The forms of Alfv¿nic fluctuations in the solar wind sometimes possess nearly constant magnetic intensities but have an approximate arc rather than circular polarization. They are also associated with layers of abrupt field rotation called rotational discontinuities (RDs) where the field changes direction by <180¿. Ion-sense and electron-sense rotations are observed in approximately equal numbers. To explore the origin of this form, we conduct a one-and-one-half-dimensional hybrid numerical simulation study of the evolution of obliquely propagating, low-frequency (≪ion cyclotron) Alfv¿n wave trains. Starting from a linearly polarized wave train, an approximate arc polarization evolves rapidly where the magnetic field moves to and fro on a less than semicircular arc. Large-amplitude (|ΔB|/B~1) wave trains steepen and produce RDs which always rotate the field by <180¿ with no preference for ion or electron sense of rotation. These properties correspond to those of Alfv¿nic fluctuations in the solar wind, and our model is the first which offers an explanation of the observed arc-shaped waves and imbedded RDs. At early times, a large density signal is also generated. For large plasma &bgr;, the signal rapidly damps, and the waveform varies little with time. For small plasma &bgr;, the generated constant-B Alfv¿n wave is parametrically unstable and causes the density signal to grow further before the instability saturates. The wave train and density signal beat strongly giving a periodic time variation of the wave amplitude and waveform. Ion heating from steepening, RD formation, relaxation to constant B, and parametric processes occurs mainly parallel to the background magnetic field and cannot explain the perpendicular heating of ions observed in the solar wind. ¿ American Geophysical Union 1996