  We use low-dispersion spectra obtained at the Magellan Observatory to study
the broad H-alpha emission from the reverse shock of the infant supernova
remnant SNR1987A. These spectra demonstrate that the spatio-kinematic structure
of the reverse shock can be distinguished from that of the circumstellar ring
and hotspots, even at ground-based spatial resolution. We measure a total
dereddened H-alpha flux of 1.99(pm0.22)e-13 ergs/s/cm2 at an epoch 18.00 years
after outburst. At 50 kpc, the total reverse shock luminosity in H-alpha is
roughly 15 Lsun, which implies a total flux of neutral hydrogen atoms across
the reverse shock of 8.9e46/s, or roughly 2.3e-3 Msun/yr. This represents an
increase by a factor ~4 since 1997. Lyman continuum radiation from gas shocked
by the forward blast wave can ionize neutral hydrogen atoms in the supernova
debris before they reach the reverse shock. If the inward flux of ionizing
photons exceeds the flux of hydrogen atoms approaching the reverse shock, this
pre-ionization will shut off the broad Ly-alpha and H-alpha emission. The
observed X-ray emission of SNR1987A implies that the ratio of ionizing flux to
hydrogen atom flux across the reverse shock is presently at least 0.04. The
X-ray emission is increasing much faster than the flux of atoms, and if these
trends continue, we estimate that the broad Ly-alpha and H-alpha emission will
vanish within about 7 years.
