Comparative Study of Solar Bursts at Sub-THz Frequencies

Solar burst radio emissions at sub-THz frequencies have revealed a new spectral component with fluxes increasing with frequency, apparently simultaneous but separated from the well known emissions at microwaves. The intriguing THz emissions bring challenges for interpretation. Observations were obtained at frequencies centered at 0.2 and 0.4 THz by the Solar Submm-w Telescope (SST) at El Leoncito, at 2550 m altitude in the Argentina Andes. We discuss the quality these observations which is poor compared to microwaves and mm-waves. Principal uncertainties arise from approximate estimates of variable atmosphere transmission, and to the irregular three SST 0.2 THz beam shapes used in the partially overlapping beams technique to determine the burst position, especially in weak bursts. Good to fair atmosphere transmission conditions at El Leoncito are found for less than 205 days/year at 0.2 THz and 180 days/year 0.4 THz. The two principal factors lead to flux uncertainties of the order of ± 25%. In 2012-2013 they were complemented by 45 GHz and 90 GHz radio-polarimeters, also operated at El Leoncito. Sub-THz bursts were compared to microwaves obtained by RSTN (2.695, 4.995, 8.8, and 15.4 GHz). Complete radio spectra were derived, when possible, from the GHz to the sub-THz range of frequencies. The selected bursts were compared to GOES soft X-ray bursts, and at other space or ground-based observations, when available. Preliminary results have shown that 5 out of 13 bursts exhibited 0.4 THz fluxes larger than 0.2 THz; 2 impulsive bursts were observed at 0.4 THz only. The observed 0.4 THz fluxes ranged from 50-200 SFU; all sub-THz bursts were associated to 9 GOES M-class and to 4 X-class bursts. Two of these bursts were also observed at 30 THz, exhibiting fluxes 1-2 orders of magnitude larger than at microwaves, mm-w and sub-THz frequencies.