Solar flare energy manifestations were believed to be the result of the same kind of particle acceleration. It is generally accepted that a population of relativistic electrons accelerated during the impulsive phase of solar flares produces microwaves by synchrotron losses in the solar magnetic field and X-rays by collisions in denser regions of the solar atmosphere. We report the discovery of a new intense solar flare spectral radiation component, peaking somewhere in the shorter submillimeter to far-infrared range, identified during the 2003 November 4 large flare. The new solar submillimeter telescope, designed to extend the frequency range to above 100 GHz, detected this new component with increasing fluxes between 212 and 405 GHz. It appears along with, but is separated from, the well-known gyrosynchrotron emission component seen at microwave frequencies. The novel emission component had three major peaks with time, originated in a compact source whose position remained remarkably steady within 15Љ. Intense subsecond pulses are superposed with excess fluxes also increasing with frequency and amplitude increasing with the pulse repetition rate. The origin of the terahertz emission component during the flare impulsive phase is not known. It might be representative of emission due to electrons with energies considerably larger than the energies assumed to explain emission at microwaves. This component can attain considerably larger intensities in the far-infrared, with a spectrum extending to the white-light emission observed for that flare.

Abstract. An atmospheric model was constructed in order to reproduce quantitatively the observations at 17 GHz from Nobeyama Radio Heliograph, namely the brightness temperature at disk center (from 1.4 to 400 GHz), center-to-limb brightening distribution, and radius derived from 17 GHz solar maps. The two dimensional solar atmospheric model, that takes into account the curvature of the Sun, includes spicules, which physical characteristics (such as size, temperature, density, position, and inclination angle) were randomly attributed. After the interferometer instrumental response is taken into account, the results showed than an atmospheric model without spicules produces 36% of limb brightening, approximately the value observed at the solar poles. However, the inferred solar radius from the model (970 ) was 6 smaller than the mean value derived from the solar maps. An improvement of the model is made by including spicules. Results from this upgraded model showed that depending on their physical parameters, limb brightening and solar radius values are obtained in agreement with the radio observations (except for polar regions).

We investigate the origin of the increasing spectra observed at submillimeter wavelengths detected in the flare on 2 November 2003 starting at 17:17 UT. This flare, classified as an X8.3 and 2B event, was simultaneously detected by RHESSI and the Solar Submillimeter Telescope (SST) at 212 and 405 GHz. Comparison of the time profiles at various wavelengths shows that the submillimeter emission resembles that of the high-energy X rays observed by RHESSI whereas the microwaves observed by the Owens Valley Solar Array (OVSA) resemble that of ∼50 keV X rays. Moreover, the centroid position of the submillimeter radiation is seen to originate within the same flaring loops of the ultraviolet and X-ray sources. Nevertheless, the submillimeter spectra are distinct from the usual microwave spectra, appearing to be a distinct spectral component with peak frequency in the THz range. Three possibilities to explain this increasing radio spectra are discussed: (1) gyrosynchrotron radiation from accelerated electrons, (2) bremsstrahlung from thermal electrons, and (3) gyrosynchrotron emission from the positrons produced by pion or radioactive decay after nuclear interactions. The latter possibility is ruled out on the grounds that to explain the submillimeter observations requires 3000 to 2 × 10 5 more positrons than what is inferred from X-ray and γ -ray observations. It is possible to model the emission as thermal; however, such sources would produce too much flux in the ultraviolet and soft X-ray wavelengths. Nevertheless we are able to explain both spectral components at microwave and submillimeter wavelengths by gyrosynchrotron emission from the same population of accelerated electrons that emit hard X rays and γ rays. We find that the same 5 × 10 35 electrons inferred from RHESSI observations are responsible for the compact submillimeter source (0.5 arcsec in radius) in a region of 4500 G low in the atmosphere, and for the traditional microwave spectral component by a more extended source (50 arcsec) in a 480 G magnetic field located higher up in the loops. The extreme values in magnetic field and source size required to account for the submillimeter emission can be relaxed if anisotropy and transport of the electrons are taken into account.

Aims. We construct a 3D solar atmospheric model to reproduce active region brightness temperature of radio observations. Methods. A 3D solar atmospheric model was developed to reproduce the radio observations at 17 and 34 GHz from the Nobeyama Radioheliograph. The model included bremsstrahlung and gyro-resonance emission mechanisms. Both potential and force-free magnetic field extrapolations from MDI magnetograms are considered, as well as the changes in the quiet Sun atmosphere (density and temperature distributions) due to the magnetic field interaction. We analyze a highly polarized active region at 17 GHz (85 ± 5% LHCP) observed 2002 June 25 (NOAA 10008). Results. Modeling of this region requires that the whole projected region between the magnetic field footpoints changes its atmospheric density and temperature constitution. The modeling at 17 GHz showed the following results: a) the intensity of the corrected MDI magnetograms is able to account for the emission as gyro-resonance, but, the problem with saturation points still persists; b) for a low number of saturation points, a simple linear correction can change the maximum brightness temperature results in the simulations completely; c) the brightness temperature maxima resulting from the linear correction in the magnetic field intensities reproduced all maxima observed during the day (1.14-1.76 × 10 5 K); and d) the spatial brightness distribution of the gyroresonance emission of NOAA 10008 was well-reproduced either by a potential or force-free field extrapolation with low positive α (0.70-1.10 × 10 −2 Mm −1 ). At 34 GHz, the emission was successfully modeled as completely free-free radiation with a brightness temperature maximum in agreement with the observations. Conclusions. In summary the model is able to account for the observations at both frequencies, which are, however, produced by distinct emission mechanisms.