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Planetary nebulae (PNe) are expected to have a ~10$^{5}$ K interface layerbetween the $\geq$10$^{6}$ K inner hot bubble and the ~10$^{4}$ K opticalnebular shell. The PN structure and evolution, and the X-ray emission dependcritically on the efficiency of mixing of material at this interface layer.However, neither its location nor its spatial extent has ever been determinedso far. Using high-spatial resolution HST STIS spectroscopic observations ofthe N V 1239,1243 lines in the Cat's Eye Nebula (NGC 6543), we have detectedthis interface layer and determined its location, extent, and physicalproperties for the first time in a PN. We confirm that this interface layer, asrevealed by the spatial distribution of the N V 1239 line emission, is locatedbetween the hot bubble and the optical nebular shell. We estimate a thicknessof 1.5$\times$10$^{16}$ cm and an electron density of ~200 cm$^{-3}$ for themixing layer. With a thermal pressure of ~2$\times$10$^{-8}$ dyn cm$^{-2}$, themixing layer is in pressure equilibrium with the hot bubble and ionized nebularrim of NGC 6543.

HST STIS observations of the mixing layer in the Cat's Eye Nebula