SubTHz Radiation Mechanisms in Solar Flares Ap JL

Sub-THz Radiation Mechanisms in Solar Flares Ap. JL 2010. V. 709. P. L 127 -L 132. Gregory D. Fleishman and Eduard P. Kontar(*) March 10, 2010 (*) Department of Physics and Astronomy, University of Glasgow, United Kingdom

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

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

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

Impulsive phase Extended phase

Distinct Sub-THz spectral component: • Firmly established • Large radiation peak flux of the order of 104 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) (x 1. 5) (x 26)

Distinct Sub-THz Component

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

Temporal Fluctuations Spatial data and context observations

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

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

Synchrotron radiation from rel. particles Relativistic positrons with E ~ 10 Me. V, 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.

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

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

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 submm 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.