  The MIRO instrument's remote sensing capability is integral in constraining
water density, temperature and velocity fields in the coma of
67P/Churyumov-Gersimenko. Our aim is to quantify the contribution to the water
density from all facets inside and outside the field-of-view (FOV) of MIRO, in
both, nadir and limb geometries. This information is crucial for understanding
the MIRO derived coma production rates and their relation to the nucleus
characteristics, and inherent spatial resolution of the data. This study relies
on a detailed 3D nucleus shape model, illumination conditions and the pointing
information of the viewing geometry. With these parameters, we can evaluate the
relative contribution of water density originating from facets directly inside
the MIRO beam as well outside of the beam as a function of distance along the
MIRO line-of-sight. We also calculate the ratio of in-beam versus out-of-beam
number density. We demonstrate that despite the rather small MIRO field-of-view
there is only a small fraction of molecules that originate from facets within
the MIRO beam. This is true for nadir, but also translated into the limb
observing geometry. The MIRO instrument cannot discriminate active from
non-active regions directly from observations. This study also suggests that
the beam averaged solar incidence angle, local time and mean normal vectors are
not necessary related to molecules within the MIRO beam. These results also
illustrate why the 1D spherical Haser model can be applied with relative
success for analyzing the MIRO data (and generally any Rosetta measurements).
The future possibilities of constraining gas activity distribution on the
surface should use 3D codes extracting information from the MIRO spectral line
shapes which contain additional information. The presented results are in fact
applicable to all relevant instruments onboard Rosetta.
