  We investigate the conversion of kinetic mixing dark photon dark matter into
gravitational waves within the magnetic field of the Sun. Our study reveals
that the WKB approximation is invalid in this scenario. We derive an analytic
solution for the conversion probability with unitary evolution feature. This
solution aligns in form with previous studies on photon-gravitational wave
conversion. Interestingly, it is applicable in situations where the WKB
approximation fails. We extend the unitary evolution solution to other
conversion processes, such as axion-photon and dark photon-photon conversions.
When the WKB approximation conditions are met, this solution reduces to the WKB
result. We compute the characteristic strain of gravitational waves resulting
from dark photon conversion in the solar magnetic field, spanning frequencies
from $10^{-5}$ Hz to $10^3$ Hz. Our findings indicate that the characteristic
strain derived from the unitary evolution solution differs significantly from
that of the WKB solution. The resulting strain signal is far below the
sensitivity of current gravitational wave interferometers. Nevertheless, we
have proposed an exotic gravitational wave source, especially at high
frequencies, from the dark photon dark matter local conversion inside the Sun.
