A magnetosonic wave with wavevector k ⊥ B _bg at two oscillation phases: when the wave field B _w = B − B _bg is aligned with B _bg (left) and antialigned with B _bg (right). The wave electric field E _w = E is always orthogonal to B _bg. The quantity B ² − E ² reaches zero when B _w = − B _bg/2. At this moment, the plasma drift speed v = c E × B /B ² approaches c, so it experiences ultrarelativistic acceleration in the direction opposite to the wave propagation direction.