  We present the construction of a new white-light coronal brightness index
(CBI) from the entire archive of observations recorded by the Large Angle
Spectrometric Coronagraph (LASCO) C2 camera between 1996 and 2017, comprising
two full solar cycles. We reduce all fully calibrated daily C2 observations of
the white light corona into a single daily coronal brightness observation for
every day of observation recorded by the instrument, with mean daily brightness
values binned into 0.1 Rsun radial x 1 degree angular regions from 2.4 -- 6.2
Rsun for a full 360-degrees. As a demonstration of the utility of the CBI, we
construct a new solar irradiance proxy that correlates well with a variety of
direct solar irradiance observations, with correlations shown to be in the
range of 0.77-0.89. We also present a correlation mapping technique to show how
irradiance correlations depend on, and relate to, coronal structure/locations,
and to demonstrate how the LASCO CBI can be used to perform long-term "spatial
correlation" studies to investigate relationships between the solar corona and
any arbitrary concurrent geophysical index. Using this technique we find
possible relationships between coronal brightness and plasma temperature,
interplanetary magnetic field magnitude and (very weakly) proton density.
