An explanation of the solar transition region
  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. 
Philip G. Judge