Global magnetic fields of flare stars can evolve rapidly, in time scale of hundreds or dozens of days. We believe, that such changes result from rapid superposition of local magnetic fields generated by differential rotation of those stars. We discuss possible mechanisms of generation and dissipation of local and global magnetic fields in sample flare stars OT Ser and YZ CMi. We propose mechanism of magnetic braking of these stars, in which differential rotation generates local magnetic fields, and eventually energy accumulated in local fields is radiated away by flares. We obtained estimates of the rotational energy and the energy of the global magnetic field of OT Ser and YZ CMi. We also show that the energy of the local magnetic fields dissipated during superflare of YZ CMi on 9 February 2008 (UT 20:22:00) did not influence the global magnetic field of this star.
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