Two-dimensional spectra of Supernova 1987A were obtained on 1998 November 14-15 ( 4282 days after outburst ) with the Space Telescope Imaging Spectrograph ( STIS ) on board the Hubble Space Telescope ( HST ) .
The slit sampled portions of the inner circumstellar ring at the east and west ansae as well as small sections of both the northern and southern outer rings .
The temperature and density at these locations are estimated by nebular analysis of [ \ion N2 ] , [ \ion O3 ] , and [ \ion S2 ] emission line ratios , and with time-dependent photoionization/recombination models .
The results from these two methods are mutually consistent .
The electron density in the inner ring is \sim 4000 ~ { } cm ^ { \small - 3 } for \ion S2 , with progressively lower densities for \ion N2 and \ion O3 .
The electron temperatures determined from [ \ion N2 ] and [ \ion O3 ] line ratios are \sim 11 , 000 K and \sim 22 , 000 K , respectively .
These results are consistent with evolutionary trends in the circumstellar gas from similar measurements at earlier epochs .
We find that emission lines from the outer rings come from gas of lower density ( n _ { e } \la 2000 ~ { } cm ^ { \small - 3 } ) than that which emits the same line in the inner ring .
The N/O ratio appears to be the same in all three rings .
Our results also suggest that the CNO abundances in the northern outer ring are the same as in the inner ring , contrary to earlier results of Panagia et al .
( 1996 ) .
Physical conditions in the southern outer ring are less certain because of poorer signal-to-noise data .
The STIS spectra also reveal a weak { H } \alpha emission redshifted by \sim 100 ~ { } km~ { } s ^ { \small - 1 } at p.a .
103 \arcdeg that coincides with the recently discovered new regions that are brightening ( [ Lawrence et al .
2000 ] ) .
This indicates that the shock interaction in the SE section of the inner ring commenced over a year before it became apparent in HST images .