The time behaviour of the continua during the initial stage of type IV bursts

The existence of a group of broad-band continua during the initial stage of some type IV bursts can be shown which are polarized in the extraordinary sense. Unfortunately interferometric records were available only for one of these continua. In this case no movement of the radiation source could be stated during the whole of its lifetime. It can be shown that the time behaviour of the broad-band continua is clearly different from that of the moving type IV bursts recorded directly by interferometers. Therefore, it must be concluded that both types of continua originate from different physical processes at the Sun, though both are caused by synchrotron radiation.Based on the author's thesis in Berlin.     

Pulsating type IV solar radio bursts

Several models for pulsating type IV radio bursts are presented based on the assumption that the pulsations are the result of fluctuations in the synchrotron emission due to small variations in the magnetic field of the source. It is shown that a source that is optically thick at low frequencies due to synchrotron self-absorption exhibits pulsations that occur in two bands situated on either side of the spectral peak. The pulsations in the two bands are 180° out of phase and the band of pulsations at the higher frequencies is the more intense. In contrast, a synchrotron source that is optically thin at all frequencies and whose low frequency emission is suppressed due to the Razin effect develops only a single band of pulsations around the frequency of maximum emission. However, the flux density associated with the later model would be too small to explain the more intense pulsations that have been observed unless the source area is considerably larger than presently seems reasonable.     

Transient magnetic disturbances from solar flare explosions and associated pulsations of type IV radio bursts

We have proposed a mechanism of arise of transient magnetic disturbances from solar flare explosion which can lead to understanding of observed pulsations of type IV radio emission with period of 0.3–3.0 s. According to the proposed mechanism the pulsation activity of the radio emission results from MHD waves accompanying the expanding diamagnetic plasma produced by the explosive flare material.     

Observations and interpretation of moving type IV solar radio bursts

Properties of 23 moving type IV bursts observed with the Culgoora Radioheliograph are summarized. Both shock and plasmoid models are examined. It is found that the theories invoking shocks have limited application and that plasmoid models have several problems with regard to plasmoid formation as well as with explanations for multiple sources and large values of circular polarization. While the synchrotron radiation mechanism is the most widely accepted for both shock and plasmoid models, it is possible that Langmuir wave emission processes may be important, at least in some events. To overcome some of the difficulties of the plasmoid theory, a new source model is proposed. This model involves synchrotron radiation from electror ; confined by rapid scattering through hydromagnetic wave particle interactions.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST-74-04129 with the National Science Foundation.     

Nonlinear wave coupling in type IV solar radio bursts

In order to explain a fine structure of parallel ridges in stationary type IV continua, the emission due to the coupling of electrostatic upper hybrid waves and Bernstein waves at the sum frequency of the upper hybrid and harmonics of the gyro frequency has been calculated. If the energy density of these electrostatic waves is of the order of 10^-3 times the thermal energy density, then the observed zebra pattern can be emitted by a region with a diameter of 10³ km.     

A coherent radiation mechanism for type IV solar radio bursts

A new coherent radiation mechanism, involving nonlinear interaction of whistler solitons with upperhybrid waves, excited by energetic electrons of energies of 10 keV–100 keV, is proposed for type IV solar bursts of both moving (type IV M) and stationary (type IV S) types. We show that the type IV M bursts occur when the interaction of whistler solitons and upperhybrid waves takes place in the coronal transients whereas the type IV S bursts originate provided this interaction takes place in stationary loops where density has been increased. The emitted radiation is right-hand circularly polarized with 100% polarization. Increase of brightness temperature, T _b , at lower frequencies and also its decrease, at all frequencies, with the passage of time is predicted for type IV M bursts; this agrees fully with the observations. Furthermore, the decrease of T _b , with time for stationary type IV component, is easily explained if the source which supplies energetic electron to the loop, becomes weaker with time.     

