  Brunel harmonics appear in the optical response of an atom in process of
laser-induced ionization, when the electron leaves the atom and is accelerated
in the strong optical field. In contrast to recollision-based harmonics, the
Brunel mechanism does not require the electron returning to the core. Here we
show that in the presence of a strong ionizing terahertz (THz) field, even a
weak driving field at the optical frequencies allow for generating Brunel
harmonics effectively. The strong ionizing THz pump suppresses recollisions,
making Brunel dominant in a wide spectral range. High-order Brunel harmonics
may form a coherent carrier-envelope-phase insensitive supercontinuum,
compressible into an isolated pulse with the duration down to 100 attoseconds.
