Large-amplitude Alfvén wave in interplanetary space: The wind spacecraft observations

We present, for the first time, measurements of arc-polarized velocity variations together with magnetic field variations associated with a large-amplitude Alfvén wave as observed by the Wind satellite. The module of the magnetic field variance is larger than the magnitude of the average magnetic field, indicating the large amplitude of these fluctuations. When converting to the deHoffman-Teller frame, we find that the magnetic field and velocity vector components, in the plane perpendicular to the minimum-variance direction of the magnetic field, are arc-polarized, and their tips almost lie on the same circle. We also find that the normalized cross helicity and Alfvén ratio of the wave are both nearly equal to unity, a result which has not been reported in previous studies at 1AU. It is worthy to stress here that pure Alfvén waves can also exist in the solar wind even near the Earth at 1AU, but not only near 0.3AU. Further study could be done to help us know more about the properties of pure Alfvén wave at 1AU that could not be figured out easily before because of the contaminations (e.g., Alfvén waves propagating in different directions, magnetic structures, and other compressional waves) on previously reported Alfvén wave cases.