We present high-resolution emission-line images of the remarkable Herbig-Haro jet HH 46/47 obtained with the Wide Field and Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope. Two narrowband filters were used, F673N which isolates the [S II] lambda lambda 6716,6731 doublet, and F656N which transmits H alpha. The exposures through each filter had a total integration time of 11,900 s. The 0''.1 spatial resolution of the WFPC2 images (corresponding to 45 AU at the distance of HH 46/47), coupled with the high signal-to-noise of these images, allows us to study this collimated outflow, driven by a young star, in unprecedented detail. In particular, we are able to resolve the sizes of emission knots and filaments, and determine the structure and scale of the cooling zone behind the shock waves in the flow. We discuss the observed structures in terms of radiative shock models. Both in the major working surfaces and along the body of the jet we identify many shocks in which the Balmer emission arises predominantly from collisional excitation in the thin heating zone at the shock front. The body of the jet is seen primarily in [S II] emission, The jet has a complicated structure consisting of a sinuous chain of emission knots and strands. The [S II] emission in the jet appears to arise where shocks are excited in the jet material by its interaction with the surrounding gas and by collisions between faster and slower moving segments of the flow. A combination of time variability in the velocity and direction of ejection may explain the jets sinuous structure. In H alpha we see with absolute clarity the delicate wisps and filaments bordering the jet that were only barely visible in the best ground-based images, We identify these wisps as Balmer-line emitting shock fronts marking where shock waves are driven into the (apparently) neutral gas surrounding the jet. These Balmer-line filaments are only 0''.2-0''.3 wide but can be up to 2 '' long. Each is associated with a [S II]-bright knot in the jet. These wisps have an arcuate morphology, with trailing wings which sweep back at an oblique angle to the direction of the flow. They thus resemble one-sided bow shocks that extend into and accelerate the ambient gas. All the knots in the jet may excite similar shock waves, however, these are only detected as an H alpha filament where the shock front lies almost tangent to the line of sight. The fact that we see strong Balmer emission from the shock fronts, and that high excitation lines such as [O III] are not detected in ground-based spectra, implies that the shocks driven into the ambient medium are weak (V-sh much less than 100 km s(-1)). However, proper motion measurements show that these shock systems propagate along the jet at similar to 300 km s(-1). Hence material alongside the jet flows away from the source at an appreciable fraction of the jet's velocity. We believe that the dominant process that accelerates the gas surrounding the jet is prompt entrainment, where the major bow shocks HH 47D and HH 47A, aided by the lesser but more frequent Balmer-arc-shocks, push material ahead and away from the axis of the jet. The very extended wings of HH 47D and KH 47A suggest that their influence may be felt far from the jet's axis. Prompt entrainment by these bow shocks might drive the weak, approaching lobe of the molecular flow associated with the HH 47 complex. The long term meandering of the jet, evident from the misalignment between the bow shocks and the jet, may further widen the flow channel. As the jet changes direction, the newly ejected gas will entrain material from the cloud, and this could produce the observed filaments in the reflection nebula that extend roughly parallel to the northern boundary of the jet. Our high-resolution images show no clear evidence for a turbulent mixing layer at the interface between the jet and its surroundings. In the HH 47A working surface at the terminus of the jet we resolve the double shock structure expected from theory. Both the forward (or bow) shock and reverse shock (or Mach disk) have collisionally excited H alpha components that delineate the positions of the respective shock fronts. Sandwiched between these two shock waves is a region, luminous in [S II], where the shock-heated gas cools. The turbulent structure seen in [S II] suggests that instabilities fragment the cooling, compressed material into clumps, as seen in some hydrodynamical simulations of jets. Our realization that the H alpha luminosity of the Mach disk arises from collisional excitation at the shock front, and our discovery of a similar H alpha-bright leading shock, leads us to reassess the relative strengths of the two shocks. We conclude that they are of comparable strength, and hence that the jet is of similar density to its surroundings. This is contrary to the earlier findings that the HH 47 jet was a rare example of a ''light'' stellar jet. (C) 1996 American Astronomical Society.
