Advantage of Forbidden Emission Lines Over Extreme Ultraviolet Lines as Coronal Diagnostic Tools

Space-based observations, starting in the late sixties and early seventies, established the frontier for exploring coronal emission (from the Sun and stars) in the ultraviolet, extreme ultraviolet and x-rays. Recently, these wavelengths have been the prime line-up for imaging experiments on solar and heliospheric space-based observatories, such as SOHO, Trace, STEREO, Hinode, and SDO. Such is also the case for the recently approved Solar Orbiter mission. Using recent examples from total solar eclipses, we show how probing the physics of the corona is seriously short-changed without the inclusion of coronal forbidden lines, such as the Fe X 637.4 nm, Fe XI 789.2 nm and Fe XIV 530.3 nm lines. The diagnostic potential of these spectral lines stems primarily from the strength of their resonantly excited component compared to their EUV and X-rays counterparts where it is absent.