Nonlinear twofluid hydromagnetic waves in the solar wind: Rotational discontinuity, soliton, and finiteextent Alfvén wave train solutions
Abstract
Nonlinear onedimensional constantprofile hydromagnetic wave solutions are obtained in finitetemperature twofluid collisionless plasmas under adiabatic equation of state. The nonlinear wave solutions can be classified according to the wavelength. The longwavelength solutions are circularly polarized incompressible oblique Alfvén 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 shortwavelength solutions include rarefaction fast solitons, compression slow solitons, Alfvén solitons and rotational discontinuities, with wavelength of several tens of ion inertial length, provided that the upstream flow speed is less than the fastmode speed.
The Alfvén solitons and rotational discontinuities are superAlfvénic compression waves if the upstream Alfvénmode speed is greater than the sound speed; otherwise, they are subAlfvénic rarefaction waves. The density and magnetic field variations of these shortwavelength waves are shown to obey the following two rules: (1) all compression waves are lefthad polarized and all rarefaction waves are righthand 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 twofluid 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 twofluid rotational discontinuity must deviate slightly from the Alfvénmode speed; the downstream flow speed is equal to the local sound speed.
The formation of the twofluid rotational discontinuity depends critically on the dispersion effect which converts the Alfvén mode to the ion acoustic mode.
 Publication:

Journal of Geophysical Research
 Pub Date:
 June 1989
 DOI:
 10.1029/JA094iA06p06523
 Bibcode:
 1989JGR....94.6523L
 Keywords:

 Magnetohydrodynamic Waves;
 Nonlinear Equations;
 Solar Wind;
 Solitary Waves;
 Collisionless Plasmas;
 Compression Waves;
 Polarization (Waves);
 Power Spectra;
 Solar Magnetic Field;
 Two Fluid Models;
 Solar Physics;
 Space Plasma Physics: Nonlinear phenomena;
 Interplanetary Physics: Discontinuities;
 Interplanetary Physics: Interplanetary magnetic fields;
 Space Plasma Physics: Discontinuities