1. Sub-THz Radiation Mechanisms in Solar Flares Gregory D. Fleishman and Eduard P. Kontar(*) March 10, 2010 (*) Department of Physics and Astronomy, University of Glasgow, United Kingdom ApJL 2010. V. 709. P. L127-L132.

2. Overview Specifics of Sub-THz observations Main observational results Properties of distinct Sub-THz component of large solar flares. Possible radiation mechanisms and related diagnostics Perspectives of the sub-THz astronomy Conclusions

3. Specifics of Sub-THz observations: strong atmospheric absorption due to quantum transitions in water molecules

4. Steady thermal emission, Impulsive flare emission (gyrosynchrotron), Flaring thermal emission

5. Impulsive phase Extended phase

6. Distinct Sub-THz spectral component: Firmly established Large radiation peak flux of the order of 10 4 sfu radiation spectrum raising with frequency, F~f  positive spectral index varying with time Less reliable spectral index varying with time within   can display a sub-second time variability with the modulation about 5% the source size is believed to be less than 20''  atm = 0.4 (212 GHz) and 3.25 (405 GHz) (x26)(x1.5)

7. Distinct Sub-THz Component

8. Temporal Fluxes in the sub-THz components. Over a time scale of minutes, the sub-THz components have been observed to fluctuate.

9. Temporal Fluctuations Spatial data and context observations

10. Thermal free-free emission Extended source, 20’’ Temporal variability: e.g., sausage mode loop oscillations:

11. Gyrosynchrotron Radiation Compact source, 1’’ All flare-accelerated electrons, are needed Accelerated electron density is very large: Temporal variability: related to el’s injection, like in MW

12. Synchrotron radiation from rel. particles Relativistic positrons with E ~ 10 MeV, produced in ion nuclear interactions, are needed in large numbers. Estimates show that their numbers are at least one-two orders of magnitude below than needed.

13. Diffusive radiation Langmuir waves Again, relativistic positrons or electrons with E ~ 10 MeV are needed in large numbers. Moreover, high level of long Langmuir waves is needed. Temporal variability: nonlinear wave-wave oscillations of Langmuir waves

14. Vavilov-Cherenkov (Cherenkov) radiation is not possible in fully ionized plasma, BUT: the chromospheric plasma is only partly ionized

15. Conclusions It is likely that the sub-THz emission originates from more than a single source and more than one mechanism is involved. Free-free emission is a plausible candidate in many cases, at least for large sources; the free-free emission is clearly always present, so other mechanisms build additional contribution on top of the free-free component. Gyrosynchrotron/synchrotron emission is likely to play a role in moderate events and also as falling with frequency extension of normal microwave bursts The role of DRL is less clear since the level of long-wave Langmuir waves is yet unknown in flares Vavilov-Cherenkov emission from compact sources located at the chromosphere level seems to be a plausible process to account the raising with frequency sub- mm component of large flares Sub-THz spectral window can be extremely informative, e.g., for diagnostics of the chromospheric chemical composition if the role of the Vavilov-Cherenkov emission is confirmed. This mechanism can also be important in IR, viz, and UV ranges. This calls for a new project bringing a sub-THz receivers/interferometers, combining good sensitivity with high spatial resolution, to a space mission, complementing on-going efforts in the microwave range.