  Growing observations of temporal, spectral, and polarization properties of
fast radio bursts (FRBs) indicate that the radio emission of the majority of
bursts is likely produced inside the magnetosphere of its central engine,
likely a magnetar. We revisit the idea that FRBs are generated via coherent
inverse Compton scattering (ICS) off low-frequency X-mode electromagnetic waves
(fast magnetosonic waves) by bunches at a distance of a few hundred times of
the magnetar radius. Following findings are revealed: 1. Crustal oscillations
during a flaring event would excite kHz Alfv\'en waves. Fast magnetosonic waves
with the same frequency can be generated directly or be converted from Alfv\'en
waves at a large radius, with an amplitude large enough to power FRBs via the
ICS process. 2. The cross section increases rapidly with radius and significant
ICS can occur at $r \gtrsim 100 R_\star$ with emission power much greater than
the curvature radiation power but still in the linear scattering regime. 3. The
low-frequency fast magnetosonic waves naturally redistribute a fluctuating
relativistic plasma in the charge-depleted region to form bunches with the
right size to power FRBs. 4. The required bunch net charge density can be
sub-Goldreich-Julian, which allows a strong parallel electric field to
accelerate the charges, maintain the bunches, and continuously power FRB
emission. 5. This model can account for a wide range of observed properties of
repeating FRB bursts, including high degrees of linear and circular
polarization and narrow spectra as observed in many bursts from repeating FRB
sources.
