  Understanding the temperature structure of protoplanetary disks is crucial
for answering the fundamental question of when and where in the disks rocky
planets like our own form. However, the thermal structure of the inner few au
of the disks is poorly understood not only because of lack of observational
constraints but also because of the uncertainty of accretion heating processes.
Here, we propose thermal tomography of the inner regions of protoplanetary
disks with the ngVLA and ALMA. The proposed approach is based on the assumption
that the inner disk regions are optically thick at submillimeter wavelengths
but are marginally optically thin at longer millimeter wavelengths. By
combining high-resolution millimeter continuum images from the ngVLA with
submillimeter images at comparable resolutions from ALMA, we will be able to
reconstruct the radial and vertical structure of the inner few au disk regions.
We demonstrate that the thermal tomography we propose can be used to constrain
the efficiency of midplane accretion heating, a process that controls the
timing of snow-line migration to the rocky planet-forming region, in the few au
regions of protoplanetary disks at a distance of 140 pc.
