I’m interested to know how we define the top and bottom of it, whether the boundaries are sharp or gradual, what fine-scale properties make its rheology different from the lithosphere (and whatever mantle material is below it, if anything), and if there’s any geophysical means of imaging it. I’m aware of the mineralogical/geophysical boundaries at 410 and 660 km depth, but those seem to be totally unrelated.
Additionally, back in my undergrad mineralogy class, I learned the term “mesosphere” for the mantle layer below the asthenosphere. I’ve never heard it since for describing anything in the mantle. Is that term still in use?