  In this work we study the ac dissipation of mesoscopic superconductors at
microwave frequencies using the time dependent Ginzburg-Landau equations. Our
numerical simulations show that the ac dissipation is strongly dependent on the
orientation of the ac magnetic field ($h_{ac}$) relative to the dc magnetic
field ($H_{dc}$). When $h_{ac}$ is parallel to $H_{dc}$ we observe that each
vortex penetration event produces a significant supression of the ac losses
because the imaginary part of the ac susceptibility as a function of $H_{dc}$
increases before the penetration of vortices, and then it decreases abruptly
after vortices have entered into the sample. In the second case, when $h_{ac}$
is perpendicular to $H_{dc}$, we observe that the jumps in dissipation occur at
the same values of $H_{dc}$ but are much smaller than in the parallel
configuration. The behavior of the dissipation in the perpendicular
configuration is similar to previous results obtained in recent microwave
experiments using mesoscopic litographed squares of Pb [J. Low Temp. Phys. 135,
119 (2004)].
