  Spectral modification of energetic magnetar flares by resonant cyclotron
scattering (RCS) is considered. During energetic flares, photons emitted from
the magnetically-trapped fireball near the stellar surface should resonantly
interact with magnetospheric electrons or positrons. We show by a simple
thought experiment that such scattering particles are expected to move at
mildly relativistic speeds along closed magnetic field lines, which would
slightly shift the incident photon energy due to the Doppler effect. We develop
a toy model for the spectral modification by a single RCS that incorporates
both a realistic seed photon spectrum from the trapped fireball and the
velocity field of particles, which is unique to the flaring magnetosphere. We
show that our spectral model can be effectively characterized by a single
parameter; the effective temperature of the fireball, which enables us to fit
observed spectra with low computational cost. We demonstrate that our single
scattering model is in remarkable agreement with Swift/BAT data of intermediate
flares from SGR 1900+14, corresponding to effective fireball temperatures of
$T_{\rm eff}=6$-$7$ keV, whereas BeppoSAX/GRBM data of giant flares from the
same source may need more elaborate models including the effect of multiple
scatterings. Nevertheless, since there is no standard physically-motivated
model for magnetar flare spectra, our model could be a useful tool to study
magnetar bursts, shedding light on the hidden properties of the flaring
magnetosphere.
