  In this paper we investigate spectral and phase coherence properties of
magnetic fluctuations in the vicinity of the spectral transition from large,
magnetohydrodynamic (MHD) to sub-ion scales using in-situ measurements of the
Wind spacecraft in a fast stream. For the time interval investigated by Leamon
et al. (1998) the phase-coherence analysis shows the presence of sporadic
quasi-parallel Alfv\'en Ion Cyclotron (AIC) waves as well as coherent
structures in the form of large-amplitude, quasi-perpendicular Alfv\'en
vortex-like structures and current sheets. These waves and structures
importantly contribute to the observed power spectrum of magnetic fluctuations
around ion scales; AIC waves contribute to the spectrum in a narrow frequency
range whereas the coherent structures contribute to the spectrum over a wide
frequency band from the inertial range to the sub-ion frequency range. We
conclude that a particular combination of waves and coherent structures
determines the spectral shape of the magnetic field spectrum around ion scales.
This phenomenon provides a possible explanation for a high variability of the
magnetic power spectra around ion scales observed in the solar wind.
