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tokens: non-thermal electrons with energy 30 keV are depositing their energy and the plasma densities are sufficient to make 0.4 THz emission optically thick. The observation of ribbon-like HXR emission ( Liu et al. 2007 ) is often viewed as an additional indication that UV emission is caused by the energy deposition of HXR producing electrons. 4.3. Time variability of sub-THz emission Relatively dense plasma n > 10 11 cm −3 heated by energetic electrons to temperatures 0.1–1 MK leads to enhanced radiation ( Raymond et al. 1976 ; Johnson et al. 2011 ; Scullion et al. 2016 ), so that the radiation losses would lead to effective cooling. The radiative cooling time is ( Cox Tucker 1969 ; Raymond et al. 1976 ; Bian et al. 2016 ) where the radiative loss function is calculated using the CHIANTI ( Dere et al. 1997 ) function rad_loss.pro 2 . The radiative loss can be estimated using the approximation Λ( T )≃1.2 × 10 −19 T −1/2 , where T is in units of kelvin and Λ( T ) in units of [erg cm 3 s −1 ], giving Λ( T )≃4 × 10 −22 for T = 10 5 K. The radiative loss time for plasma temperatures between T = 10 4 − 10 6 K is shown in Fig. 3 . The results suggest that the plasma can quickly cool if the heating time is greater than the radiation loss time. The observations by Kaufmann et al. (2009a) show that the flux changes with a characteristic time of 0.1 − 1 s. This suggests that the interplay between non-thermal electron heating and the radiative cooling of dense plasma can explain the observed variability of sub-THz emission. Quasi-synchronised broadband solar flare rapid pulsations at sub-second timescales are observed at HXR and centimetre-millimetre wavelengths ( Takakura et al. 1983 ; Cornell et al. 1984 ; Kaufmann et al. 2000 ). These short-timescale features are often called “elementary bursts” following de Jager de Jonge (1978) . In particular, efforts to search for small-scale structures have also been made in optical wavelengths, typically H α observations ( Wang et al. 2000 ; Trottet et al. 2000 ; Kurt et al. 2000 ; Altyntsev et al. 2008 ; Fleishman et al. 2016 ) For example, Wang et al. (2000) reported fine temporal structures of the order of 0.3 − 0.7 s in H α off-band emission, and it is found that these fine structures only occur at flare kernels whose light curves are closely correlated with HXR emission. This close correlation suggests that the sub-THz pulsations are caused by fast electron energy deposition or by thermal conduction in the transition region and heated chromosphere. In such a scenario, the finite temperature of the ambient plasma plays an important role in determining the energy deposition ( Kontar et al. 2015 ; Jeffrey et al. 2015 ). An alternative way proposes heating by fast electrons accelerated in situ in the chromosphere due to the magnetic Rayleigh-Taylor instability ( Zaitsev Stepanov 2015 , 2017 ). 5. The sub-THz emission model During solar flares the transition region defined as the plasma between the chromosphere and