Fine structure of solar radio bursts at meter wavelengths: A survey

Conclusions Our survey indicates that diverse fine structure is an important (and frequently decisive) property of many radio bursts. As far as we can judge from the available data, however, this type of structure is hardly observed in every phenomenon. One of the major tasks for future research, therefore, is to determine the specific conditions over the active regions that result in the appearance of particular types of fine structure.In particular, the solution of this problem calls for combining observations with a high degree of time and frequency resolution with instrument observations with high angular resolution, which are capable of localizing sources of fine structure within the limits of burst generation regions.Information on the specific conditions under which fine structure arises will help to formulate more definite opinions about possible generation mechanisms both of the fine structure itself and of the associated radio bursts. This information is also necessary in order to link conceptions of radio-burst fine structure with the overall development pattern of solar activity and, in particular, with the phenomena that accompany bursts as a whole.Institute of Terrestrial Magnetism, the Ionosphere, and Radio Wave Propagation, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 20, No. 9, pp. 1255–1301, September, 1977.     

Diagnostics of the solar corona structure by the refraction effects on radio emission

The solar disc radio brightness distribution observed at RATAN-600 can be interpreted as the angular directivity of the disk center emission at viewing anglesΘ=arcsin(R/R₀) over an angular range of R<0.9R ₀ at several wavelengths in the 2.0–32 cm region. These spectra can be treated as an emission enhancement due to refraction effects in free-free corona emission. The model simulations show good agreement with the observations and lead to estimation of the electron density N_e=1·10 ⁹ cm^?3 and emission measureEM=6·10 ²⁶ cm^?5 for the quiet Sun.     

Polarization of radio emission of the turbulent plasma of solar flares

We present a mechanism for the appearance of circular polarization due to the scattering of electromagnetic radiation by the turbulent pulsations of a plasma with a magnetic field. The application of this mechanism to the turbulent plasma of solar flares is discussed.     

Interpretation of harmonic structure in solar type II radio bursts

Spectrographic and partly imaging observations of three Type II solar radio bursts displaying three drifting bands with frequencies related as 1¬2¬3 are discussed. The radio data of two of these events were simultaneousely recorded by the digital radiospectrograph of the Observatory of Solar Radioastronomy in Potsdam-Tremsdorf and the radioheliograph of the Paris-Meudon Observatory in Nançay. The data allow the brightness temperatures of radio emission in the three frequency bands to be determined. The second harmonic is traditionally explained as a result of coalescence of two plasma waves into an electromagnetic wave at twice the plasma frequency. Two nonlinear merging processes—the coalescence of three plasma waves, and of a plasma wave and an electromagnetic wave at twice the plasma frequency—are considered to explain the occurrence of the third harmonic on Type II dynamic spectra. The analysis shows that both processes can fit the observed brightness temperatures of the second and third harmonic. The first process acts preferably at low phase velocities of plasma waves and sharp electron density gradients in the source, and the second process dominates in the case of high plasma wave phase velocities. It is shown that the occurrence of the third harmonic in type II bursts due to nonlinear processes in the coronal plasma indicates not only a powerful event but also some specific conditions in the shock or foreshock region. Finally, we propose a method to distinguish between the two invoked nonlinear processes by a statistical investigation of Type II burst data.     

Interpretation of harmonic structure in solar type II radio bursts

Spectrographic and partly imaging observations of three Type II solar radio bursts displaying three drifting bands with frequencies related as 1?2?3 are discussed. The radio data of two of these events were simultaneousely recorded by the digital radiospectrograph of the Observatory of Solar Radioastronomy in Potsdam-Tremsdorf and the radioheliograph of the Paris-Meudon Observatory in Nan?ay. The data allow the brightness temperatures of radio emission in the three frequency bands to be determined. The second harmonic is traditionally explained as a result of coalescence of two plasma waves into an electromagnetic wave at twice the plasma frequency. Two nonlinear merging processes—the coalescence of three plasma waves, and of a plasma wave and an electromagnetic wave at twice the plasma frequency—are considered to explain the occurrence of the third harmonic on Type II dynamic spectra. The analysis shows that both processes can fit the observed brightness temperatures of the second and third harmonic. The first process acts preferably at low phase velocities of plasma waves and sharp electron density gradients in the source, and the second process dominates in the case of high plasma wave phase velocities. It is shown that the occurrence of the third harmonic in type II bursts due to nonlinear processes in the coronal plasma indicates not only a powerful event but also some specific conditions in the shock or foreshock region. Finally, we propose a method to distinguish between the two invoked nonlinear processes by a statistical investigation of Type II burst data. Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia; Astrophysical Institute, Potsdam, Germany; Paris-Meudon Observatory, France. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 1, pp. 61–83, January, 1998.     

