  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 3\sigma\ 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.
