Across the microwave band, broadly considered here as 1100 GHz, the two dominant mechanisms will be thermal bremsstrahlung and thermal (i.e., vibrational) dust emission.
The most commonly observed of these is that of thermal bremsstrahlung, or free-free, emission.
A model including a thermal bremsstrahlung emission component suffering from dust absorption at high frequencies (scenario 2; abFF+GB), see Section 2.2.1, can account for the dip in flux density at 94 GHz, see Figure 3, relative to lower frequencies.
The process is referred to as free-free transition, or thermal bremsstrahlung
. The S-component is generated above active areas in the Sun's chromosphere (most notably plages and sunspots) by bremsstrahlung emission, and by gyromagnetic radiation--a process in which energized electrons release photons as they spiral around strong magnetic fields.