Polarization changes with time during solar microwave impulsive bursts

The evolution with time of circular polarization (t) from solar bursts at 7 GHz presents, in the majority of cases, a polarization degree peak before the maximum flux time. The subsequent evolution of (t) is continuous and usually increasing. The changes could be caused by superimposed polarization effects, due to the fast emissive electrons (dominant in the first phase), and to the propagation effects caused by the coronal condensation where the event occurred (dominant in the second phase). In an approximate approach, (t) is connected to the movement of the source in the second phase, being qualitatively sound, but limited to the lack of knowledge on acceleration processes and on magnetic field topology in the active region where the flares take place.     

Unpolarized impulsive solar bursts observed at 7 GHz

The occurrence of very faintly polarized, or unpolarized impulsive bursts observed at 7 GHz is discussed. It appears that some of them show a peculiar spectral peak somewhere between 5 GHz and 7 GHz. Possible interpretations are suggested, emphasizing the need to associate to the burst the state of polarization of the S-component in which it occurred.     

Diffuse Component Spectra of Solar Active Regions at Submillimeter Wavelengths

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 × 10⁴ K, for source diameters of 2′–7′.     

Time delays in solar bursts measured in the mm-cm range of wavelengths

Various solar bursts have been analysed with high sensitivity (0.03 sfu, rms) and high-time resolution (1 ms) at two frequencies in the millimeter wave range (22 GHz and 44 GHz), and with moderate time resolution (100 ms) by a patrol telescope at a frequency in the microwave range (7 GHz). It was found that, in most cases, burst maximum emission is not coincident in time at those frequencies. Preceding maximum emission can be either at the higher or at the lower frequency. Time delays ranged from about 3 s to near coincidence, defined within 10 ms. Some complex bursts presented all kinds of delays among different time structures, and sometimes nearly uncorrelated time structures.Large time delays favour the association of the dynamic effects to shock wave speeds. Directional particle acceleration in complex magnetic configuration could be considered to explain the variety of the dynamic effects. Fastest burst rise times observed, less than 50 ms at 44 GHz and at 22 GHz, might be associated to limiting formation times of emission sources combined with various absorption mechanisms at the source and surrounding plasma.In memoriam, 1942–1981.INPE operates Itapetinga Radio Observatory and CRAAM.     

Analysis of the impulsive phase of a solar flare at submillimeter wavelengths

We present a report on the strong X5.3 solar flare which occurred on 25 August 2001, producing high-level γ-ray activity, nuclear lines and a dramatic long-duration white-light continuum. The bulk of millimeter radio fluxes reached a peak of ∼100 000 solar flux units at 89.4 GHz, and a few thousands of solar flux units were detected in the submillimeter range during the impulsive phase. In this paper we focus on and discuss (i) the implications inferred from high frequency radio observations during the impulsive phase; (ii) the dynamics of the low corona active region during the impulsive phase. In particular we found that 4–5 × 10³⁶ accelerated (>20 keV) electrons s⁻¹ radiating in a 1000–1100 G region, are needed to explain the millimeter to submillimeter-wave emissions. We present evidence that the magnetic field in the active region was very dynamic, and that strong non-thermal processes were triggered by the appearance of new, compact, low-lying (few thousand kilometers) loop systems, suggesting the acceleration site(s) were also located in the low solar atmosphere.     

The use of the large MM-wave antenna at Itapetinga in high-sensitivity solar research

Large dishes used in solar radio astronomy are becoming an essential tool for the analysis of low level activity and fine time structures in solar bursts. Some front-end and back-end arrangements have been added to the Itapetinga 13.7-m radome-enclosed antenna to allow for simultaneous 22 GHz and 44 GHz observations; 22 GHz right- and left-handed circular polarization (or two linear orthogonal), with sensitivities of the order of 0.03 s.f.u., and time resolution of 1 ms. Full Sun maps can be obtained every 6 min, and selected active region maps every 3 min. Spatial angular definition of positions of active-region hot spots is close to 10 arc sec. This system is being used in a number of specific investigations, in SMM satellite related research, and in other internationally coordinated works. Examples of results are shown.In memoriam, 1942–1981.     

Five minute microwave solar oscillations

Oscillations with a period of 5.6 min were observed on 10 July, 1978 while tracking at 22 GHz the active region McMath 15403. The oscillations were strong, clearly defined, had no damping, and lasted for about two hours. The rarity of the phenomenon is indicated by the fact that it occurred only once in more than 250 hr of solar observations. The possibility that these oscillations are due to a standing Alfvén wave driven by the photospheric velocity field is discussed.On sabbatical leave from Technion, Haifa, Israel.Formerly: Centro de Radio-Astronomia e Astrofísica Mackenzie, now with Brazilian National Research Agency CNPq, National Observatory.