  Visible and near-infrared (V+NIR) emission lines were the first to be
discovered in the corona, during total solar eclipses, and they continue to
offer unique opportunities to study the physical properties of the corona. The
most commonly observed coronal emission lines today are in the extreme
ultraviolet, which are dominated by collisionally excited emission. V+NIR lines
on the other hand, are radiatively excited out to high helioprojective
distances. Indeed, recent eclipse observations have demonstrated the diagnostic
potential of V+NIR lines, which are still observable out to at least 3.4 Rs.
V+NIR lines can be used to infer key plasma parameters such as: the electron
and ion temperatures, magnetic field strength and morphologies, the ionic
freeze-in distances, Doppler motions of coronal plasmas, and the dynamics of
coronal mass ejections through time variations of these parameters. Current and
planned space-based coronagraphs, such as Solar Orbiter and Proba 3, will have
some filters for V+NIR lines, but will only have an exceptionally small
selection. They will thus be limited in their ability to infer electron or ion
temperatures, as well as other crucial physical properties of the corona. The
ground-based DKIST and UCoMP will soon offer V+NIR line observations, but they
will be limited to a maximum helioprojective distance of about 1.5 Rs. To
better explore the middle corona, and to understand the formation of the solar
wind and space weather events, it is essential that we deploy additional
space-based assets to measure a wide selection of V+NIR emission lines at
helioprojective distances beyond 1.5 Rs. Occulting of the solar disk could be
achieved by a conventional coronagraph, by novel methods such as an external
occulter, by lunar occultations in situ in orbit around the Moon, or by lunar
based observations of lunar eclipses (i.e., total solar eclipse on the Moon).

Instrumentation and Methods for Astrophysics;Solar and Stellar Astrophysics