  Brunel radiation appears as a result of a two-step process of
photo-ionization and subsequent acceleration of electron, without the need of
electron recollision. We show that for generation of Brunel harmonics at all
frequencies the subcycle ionization dynamics is of critical importance. Namely,
such harmonics disappear at low pump intensities when the ionization dynamics
depends only on the slow envelope (so called multiphoton ionization regime) and
not on the instantaneous field. Nevertheless, if the pump pulse contains
incommensurate frequencies, Brunel mechanism does generate new frequencies even
in the multiphoton ionization regime.
