  A three-dimensional picture of the solar atmosphere's thermodynamics can be
obtained by jointly analyzing multiple spectral lines that span many formation
heights. In paper I, we found strong correlations between spectral shapes from
a variety of different ions during solar flares in comparison to the quiet Sun.
We extend these techniques to address the following questions: which regions of
the solar atmosphere are most connected during a solar flare, and what are the
most likely responses across several spectral windows based on the observation
of a single Mg II spectrum? Our models are derived from several million IRIS
spectra collected from 21 M- and X-class flares. We applied this framework to
archetypal Mg II flare spectra, and analyzed the results from a multi-line
perspective. We find that (1) the line correlations from the photosphere to the
transition region are highest in flare ribbons. (2) Blue-shifted reversals
appear simultaneously in Mg II, C II, and Si IV during the impulsive phase,
with Si IV displaying possible optical depth effects. Fe II shows signs of
strong emission, indicating deep early heating. (3) The Mg II line appears to
typically evolve a blue-shifted reversal that later returns to line center and
becomes single peaked within 1-3 minutes. The widths of these single peaked
profiles slowly erode with time. During the later flare stages, strong red wing
enhancements indicating coronal rain are evident in Mg II, C II, and Si IV. Our
framework is easily adaptable to any multi-line data set, and enables
comprehensive statistical analyses of the atmospheric behavior in different
spectral windows.
