  We present optical spectroscopic data of the inner circumstellar ring around
SN 1987A from the Anglo-Australian Telescope (AAT) and the Very Large Telescope
(VLT) between ~1400 and ~5000 days post-explosion. We also assembled the
available optical and near-infrared line fluxes from the literature between
~300 and ~2000 days. These line light curves were fitted with a photoionization
model to determine the density structure and the elemental abundances for the
inner ring. We found densities ranging from 1x10^3 to 3x10^4 atoms cm^-3 and a
total mass of the ionized gas of ~0.058 Msun within the inner ring. Abundances
inferred from the optical and near-infrared data were also complemented with
estimates of Lundqvist & Fransson (1996) based on ultraviolet lines. This way
we found an He/H-ratio (by number of atoms) of 0.17+-0.06 which is roughly 30%
lower than previously estimated and twice the solar and the Large Magellanic
Cloud (LMC) value. We found an N/O-ratio of 1.5+-0.7, and the total
(C+N+O)/(H+He) abundance about 1.6 times its LMC value or roughly 0.6 times the
most recent solar value. An iron abundance of 0.20+-0.11 times solar was found
which is within the range of the estimates for the LMC. We also present late
time (~5000 - 7500 days) line light curves of [O III], [Ne III], [Ne IV], [Ar
III], [Ar IV], and [Fe VII] from observations with the VLT. We compared these
with model fluxes and found that an additional 10^2 atoms cm^-3 component was
required to explain the data of the highest ionization lines. Such low density
gas is expected in the H II-region interior to the inner ring which likely
extends also to larger radii at higher latitudes (out of the ring plane). At
epochs later than ~5000 days our models underproduce the emission of most of
these lines as expected due to the contribution from the interaction of the
supernova ejecta with the ring.
