Coronal plasma diagnostics from ground-based observations

In this paper we discuss the potential of ground-based visible observations of the solar corona to address the key open problems in the physics of the solar atmosphere and of solar activity. We first compare the diagnostic potential of visible observations with those of high-resolution spectrometers and narrowband imagers working in the EUV and X-ray wavelength ranges. We then review the main diagnostic techniques (and introduce a few new ones) that can be applied to line and continuum emission in the solar atmosphere, and the physical problems that they enable us to address. Finally, we briefly review the main features of ground-based coronographic instrumentation currently being developed and planned.