The nonlinear mechanism for kinetic-Alfvén-wave (KAW) excitation
by upper-hybrid
waves (UHWs) is discussed. Taking into account perpendicular dispersion
of KAWs, caused by effects of finite ion Larmor radius and electron inertia,
we
examine a new channel for UHW decay, in which a pump UHW decays into another
UHW and an ultralow-frequency wave, KAW: UHW→KAW+UHW. A nonlinear
dispersion relation is derived, and the growth rate of the parametric decay
instability
is calculated for a pump UHW propagating at an arbitrary angle to the background
magnetic field. We find that the resulting KAWs often have a two-peaked
spectrum with different perpendicular dispersions. Using satellite observations,
the
analytical results are applied to show that the considered process represents
an
effective mechanism for KAW generation and the consequent spreading of
the UHW
spectrum in the Earth's magnetosphere and solar corona.