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 ( \pm 0.22 ) \times 10 ^ { -13 } ergs s ^ { -1 } cm ^ { -2 } at an epoch 18.00 years after outburst .
At 50 kpc , the total reverse shock luminosity in H \alpha is roughly 15 L _ { \odot } , which implies a total flux of neutral hydrogen atoms across the reverse shock of 8.9 \times 10 ^ { 46 } s ^ { -1 } , or roughly 2.3 \times 10 ^ { -3 } M _ { \odot } yr ^ { -1 } .
This represents an increase by a factor \sim 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 in \lesssim 7 years .