Density Structure of the Solar Corona From Radio Occultation Measurements

Starting with angular broadening measurements five decades ago, a wide variety of radio propagation and scattering phenomena have been observed when natural radio sources or spacecraft radio signals happened to pass behind or were occulted by the solar corona. While yielding information on density, velocity and magnetic fields, these unique measurements probe density most directly, and with unprecedented sensitivity, spatial and temporal resolution. Coronal density takes on added importance because it provides insight into magnetic field topology at a time when few measurements of the coronal magnetic field are available. While complexity of the corona and paucity of measurements have certainly contributed, lack of understanding of the nature of the inferred density variations is the main reason why it has taken so long for radio occultation measurements of density to finally realize their perennial potential. The purpose of this paper is to summarize the salient features and highlights of radio occultation measurements of density that include: propagation of shock waves driven by coronal mass ejections, origin and evolution of the solar wind, coronal magnetic field topology, and fine-scale filamentary structures that are more than two orders of magnitude smaller than those observed in highest spatial resolution images of the solar corona.