The morphology of the ejecta of SN 1987A at 31 years from 1150 to 10000
  {\AA}
  We present spectroscopy of the ejecta of SN 1987A in 2017 and 2018 from the
Hubble Space Telescope and the Very Large Telescope, covering the wavelength
range between $1150$ and $10000$ {\AA}. At 31 years, this is the first epoch
with coverage over the ultraviolet-to-near-infrared range since 1995. We create
velocity maps of the ejecta in the H$\alpha$, Mg II $\lambda\lambda2796,2804$
and [O I] $\lambda\lambda6302,6366$ (vacuum) emission lines and study their
morphology. All three lines have a similar morphology, but Mg II is blueshifted
by $\sim$1000 km s$^{-1}$ relative to the others and stronger in the northwest.
We also study the evolution of the line fluxes, finding a brightening by a
factor of $\sim$9 since 1999 in Mg II, while the other line fluxes are similar
in 1999 and 2018. We discuss implications for the power sources of emission
lines at late times: thermal excitation due to heating by the X-rays from the
ejecta-ring interaction is found to dominate the ultraviolet Mg II lines, while
the infrared Mg II doublet is powered mainly by Ly$\alpha$ fluorescence. The
X-ray deposition is calculated based on merger models of SN 1987A.
Far-ultraviolet emission lines of H$_2$ are not detected. Finally, we examine
the combined spectrum of recently-discovered hotspots outside the equatorial
ring. Their unresolved Balmer emission lines close to zero velocity are
consistent with the interaction of fast ejecta and a clumpy, slowly moving
outflow. A clump of emission in this spectrum, south of the equatorial ring at
$\sim$1500 km s$^{-1}$, is likely associated with the reverse shock.
