Abstract
Abstract We use magnetic field and ion moment data from the MFI and SWE instruments on board 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 5.4 Hz during 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 , rather than the proton inertial length, d i , or proton gyroscale, ρ i . This agreement is strongest when we consider periods where , and is consistent with a spectral break at d i for and at ρ i for . 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.
Highlights
The solar wind supports a turbulent energy cascade where the spectrum of magnetic field fluctuations follows a Kolmogorov inertial range scaling of k−5/3 extending over several decades (Goldstein et al 1995; Tu & Marsch 1995; Bruno & Carbone 2013)
We present a rigorous analysis of solar wind turbulence at ion-kinetic frequencies using a combined identification of the frequency of the spectral break and onset of the magnetic helicity signature
Our results indicate that the transition range often observed in fast wind streams, which follows a break in the power spectrum of the magnetic field fluctuations at ion-kinetic scales, may result from proton cyclotron resonance
Summary
The solar wind supports a turbulent energy cascade where the spectrum of magnetic field fluctuations follows a Kolmogorov inertial range scaling of k−5/3 extending over several decades (Goldstein et al 1995; Tu & Marsch 1995; Bruno & Carbone 2013). We can convert the wavenumber, k, of the turbulent fluctuations along the sampling direction of the solar wind flow to a frequency, f, using Taylor’s hypothesis (Taylor 1938): f ∼ kvsw/2π, where vsw is the solar wind speed. At frequencies in the plasma frame of the order of the ion gyrofrequency, Ωi = qiB0/mi, typically measured around 0.1–1 Hz in the spacecraft frame at 1 au, the spectrum steepens (e.g., Coleman 1968; Russell 1972). In situ data from spacecraft have revealed a bimodal distribution in solar wind speed with two distinct peaks, leading to the designation of two types of wind: slow (∼350 km s−1) and fast (∼600 km s−1), attributed to different source regions in the solar corona The observed spectral break in the power spectra at these so-called ion-kinetic frequencies has been attributed to the onset of kinetic effects such as dispersion or turbulent dissipation (see Alexandrova et al 2013; Kiyani et al 2015; Chen 2016, and references therein), the actual physical mechanisms behind the steepening remain poorly understood.
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