  In the solar wind, magnetic field power spectra usually show several
power-laws. In this paper, magnetic field data from the Cluster mission during
an undisturbed interval of slow solar wind is analyzed at 0.28Hz, near the
spectral break point between the ion inertial and dissipation/dispersion
ranges. Assuming Taylor's frozen-in condition, it corresponds to a proton
kinetic scale of $kv_A/\Omega_p \sim 0.38$, where $v_A$ and $\Omega_p$ are the
Alfv\'en speed and proton angular gyrofrequency, respectively. Data show that
the Cluster spacecraft passed through a series of wavepackets. A strong
isolated wavepacket is found to be in accordance with the four Cluster
satellites crossing an Alfv\'en vortex, a nonlinear solution to the
incompressible MHD equations. A strong agreement is seen between the data from
four satellites and a model vortex with a radius of the order of $40$ times the
local proton gyro-radii. The polarization at different spacecraft is compared
and is found to agree with the vortex model, whereas it cannot be explained
solely by the linear plane wave approach.
