  The dc magnetoconductivity of the multisubband two-dimensional electron
system formed on the liquid helium surface in the presence of resonant
microwave irradiation is described, and a new mechanism of the negative linear
response conductivity is studied using the self-consistent Born approximation.
Two kinds of scatterers (vapor atoms and capillary wave quanta) are considered.
Besides a conductivity modulation expected near the points, where the
excitation frequency for inter-subband transitions is commensurate with the
cyclotron frequency, a sign-changing correction to the linear conductivity is
shown to appear for usual quasi-elastic inter-subband scattering, if the
collision broadening of Landau levels is much smaller than thermal energy. The
decay heating of the electron system near the commensurability points leads to
magnetooscillations of electron temperature, which are shown to increase the
importance of the sign-changing correction. The line shape of
magnetoconductivity oscillations calculated for wide ranges of temperature and
magnetic field is in a good accordance with experimental observations.
