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Probing the Structure and Kinematics of the transition Layer between the Magellanic Stream and the Halo in HI

The Magellanic Stream (MS) is a nearby laboratory for studying the fate of cool gas streams injected into a gaseous galactic halo. We investigate properties of the boundary layer between the cool MS gas and the hot Milky Way halo with 21 cm HI observations of a relatively isolated cloud having circular projection in the northern MS. Through averaging and modeling techniques, our observations obtained with the Robert C. Byrd Green Bank Telescope (GBT), reach unprecedented 3σ sensitivity of ~10^17/cm^2, while retaining the telescope's 9.1' resolution in the essential radial dimension. We find an envelope of diffuse neutral gas with FWHM of 60 km/s, associated in velocity with the cloud core having FWHM of 20 km/s, extending to 3.5 times the core radius with a neutral mass seven times that of the core. We show that the envelope is too extended to represent a conduction-dominated layer between the core and the halo. Its observed properties are better explained by a turbulent mixing layer driven by hydrodynamic instabilities. The fortuitous alignment of the NGC 7469 background source near the cloud center allows us to combine UV absorption and HI emission data to determine a core temperature of 8350 +/- 50 K. We show that the HI column density and size of the core can be reproduced when a slightly larger cloud is exposed to Galactic and extragalactic background ionizing radiation. Cooling in the large diffuse turbulent mixing layer envelope extends the cloud lifetime by at least a factor of two relative to a simple hydrodynamic ablation case, suggesting that the cloud is likely to reach the Milky Way disk.