We present a simplified model of the outer heliosheath to help interpret the
observations of interstellar neutral helium by the IBEX-Lo instruments. We
assume that the measured particles are composed of the superposition of a
primary beam population, with the properties of the local interstellar medium,
and a secondary population, created by charge exchange between the primary beam
neutrals and the ions that have been deflected as they approach the heliopause.
We extract information on the large-scale shape of the heliopause by comparing
the helium flux measured at IBEX along four different look directions with
simple models of deflected plasma flow around hypothetical obstacles of
different aspect ratios to the flow. As a first step in this paper, we model
the deflected plasma flow with the analytical solutions for compressible gas
flow around a series of oblate ellipsoidal obstacles. Our comparisons between
the model results and the observations indicate that the heliopause is very
blunt in the vicinity of the heliospheric nose, especially compared to a
Rankine half-body or cometary shape. The upstream heliopause seems to be highly
elongated in the directions parallel to the interstellar magnetic field, and
relatively more compact and symmetric in the directions transverse to that
field. The IBEX-Lo helium observations are not consistent with a heliopause
elongated in directions parallel to the solar rotation axis.
