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

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 ρ i . This agreement is strongest when we consider periods where β i,⊥ ∼ 1, and is consistent with a spectral break at d i for β i,⊥ ≪ 1 and ρ i for β i,⊥ ≫ 1. 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 ρ i . These results hold in both slow and fast wind streams, but with a better correlation in the more Alfvénic 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.

Three-dimensional density and compressible magnetic structure in solar wind turbulence

“…Gary (1986). The results here also complement the studies of the magnetic helicity (He et al, 2011;Podesta and Gary, 2011;Klein et al, 2014b;Bruno and Telloni, 2015;Woodham et al, 2018), which have shown a signature consistent with ICWs for intervals of fast solar wind. Bruno and Telloni (2015) also demonstrated the reduction and disappearance of this component as the velocity transitioned from fast to slow.…”

“…Although the plasma β values are much less extreme than the cases reported in Chen et al (2014), we demonstrate that such a study is currently possible in the wave-vector domain with multi-point measurements. However, the break in the parallel component is closer to the combined inertial and gyro scales suggesting that cyclotron damping may be important (Roberts and Li, 2015;Woodham et al, 2018).…”

The Radial Dependence of Proton-scale Magnetic Spectral Break in Slow Solar Wind during PSP Encounter 2

“…The ratio f b /f c approaches unity near the Sun, which may be due to the increased activity of the solar wind plasma close to the Sun generating ion cyclotron waves (Bowen 2020, submitted, this volume). Regarding that f b /f c deviates slightly from unity (less than unity) in the slow solar wind at 1 AU (Woodham et al 2018) and that f b /f c increases slightly with decreasing heliocentric distance in the slow solar wind, it seems to be a natural result for f b /f c to approach unity near the Sun.…”

Ion-scale Electromagnetic Waves in the Inner Heliosphere

“…where Ω p = qB 0 /m p the ± relates to the direction of the wave propagation relative to the particles motion (Woodham et al 2018).…”

“…In the solar wind, observations of non-zero magnetic helicity at ion scales have been interpreted as evidence for active cyclotron damping of turbulent fluctuations which contribute to the dissipation and heating at kinetic scales (Isenberg 1990;Leamon et al 1998;Woodham et al 2018). In addition to proton-cyclotron dynamics, the cyclotron resonance of doubly ionized helium (α-particles) and heavy ions has been studied extensively (Isenberg, & Hollweg 1983;Isenberg 1984).…”