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tokens: TiO and FeH lines using the four free model parameters revealed for most stars T mag between 2000 and 2300 K with an average of 2100 K, which is approximately constant across the M1 to M7 spectral classes ( Fig. 4 b). These cool spots are also highly magnetic (see Sect. 5.3 ). Here, the difference between T phot and T mag reduces on average from 1700 K for an M1 dwarf to about 1000 K for an M7 dwarf. Interestingly, this range of spot temperature differences is typical for sunspots of various sizes (e.g., Berdyugina 2005 ). For a young, active M8.5 brown dwarf, Berdyugina et al. (2017) and Kuzmychov et al. (2017) determined T phot = 2800 K and T mag = 2200 K using also atomic and molecular lines. This result agrees with the found trend and extends our finding to ultra-cool dwarfs. The spot temperature difference with respect to the photosphere determined here for M dwarfs differs from that found from broad-band photometric measurements, as summarized by Berdyugina (2005) . This is probably due to a height-dependence of spot parameters as well as the higher sensitivity of molecular lines to the temperature than that of photometry (see Sect. 6 ). In summary, thanks to the variety of our spectral diagnostics, we found two kinds of magnetic regions with clearly different temperatures in M dwarf atmospheres as a function of the spectral class: one is cold (starspots), and the other is warm (network). Fig. 2 Illustration of the high temperature and magnetic sensitivity of FeH lines which is reproduced by our models. Observed (black) and synthetic (red) intensity and circular polarization profiles for FeH at 9897–9913 Å in the considered sample of M dwarfs. The synthetic profiles were calculated using two components: one non-magnetic (“photospheric”) and one magnetic (“spot”) atmosphere model ( Allard et al. 2001 ) with best-fit parameters obtained from fits of Stokes I and V spectra for FeH in the considered wavelength region only. All the following parameters were obtained simultaneously in one χ 2 minimization: the photosphere and spot temperatures, the magnetic field strength, the spot filling factor for the intensity f I , and the circular polarization f V . 5.3 The magnetic field strength The magnetic field strength B was determined during the same fitting procedure as described for T mag in Sect. 5.2 . It corresponds therefore to an average field strength of a given magnetic structure at a given layer of the atmosphere,as defined by the corresponding species. The B values found from atomic and molecular lines are confined to the range of 3–6 kG (there are only three outliers), with an average of about 5 kG ( Fig. 4 d). This average is practically constant throughout the considered spectral class range of M1–M7 dwarfs. Berdyugina et al. (2017) and Kuzmychov et al. (2017) similarly found B ~ 5 kG in their study of a young, active M8.5 brown dwarf. In summary, a cooler starspot atmosphere probed by the mol1 set and a hotter network atmosphere