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 0.28 Hz , 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 Alfvén 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 Alfvén 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 can not be explained solely by the linear plane wave approach .