FWHM of the broad Balmer line as a function of the CR acceleration efficiency. Different lines show different electron-ion equilibrium levels. The plots on the left (right) are obtained for V_sh = 4000 km s^-1 (V_sh = 5000 km s^-1). The top (bottom) panels refer to neutral fraction h_N = 50% (h_N = 10%). The total gas density is n₀ = 0.6 cm^-3 (Badenes et al. 2008). The shaded area shows the FWHM as measured for the SW shock by Helder et al. (2010) at 1σ level.