Formation of arc-shaped Alfven waves and rotational discontinuities from oblique linearly polarized wave trains

The forms of Alfvenic 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 degrees. 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 (much less than ion cyclotron) Alfven wave trains. Starting from a linearly polarized wave train, an approximate are polarization evolves rapidly where the magnetic field moves to and fro on a less than semicircular are. Large-amplitude (\delta B\/B similar to 1) wave trains steepen and produce RDs which always rotate the field by < 180 degrees with no preference for ion or electron sense of rotation. These properties correspond to those of Alfvenic fluctuations in the solar wind, and our model is the first which offers an explanation of the observed are-shaped waves and imbedded RDs. At early times, a large density signal is also generated. For large plasma beta, the signal rapidly damps, and the waveform varies little with time. For small plasma beta, the generated constant-B Alfven 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.