft data to analyze spectral features and magnetic helicity signatures.

Contribution

It provides the first in-flight measurement of the MFI noise-floor and links spectral breaks to proton-cyclotron resonance scales, highlighting its role in solar wind turbulence dissipation.

Findings

Spectral break correlates with proton-cyclotron resonance scale, especially at β_{i,⊥}≈1.

Magnetic helicity signatures are bounded by the proton-cyclotron scale and peak at the proton gyroscale.

Proton-cyclotron resonance is a significant dissipation mechanism in solar wind turbulence, occurring at least half the time.

Abstract

We use magnetic field and ion moment data from the MFI and SWE instruments onboard the Wind spacecraft to study the nature of solar wind turbulence at ion-kinetic scales. We analyze the spectral properties of magnetic field fluctuations between 0.1 and 5.5 Hz over 2012 using an automated routine, computing high-resolution 92 s power and magnetic helicity spectra. To ensure the spectral features are physical, we make the first in-flight measurement of the MFI `noise-floor' using tail-lobe crossings of the Earth's magnetosphere during early 2004. We utilize Taylor's hypothesis to Doppler-shift into the spacecraft frequency frame, finding that the spectral break observed at these frequencies is best associated with the proton-cyclotron resonance scale, $1/k_c$, compared to the proton inertial length $d_i$ and proton gyroscale $\rho_i$. This agreement is strongest when we consider periods where $\beta_{i,\perp}\sim1$, and is consistent with a spectral break at $d_i$ for $\beta_{i,\perp}\ll1$ and $\rho_i$ for $\beta_{i,\perp}\gg1$. We also find that the coherent magnetic helicity signature observed at these frequencies is bounded at low frequencies by $1/k_c$ and its absolute value reaches a maximum at $\rho_i$. These results hold in both slow and fast wind streams, but with a better correlation in the more Alfv\'enic fast wind where the helicity signature is strongest. We conclude that these findings are consistent with proton-cyclotron resonance as an important mechanism for dissipation of turbulent energy in the solar wind, occurring at least half the time in our selected interval. However, we do not rule out additional mechanisms.