Formation of pressure-balanced structures and fast waves from nonlinear Alfven waves

In the solar wind, Alfvenic fluctuations are typically observed in associat ion with small fluctuations of the density (rho) and magnetic field strengt h (B), which tend to be anticorrelated and in approximate pressure balance. One would not expect any finite delta rho and delta B among pure Alfven wa ves propagating strictly outward from the Sun. Our paper shows how Alfven w aves can nonlinearly produce structures in pressure balance. We present a s econd-order analysis of the pure magnetohydrodynamic equations and hybrid s imulations which show that nonlinear Alfven waves traveling in different di rections but with equal group velocity can generate pressure-balanced struc tures with wave vectors perpendicular to the background magnetic field B-o. Homogeneous fast waves are also generated in this direction in order to sa tisfy initial conditions. They cannot be Landau or transit-time damped and so cause the values of B and rho to vary with time as they beat with the pr essure-balanced structures. However, we find delta rho delta B < 0 is satis fied most of the time, and this can partly explain the tendency for anticor relation observed in the solar wind. In directions away from the perpendicu lar one, Alfven waves produce driven fast waves which give constant B and r ho to second order. Homogeneous fast and slow waves are also produced in th ese directions but Landau damp away in large beta plasmas. Thus an equilibr ium or steady propagating waveform at second order can be produced where B and rho vary only in the perpendicular direction. If transverse magnetic st ructures with wave vectors perpendicular to B-o are included at the same or der as the initial Alfven waves, then these evolve to pressure-balanced str uctures and can. also coexist with the Alfven waves. However, an equilibriu m is obtained generally only when these structures also have velocity fluct uations equivalent of those of the Alfven waves.