Interplay between Physics and Geometry in Balmer filaments: the Case of
  SN 1006
  The analysis of Balmer-dominated emission in supernova remnants is
potentially a very powerful way to derive information on the shock structure,
on the physical conditions of the ambient medium and on the cosmic-ray
acceleration efficiency. However, the outcome of models developed in
plane-parallel geometry is usually not easily comparable with the data, since
they often come from regions with rather a complex geometry. We present here a
general scheme to disentangle physical and geometrical effects in the data
interpretation, which is especially powerful when the transition zone of the
shock is spatially resolved and the spectral resolution is high enough to allow
a detailed investigation of spatial changes of the line profile. We then apply
this technique to re-analyze very high quality data of a region along the
northwestern limb of the remnant of SN~1006. We show how some observed
features, previously interpreted only in terms of spatial variations of
physical quantities, naturally arise from geometrical effects. With these
effects under control, we derive new constraints on physical quantities in the
analyzed region, like the ambient density (in the range 0.03-$0.1{\,\rm
cm^{-3}}$), the upstream neutral fraction (more likely in the range 0.01-0.1),
the level of face-on surface brightness variations (with factors up to $\sim
3$) and the typical scale lengths related to such variations ($\ge 0.1{\,\rm
pc}$, corresponding to angular scales $\ge 10{\,\rm arcsec}$).
