Solitary waves in a Hall solar wind plasma

In the solar wind, the effect of the Hall current generated perpendicular to the ambient magnetic field can influence the plasma behaviour. In particular, the Hall current introduces wave dispersion which may compensate the nonlinear steepening of waves. In the presence of viscosity, these effects lead to a slowly decaying KdV soliton. Here we investigate a solitary wave propagating obliquely to an ambient magnetic field. Our results show that the nonlinear slow magnetoacoustic wave, that arises from nonlinear steepening, propagates faster than the linear slow mode speed; by contrast, the nonlinear fast magnetoacoustic wave propagates slower than the linear fast wave speed. We apply our calculations to the observed non-thermal Doppler broadening of the ion emission line, measured by the SoHO UVCS telescope in the acceleration region of the solar wind. The observed line broadening is recovered in our theory by the nonlinear slow magnetoacoustic waves; by contrast, fast magnetoacoustic produce too narrow broadening.