  Using data from the Chandra X-Ray Observatory, we revisited the reverse shock
in the supernova remnant (SNR) Cassiopeia A.Based on the spectroscopic of a
series of annuli in the northwest (NW) and southeast (SE), we get the radial
profiles of the S/Si K-alpha line flux ratio and Fe K-alpha line centroid
energy. They both show monotonic increase, confirming that the Si- and Fe-rich
ejecta are heated by the reverse shock.The abrupt change of the S and Si line
flux ratio is clearly observed in Cassiopeia A, leading to the determination of
the reverse shock location (~1.71+-0.16 arcmin and ~1.35+-0.18 arcmin in the NW
and SE, with respect to the central source). By comparing the radial profiles
of S and Si line flux, we find that the reverse shock is moving outward in the
frame of the observer, and the velocities are ~3950+-210 km/s and ~2900+-260
km/s in the NW and SE, respectively. In contrast, the velocities become ~1150
km/s (NW) and ~1300 km/s (SE) in the ejecta frame. Our measured reverse shock
velocities are quite consistent with those obtained from the X-ray and/or
optical images. It therefore supplies a crosscheck of the accuracy for the two
available methods to measure the reverse shock velocity in SNRs. Both the
location and the velocity of the reverse shock show apparent asymmetry,
suggesting that the asymmetric explosion of the progenitor plays a key role in
the interaction between the reverse shock and the ejecta, ultimately shaping
complex features observed in SNRs.
