Diffuse Component Spectra of Solar Active Regions at Submillimeter Wavelengths

Abstract

Solar maps at 212 and 405 GHz obtained by the Solar Submillimetric Telescope (SST) show regions of enhanced brightness temperature, which coincide with the location of active regions. A statistical study of the radio emission from these active regions was performed for the first time at such high frequencies during 23 days on June and July 2002, when the atmospheric opacity was low. The brightest regions on the maps were chosen for this study, where the brightness excess observed varies from 3 to 20% above quiet Sun levels (i.e., 200 -1000 K) at both wavelengths. Sizes of the regions of enhanced emission calculated at half the maximum value were estimated to be between 2 and 7 . These sizes agree with observed sizes of active regions at other wavelengths such as Hα and ultraviolet. An important result is that the flux density spectra of all sources increase toward submillimeter frequencies, yielding flux density spectral index with an average value of 2.0. The flux density of the active region sources were complemented with that from maps at 17 and 34 GHz from the Nobeyama Radio Heliograph. The resulting spectra at all four frequencies were fit considering the flux density to be due to thermal bremsstrahlung from the active region. In the calculations, the source radius was assumed to be the mean of the measured values at 212 and 405 K. The effective temperatures of the radio emitting source, assumed homogeneous, obtained from this fit were 0.6 -2.9 × 10 4 K, for source diameters of 2 -7 .

SUB-THz AND Hα ACTIVITY DURING THE PREFLARE AND MAIN PHASES OF AGOESCLASS M2 EVENT

“…These values are larger than the corresponding 7 GHz long-enduring component (∼ 10 SFU). This result may suggest the presence of two thermal sources, one hot at microwaves and another cold at sub-THz, as suggested by Trottet et al (2011), and (for quiescent active regions) by Silva et al (2005). On the other hand, the impulsive microwave radio burst exhibiting peak spectral emission at about 7 GHz is typical for most of flares of this size and is probably caused by gyrosynchrotron emission of mildly relativistic electrons (Dulk 1985).…”

“…At the lower ≤ 0.2 THz frequencies the radio emission is reduced, either by the synchrotron emission mechanism (self absorption) and/or associated plasma effects (Razin suppression). Another alternative is that there might be a rapid heating process linked to the Hα fast flash, lower in the solar atmosphere, where the high electron density profile is such that it is optically thin at 0.4 THz and thick at 0.2 THz (as suggested for different scenarios, and slower processes, by Silva et al 2005, andTrottet et al 2011). In this particular case, the high energy process would be confined to a short interval of time, producing the sub-THz spike-like burst, probably occurring in a small region delimited by the first small Hα flash.…”

New telescopes for ground-based solar observations at submillimeter and mid-infrared

“…Although their presentation is beyond the scope of this paper, it is relevant to mention the detection of burst superimposed subsecond pulsations, with repetition rates time profiles well correlated to fluxes, as well to hard X-and -rays time profiles [25][26][27][28] ; the submm-wave pulsations onset association to the launch time of Coronal Mass Ejections 29 ; the excess brightness of quiescent active centers for the higher SST frequency 30 ; and the discovery of a new burst spectral component extending into THz frequencies spectrally separated from the well known microwave spectra [31][32][33] (see example in Fig. 8).…”

The 6 September 2017 X9 Super Flare Observed From Submillimeter to Mid‐IR

“…Between 11:56:30 and 11:58:00 UT, the bursting area was unresolved, that is, it remained within the e −2 width region of the beams. Maps are calibrated against the quiet Sun, adopting a brightness temperature 5900 and 5100 K for 212 and 405 GHz, respectively (Silva et al, ).…”