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We use magnetic field and ion moment data from the MFI and SWE instrumentsonboard the Wind spacecraft to study the nature of solar wind turbulence ation-kinetic scales. We analyze the spectral properties of magnetic fieldfluctuations between 0.1 and 5.5 Hz over 2012 using an automated routine,computing high-resolution 92 s power and magnetic helicity spectra. To ensurethe spectral features are physical, we make the first in-flight measurement ofthe MFI `noise-floor' using tail-lobe crossings of the Earth's magnetosphereduring early 2004. We utilize Taylor's hypothesis to Doppler-shift into thespacecraft frequency frame, finding that the spectral break observed at thesefrequencies 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 thatthe coherent magnetic helicity signature observed at these frequencies isbounded at low frequencies by $1/k_c$ and its absolute value reaches a maximumat $\rho_i$. These results hold in both slow and fast wind streams, but with abetter correlation in the more Alfv\'enic fast wind where the helicitysignature is strongest. We conclude that these findings are consistent withproton-cyclotron resonance as an important mechanism for dissipation ofturbulent energy in the solar wind, occurring at least half the time in ourselected interval. However, we do not rule out additional mechanisms.

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