  We report on a new contribution to the microwave response of a
two-dimensional electron system in magnetic field which originates from
excitation of virtual Bernstein modes. These collective modes emerge as a
result of interaction between usual magnetoplasmon mode and cyclotron resonance
harmonics. The electrons are found to experience a strongly enhanced radiation
field when its frequency falls in a gap of the Bernstein modes spectrum. This
field can give rise to nonlinear effects, one of which, the parametric
cyclotron resonance, is discussed. We argue that this resonance leads to a
plasma instability in the ultraclean system. The instability-induced heating is
responsible for the giant photoresistivity spike recently observed in the
vicinity of the 2nd cyclotron resonance harmonic.
