One can draw two interesting conclusions from the data shown in Figure 20. First, in early-M dwarfs ( M6), the relation between magnetic field and chromospheric activity follows a curve similar to the rotation-activity relation; chromospheric activity grows with average field strength in the low-field regime () but saturates at a critical field strength. The critical field strength in early M stars seems to be close to 2000 G. At this point, however, the data sample is rather sparse and uncertainties are high so that such conclusions can only be preliminary. This saturation – if existent – is different from the saturation of the field itself (at ). The rotation-activity relation implies that fields cannot be stronger than 3 – 4 kG in general. A saturation of chromospheric activity at 2000 G would mean that additionally, H emission saturates at even lower rotation rates when the field is sufficiently strong.
Figure 20 also contains data for cooler stars of spectral type M7 – M9. In these stars, the level of is lower on average than in hotter stars, a reason may be the growing atmospheric neutrality that weakens the coupling between the ionized atmosphere and magnetic fields hence rendering magnetic heating ineffective. Interestingly, there is a hint in Figure 20 that the field strength at which saturation occurs may grow to larger fields in cooler stars. In other words, cooler stars need stronger field strengths to generate the same level of activity than hotter stars, and the saturation is not limited by a fixed field strength but by a maximum level of chromospheric emission ().
Living Rev. Solar Phys. 8, (2012), 1
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