  Parker Solar Probe measurements have recently shown that coherent fast
magnetosonic and Alfv\'{e}n ion-cyclotron waves are abundant in the solar wind
and can be accompanied by higher-frequency electrostatic fluctuations. In this
letter we reveal the nonlinear process capable of channelling the energy of
low-frequency electromagnetic to higher-frequency electrostatic fluctuations
observed aboard Parker Solar Probe. We present Hall-MHD simulations
demonstrating that low-frequency electromagnetic fluctuations can resonate with
the ion-sound mode, which results in steepening of plasma density fluctuations,
electrostatic spikes and harmonics in the electric field spectrum. The
resonance can occur around the wavenumber determined by the ratio between local
sound and Alfv\'{e}n speeds, but only in the case of {\it oblique} propagation
to the background magnetic field. The resonance wavenumber, its width and
steepening time scale are estimated, and all indicate that the revealed
two-wave resonance can frequently occur in the solar wind. This process can be
a potential channel of energy transfer from cyclotron resonant ions producing
the electromagnetic fluctuations to Landau resonant ions and electrons
absorbing the energy of the higher-frequency electrostatic fluctuations.
