  We take a broad look at the problem of identifying the magnetic solar causes
of space weather. With the lackluster performance of extrapolations based upon
magnetic field measurements in the photosphere, we identify a region in the
near UV part of the spectrum as optimal for studying the development of
magnetic free energy over active regions. Using data from SORCE, Hubble Space
Telescope, and SKYLAB, along with 1D computations of the near-UV (NUV) spectrum
and numerical experiments based on the MURaM radiation-MHD and HanleRT
radiative transfer codes, we address multiple challenges. These challenges are
best met through a combination of near UV lines of bright \ion{Mg}{2}, and
lines of \ion{Fe}{2} and \ion{Fe}{1} (mostly within the $4s-4p$ transition
array) which form in the chromosphere up to $2\times10^4$ K. Both Hanle and
Zeeman effects can in principle be used to derive vector magnetic fields.
However, for any given spectral line the $\tau=1$ surfaces are generally
geometrically corrugated owing to fine structure such as fibrils and spicules.
By using multiple spectral lines spanning different optical depths, magnetic
fields across nearly-horizontal surfaces can be inferred in regions of low
plasma $\beta$, from which free energies, magnetic topology and other
quantities can be derived.
  Based upon the recently-reported successful suborbital space measurements of
magnetic fields with the CLASP2 instrument,
  we argue that a modest space-borne telescope will be able to make significant
advances in the attempts to predict solar eruptions. Difficulties associated
with blended lines are shown to be minor in an Appendix.
