  Observations in the sub-THz range of large solar flares have revealed a
mysterious spectral component increasing with frequency and hence distinct from
the microwave component commonly accepted to be produced by gyrosynchrotron
(GS) emission from accelerated electrons. Evidently, having a distinct sub-THz
component requires either a distinct emission mechanism (compared to the GS
one), or different properties of electrons and location, or both. We find,
however, that the list of possible emission mechanisms is incomplete. This
Letter proposes a more complete list of emission mechanisms, capable of
producing a sub-THz component, both well-known and new in this context and
calculates a representative set of their spectra produced by a) free-free
emission, b) gyrosynchrotron emission, c) synchrotron emission from
relativistic positrons/electrons, d) diffusive radiation, and e) Cherenkov
emission. We discuss the possible role of the mechanisms in forming the sub-THz
emission and emphasize their diagnostics potential for flares.
