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tokens: other hand (lower panel), the I b / I n ratio is only slightly dependent on C . 6 APPLICATION TO A REAL CASE: SN 1006 6.1 Why SN 1006 In the following, we shall apply the methods derived so far to a real case, refining them whenever required. The Balmer emission along the northwestern limb of SN 1006 represents for this an almost perfect case: observations with high spatial and/or spectral resolution, on which to test and use our models, are available; a shock velocity () high enough to limit the role of CE processes (this simplifies the models and makes them more reliable); structures near the limb that appear rather well ordered; no evidence of efficient CR acceleration, which justifies neglecting the effect of CRs in the analysis. With reference to the shock speed, Ghavamian et al. ( 2001 ) measured a spectral width of , from which for a low T e / T p ratio they derived a shock speed of . A lower shock velocity, , follows instead from the model of Morlino et al. ( 2013b ): for a more detailed and updated discussion, see Raymond et al. ( 2017 ). In the following analysis, we shall fix V sh to . With reference to CR acceleration, a low efficiency in the northwestern limb can be deduced from several results of observations: (1) the absence of non-thermal X-ray emission (Bamba et al. 2003 ; Winkler et al. 2014 ); (2) a non-detection of TeV (Acero et al. 2010 ) and GeV gamma-rays (Xing et al. 2016 ); moreover, from the analysis of Balmer emission, two additional pieces of information point towards a low CR acceleration efficiency: (3) the FWHM (full width at half-maximum) of the narrow line is (Sollerman et al. 2003 ), compatible with being produced by unperturbed ISM with T = 10 4 K, pointing toward the absence of a CR precursor able to heat the upstream plasma; and (4) an absence of evidence of any Hα precursor in front of the shock ( R + 07 ). 6.2 Some recent high-quality observations In this section we summarize some results of two relevant papers, Raymond et al. ( 2007 , hereafter R + 07 ) and Nikolić et al. ( 2013 , hereafter N + 13 ), in which Balmer emission from some areas along the northwestern limb of SN 1006 has been observed in great detail, and that contain a wealth of information, which will be used in the present work. The former paper is based on a very deep HST /ACS image covering most of the Hα emission from that limb, and the superb spatial resolution of that image (an FWHM of about 0.13 arcsec, corresponding to about , where d 2 is the distance of SN 1006 in units of ) allows one to probe scales shorter than the collisional mean-free paths, and therefore to resolve the physical structure of the filaments. The latter paper, instead, presents a map of a selected portion of the northwestern Hα limb of SN1006, roughly corresponding to ‘Regions 26–29’ (as dubbed by R + 07 ). The instrument used, VIMOS in the IFU mode on the Very Large Telescope (VLT), allows both good spatial resolution (with a pixel size 0.67 arcsec,