We investigate the nature of stationary structures streaming at subfast
magnetosonic speeds perpendicular to the magnetic field in a bi-ion plasma consisting
of protons and a heavy ion species in which the magnetic field is frozen into the
electrons, whose inertia may be neglected. The study is based on the properties of
the structure equation for the system, which is derived from the equations of motion
and the Maxwell equations, and therefore reflects the coupling between the two ion
fluids and the electrons through the Lorentz forces and charge neutrality. The basic
features of the structure equation are elucidated by making use of conservation of
total momentum and charge neutrality, which provide relations between the ion
speeds in the unperturbed flow direction and the electron speed. This combination
of relations, which we call the momentum hodograph of the system, reveals the
structure of the flow and the magnetic field in a solitary-type pulse. In particular,
we find that in the initial portion of a compressive soliton, heavy ions run ahead
of the electrons and the protons lag between them until a point is reached where
they all once more attain the same speed, after which the protons run ahead and
are accelerated whereas the heavies now lag behind the continuously decelerating
electrons. The second half of the wave is a mirror image of the first portion. The
strength of the compression (the amplitude of the wave) is determined from the
momentum hodograph, and depends upon the initial Mach number, abundance ratio
of heavies to protons and the mass ratio. The analysis is relevant to subfast
flows of mass-loaded plasmas and pile-up boundaries, which appear near comets
and non-magnetic planets.