  We present a spectrophotometric imaging study of the emission bubble N70 (DEM
301) in the Large Magellanic Cloud. N70 is approximately 100 pc in size with a
nearly circular shell-like morphology. The nebular emission is powered by an
uncertain combination of EUV photons, intense winds, and supernova shock waves
from the central population of high-mass stars (the OB association LH 114). We
have obtained narrow-band images (FWHM~6A) of N70 in the light of H-alpha
(6563A), [N II] (6584A), [S II] (6717,6731A), and [O III] (5007A) along with
the corresponding red and green continua. The resulting line fluxes and flux
ratios are used to derive ionization rates, nebular densities, volume filling
fractions, and excitation indices. Comparison with similar spectrophotometry of
the Orion nebula indicates that the outer shell of N70 is especially enhanced
in [SII] emission compared to its H-alpha and [N II] emission. The measured
intensity ratios in N70 more closely match the range of excitation spanned by
giant and supergiant H II shells and even by some of the supernova remnants
observed in the LMC. From these spectral diagnostics, we conclude that EUV
photons probably contribute most of the inner nebula's ionization, whereas a
combination of photoionization plus collisional ionization and excitation of
sulfur atoms by low-velocity shocks is driving the emission-line luminosities
and intensity ratios observed in the outer shell. Considerations of the
radiative and mechanical energetics that are involved may indicate the need for
one or two supernova explosions having occurred during the last ~Myr.
