Nonlinear one-dimensional constant-profile hydromagnetic wave solutions are obtained in finite-temperature two-fluid collisionless plasmas under adiabatic equation of state. The nonlinear wave solutions can be classified according to the wavelength. The long-wavelength solutions are circularly polarized incompressible oblique Alfv¿n wave trains with wavelength greater than hudreds of ion inertial length. The oblique wave train solutions can explain the high degree of alignment between the local average magnetic field and the wave normal direction observed in the solar wind. The short-wavelength solutions include rarefaction fast solitons, compression slow solitons, Alfv¿n solitons and rotational discontinuities, with wavelength of several tens of ion inertial length, provided that the upstream flow speed is less than the fast-mode speed.

The Alfv¿n solitons and rotational discontinuities are super-Alfv¿nic compression waves if the upstream Alfv¿n-mode speed is greater than the sound speed; otherwise, they are sub-Alfv¿nic rarefaction waves. The density and magnetic field variations of these short-wavelength waves are shown to obey the following two rules: (1) all compression waves are left-had polarized and all rarefaction waves are right-hand polarized, due to the ion inertial effect, (2) the density variation and the magnetic field magnitude variation are in phase if the flow is supersonic, but out of phase if the flow is subsonic, which is a consequence of conservation of the momentum flux. The two-fluid rotational discontinuity solution obtained in this study is highly circularly polarized, with a variable angular rotation rate. The total angle of rotation is limited to less than or equal to 180¿, which is consistent with the rotational discontinuity observed in the solar wind. The upstream flow speed of the two-fluid rotational discontinuity must deviate slightly from the Alfv¿n-mode speed; the downstream flow speed is equal to the local sound speed.

The formation of the two-fluid rotational discontinuity depends critically on the dispersion effect which converts the Alfv¿n mode to the ion acoustic mode. ¿ American Geophysical Union 1989