Mesosphere
The mesosphere refers to the mantle in the region under the lithosphere and the asthenosphere, but above the outer core.
The upper boundary is defined as the sharp increase in seismic wave velocities and density at a depth of 660 kilometers (410 mi).
At a depth of 660 km, ringwoodite (gamma-(Mg,Fe)2SiO4) decomposes into Mg-Si perovskite and magnesiowustite.
This reaction marks the boundary between upper mantle and lower mantle.
This measurement is estimated from seismic data and high-pressure laboratory experiments.