  The persistent radio counterpart of FRB 121102 is estimated to have $N\sim
10^{52}$ particles, energy $E_N\sim 10^{48}$ erg, and size $R\sim 10^{17}$ cm.
The source can be a nebula inflated and heated by an intermittent outflow from
a magnetar --- a neutron star powered by its magnetic (rather than rotational)
energy. The object is young and frequently liberating energy in magnetic flares
driven by accelerated ambipolar diffusion in the neutron star core, feeding the
nebula and producing bright millisecond bursts. The particle number in the
nebula is consistent with ion ejecta from giant flares. The nebula may also
contain the freeze-out of electron-positron pairs $N_\pm\sim 10^{51}$ created
months after the neutron star birth; the same mechanism offers an explanation
for $N_\pm$ in the Crab nebula. The persistent source around FRB 121102 is
likely heated by magnetic dissipation and internal waves excited by the
magnetar ejecta. The volumetric heating by waves explains the nebula's enormous
efficiency in producing radio emission. The repeating radio bursts are
suggested to occur much closer to the magnetar, whose flaring magnetosphere
drives ultrarelativistic internal shocks into the magnetar wind. The shocks are
mediated by Larmor rotation that forms a GHz maser with the observed ms
duration. Furthermore, the flare ejecta can become charge-starved and then
convert to electromagnetic waves.
