Résumé
With a long-term monitoring of the large-scale magnetic field, we can search for temporal evolution of the topology and uncover stellar dynamo theories. The Sun is an important benchmark: the magnetic cycle is characterised by a 11-yr variation in sunspot number, size and latitude of emergence, and magnetic polarity reversal. Spectropolarimetric observations revealed analogous polarity flips also on other stars. At the lower end of the main sequence, M dwarfs represent excellent laboratories to study non-solar dynamo mechanisms underlying the generation and preservation of magnetic fields. The magnetic field generation occurs under different interior physical conditions, as the transition between partly - and fully-convective occurs at spectral type M3. Observational constraints for M dwarfs have been set by high-resolution spectroscopy field measurements in unpolarised light and by tomographic inversion techniques (Zeeman-Doppler Imaging), revealing a dichotomy in the full-convective regime: strong, dipolar axisymmetric and weak, complex non-axisymmetric fields. To explain this phenomenon, either dynamo bistability or magnetic cycles are invoked, but no definite conclusion has been reached so far.