Comparison of the spectra ratios obtained by dividing each spectrum by the average spectrum of the no-DRS region (H11_x with x = [2,3,4,5]). The slope is calculated considering the wavelength ranges 1.51−1.63 μm and 2.03−2.30 μm (red and blue parts for 2007 and 2011, respectively) and a simple linear regression. All spectra have been normalized at 2.20 μm (dashed line). The variation of the spectral slopes obtained for each spectra as a function of time is represented in Fig. 4.