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Magnetic field rotation and transition width in rotational discontinuities and Alfven wave trains


A hybrid simulation is carried out to study the evolution of magnetic field rotations in rotational discontinuities (RDs) and large-amplitude Alfven wave trains (AWTs) whose initial field rotation angle \Delta Phi\ > 180 degrees. For a given initial half width D, the RDs or AWTs lose a multiple of +/-360 degrees field rotation at a critical time, until a smallest rotation angle \Delta Phi\ < 180 degrees is reached. The critical time t(c) for losing the first 360 degrees field rotation is found to be t(c) similar or equal to F(Delta Phi, beta(1), theta(Bn))D3.3+/-0.8, where the function F decreases linearly with Delta Phi, decreases linearly with root beta(1) for upstream ion beta beta(1) > 0.1, and decreases with decreasing shock normal angle theta(Bn). At t < t(c), however, the rotation angle Delta Phi remains unchanged. A search is conducted for minimum widths required for RDs to preserve their \Delta Phi\ > 180 degrees for a given transient convection time at the dayside magnetopause and minimum widths of AWTs per 360 degrees field rotation in the solar wind.