  Prompted by high resolution observations, I propose an explanation for the
40+ year old problem of structure and energy balance in the solar transition
region. The ingredients are simply cross-field diffusion of neutral atoms from
cool threads extending into the corona, and the subsequent excitation,
radiation and ionization of these atoms via electron impact. The processes
occur whenever chromospheric plasma is adjacent to coronal plasma, and are
efficient even when ion gyro-frequencies exceed collision frequencies. Cool
threads - fibrils and spicules perhaps - grow slowly in thickness as a neutral,
ionizing front expands across the magnetic field into coronal plasma. Radiative
intensities estimated for H L$\alpha$ are within an order of magnitude of those
observed, with no ad-hoc parameters - only thermal parameters and geometric
considerations are needed. I speculate that the subsequent dynamics of the
diffused material might also explain observed properties of trace elements.
