  Spicule oscillations involve high-frequency components with a typical period
approximately corresponding to $40-50$ s. The typical time scale of the
photospheric oscillation is a few minutes, and thus, the origin of this
high-frequency component is not trivial. In this study, a one-dimensional
numerical simulation is performed to demonstrate that the observed spicule
oscillations originate from longitudinal-to-transverse mode conversion that
occurs around the equipartition layer in the chromosphere. Calculations are
conducted in a self-consistent manner with the exception of additional heating
to maintain coronal temperature. The analyses indicate the following features:
(1) mode conversion efficiently excites high-frequency transverse waves; (2)
the typical period of the high-frequency waves corresponds to the
sound-crossing time of the mode conversion region; and (3) simulated
root-mean-square velocity of the high-frequency component is consistent with
the observed value. These results indicate that the observation of spicule
oscillation provides direct evidence of mode conversion in the chromosphere.
