  Recent solar flare observations in the sub-THz range have provided evidence
of a new spectral component with fluxes increasing for larger frequencies,
separated from the well-known microwave emission that maximizes in the GHz
range. Suggested interpretations explain the THz spectral component, but do not
account for the simultaneous microwave component. We present a mechanism for
producing the observed double-spectra. Based on coherent enhancement of
synchrotron emission at long wavelengths in laboratory accelerators, we
consider how similar processes may occur within a solar flare. The instability
known as microbunching arises from perturbations that produce electron beam
density modulations, giving rise to broadband coherent synchrotron emission at
wavelengths comparable to the characteristic size of the microbunch structure.
The spectral intensity of this coherent synchrotron radiation (CSR) can far
exceed that of the incoherent synchrotron radiation (ISR), which peaks at
higher frequency, thus producing a double-peaked spectrum. Successful CSR
simulations are shown to fit actual burst spectral observations, using typical
flaring physical parameters and power-law energy distributions for the
accelerated electrons. The simulations consider an energy threshold below which
microbunching is not possible because of Coulomb repulsion. Only a small
fraction of the radiating charges accelerated to energies above the threshold
is required to produce the microwave component observed for several events. The
ISR-CSR mechanism can occur together with other emission processes producing
the microwave component. It may bring an important contribution at microwaves
at least for certain events where physical conditions for the occurrence of the
ISR-CSR microbunching mechanism are possible.
