  The magneto-optical imaging technique is used to visualize the penetration of
the magnetic induction in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin films during
surface resistance measurements. The in-situ surface resistance measurements
were performed at 7 GHz using the dielectric resonator method. When only the
microwave magnetic field $H_{rf}$ is applied to the superconductor, no
$H_{rf}$-induced vortex penetration is observed, even at high rf power. In
contrast, in the presence of a constant magnetic field superimposed on $H_{rf}$
we observe a progression of the flux front as $H_{rf}$ is increased. A local
thermometry method based on the measurement of the resonant frequency of the
dielectric resonator placed on the YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin film
shows that the $H_{rf}$--induced flux penetration is due to the increase of the
film temperature.