Fine structure in 14C cluster emission from 225Ac

A fine structure in the ¹⁴C decay of ²²⁵Ac is predicted quantitatively by accounting dynamical aspects during the disintegration process. Transitions to the excited states of the daughter nucleus are considered to be mainly directed by the Landau–Zener promotion mechanism in the region of avoided crossing levels. The level scheme is evaluated with the superasymmetric two–center shell model. The half–lives are computed considering the cluster decay as a superasymmetric fission process. Received: 31 July 1998 / Revised version: 27 November 1998     

Quasiperiodic structure of solar type IIId radio bursts with echo components

By solar observations with an UTR 2 antenna, we detected a new variety of type IIId radiation in which the narrow-band spectral elements have an explicit quiasiperiodic structure and contain an additional component in the form of a triple echo. According to amplitude records at 25 MHz, the initial short radio burst looks like a weak precursor of a more powerful echo-like burst accompanied by a pair of final pulses which exceed slightly the background. Nine events of this kind were registered in August 19, 1992 during a type IIId noise storm related to the active region in the central heliolongitude sector. In each event, all the three time intervals between successive peaks of the complex amplitude-time profile were almost identical and were equal to 4.7, 5.1, and 5.7 s on the average. They were somewhat (by 1 or 2 s) shorter than the maximum delay in the known events with a two-hump amplitude profile and time-split narrow-band spectral elements in which the delay t₂−t₁ and relative intensity (I₂/I₁≲1) of echo-like bursts depend on the heliolongitude of the type IIId sources. The discovery of a new complex variety of decameter type IIId radiation confirms the hypothesis of multiple quiasiperiodic echoes after short radio bursts at the second harmonic of the local plasma frequency in the solar atmosphere. Radioastronomical Institute, Kharkov, Ukraine; Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 9, pp. 1073–1085, September, 1997.     

Fine structure of emission spectrum of a single-frequency injection laser

A theoretical analysis of single-frequency lasing is carried out in the framework of the quasimonochromatic field method with variable field amplitude and phase. The spectral optical-line profile, the spectral distribution of the laser output-power fluctuations, and the spectral distribution of the active-region density fluctuations (of the p-n voltage fluctuations) are obtained. In contrast to the known work in this field, the analysis in the present paper is carried out consistently starting with the wave equation and with account taken of the wavelike character of the radiation propagation in the active region. This has made it possible to express the final distributions in terms of quantities that are most readily measurable. Allowance for thermodynamic fluctuations has revealed an additional lasing-line broadening independent of the laser power. The analysis is not confined to lasing in the diode's own cavity, but takes into account also the possibility of using an external cavity, and corresponding varying the optical line spectral distributions, the output-power fluctuations, and p-n junction voltage fluctuations.Optoelectronics Laboratory, Lebedev Physics Institute Academy of Sciences of the USSR. Translated from Preprint No. 256 of the Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1988.     

Fine structure on the excitonic emission in AgI nanoparticles embedded in silica glass

Stable photoluminescence (PL) from AgI nanoparticles embedded in silica glass was investigated at room temperature. The Z_1,2 excitonic emission of AgI exhibits fine structure with spacing of ∼0.20 eV (1610 cm⁻¹), which is assigned to the frequency of vibration in interfacial water species. The PL excitation spectrum displays two newly observed bands at 3.45 and 4.35 eV associated with AgI-silica interaction. We suggest that the excitons in AgI are localized in the AgI/SiO₂ interface region before radiative recombination.     

Fine structure in proton emission from 145Tm discovered with digital signal processing

Fine structure in proton emission from the 3.1(3) mus activity of 145Tm was discovered by using a novel technique of digital processing of overlapping recoil implantation and decay signals. Proton transitions to the ground state of 144Er and to its first excited 2(+) state at 0.33(1) MeV with a branching ratio I(p)(2(+))=9.6+/-1.5% were observed. The structure of the 145Tm wave function and the emission process were analyzed by using particle-core vibration coupling models.