A coherent radiation mechanism for type IV dm radio bursts

An interpretation is presented of the decimetric type IV continuum with fine structure on March 6, 1972 and of the corresponding source region, in terms of ?erenkov plasma radiation and alternatively of synchrotron radiation, both in case of coherent and incoherent generation. If the magnetic field strength in the source region is a few gauss, in a stationary situation a loss cone instability develops which generates electron plasma waves coherently. The amount of energetic electrons required for consecutive induced scattering of the plasma waves at the thermal ions into electromagnetic waves is less than in case of synchrotron radiation. It is concluded that the former mechanism provides the explanation of type IV continua with fine structure such as intermediate drift bursts and sudden reductions of the continuum level.     

The generation of solar type II radio bursts in coronal magnetic loops

It is shown that the existing theory of type II bursts, based on a model of the emission from the shock wave front, has difficulties when compared with observational data. We suggests a new model for type II bursts. According to this model, in an expanding magnetic loop a cluster of energetic electrons acts to excite the cyclotron instability of plasma waves. The waves are excited on surfaces where the cyclotron resonance condition is satisfied, and are then transformed into electromagnetic emission by merging. Our proposed model may be useful to explain some observational facts, such as the narrow-band character of the emission and the space-time relationship between the harmonics. Some tests to check the validity are proposed.     

A model for the coherent emission of solar radio moving type IV bursts

A theoretical model is proposed for interpreting the coherent emissionmechanism of solar radio moving type IV bursts. Energetic electrons produced in flares captured by an expanding and rising magnetic flux tube exhibit a beam-like distribution of velocities on the top of the flux tube. These excite beaming plasma instability and directly amplifies O-mode electromagnetic waves. The instability growth rate sensitively depends on the coronal plasma parameter, ƒ_pe/ƒ_ce and the beam-temperature T_b. This can qualitatively explain the high brightness temperature and high degree of polarization as well as the broad spectrum observed in this type of solar radio bursts.     

The time-latitude distribution of solar flares accompanied by type IV radio bursts during the period 1956 to 1969

A list of nearly 350 flares accompanied by type IV radio bursts by Krüger et al. (1971), which covers a period of 14 yr (1956–1969), was expanded to include all PCA and solar cosmic ray events during this entire period. This list, which includes practically all of the most energetic events during the maxima of two consecutive solar cycles, was used to investigate the latitudinal distribution of the above-mentioned flares, as well as of all PCA events, solar cosmic ray events and plage regions associated with them.Histograms of these occurences show clearly the appearance of two peaks in both solar maxima, which confirm the observations of Gnevyshev (1967). Latitudinal analysis of these histograms shows that in cycle 20 the two peaks are independent and their relative strength varies strongly with latitude. In cycle 19, however, this effect is not clearly evident, possibly because of the extremely high level of activity during this cycle. In both cycles, the second maximum shows the highest concentration of the most energetic events.During 1971–1972 visiting Professor of Astrophysics at the National University of Athens, Athens, Greece.     

Beat structure in pulsating type IV solar radio bursts and a possible mechanism

Pulsating type IV solar radio bursts with beat structure are presented and analysed in this paper. Based upon the theory of whistler soliton emission we interpret the beat structure by the combination of two components with different pulsation frequencies due to radial oscillations of two legs of the magnetic loop. The large depth of pulsation is also explained in this model.Proceedings if the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.On leave from the Department of Astronomy, Nanjing University, Nanjing, The People's Republic of China.     

A moving Type IV radio burst and its relation to the coronal magnetic field

A moving Type IV burst, observed with the Culgoora radioheliograph on 1970 April 29, moved out to about 3 R and attained high circular polarization before fading. The appearance of the moving Type IV source suggests an isolated, self-contained, synchrotron emitting plasmoid. Magnetic field maps of the corona derived from photospheric observations indicate that the plasmoid moved almost radially outward from the flare region along open field lines. To explain the observed source structure and high unipolar polarization, we suggest that a ring of electric current was ejected from the low corona and guided by coronal magnetic field lines; the radio emission was synchrotron radiation generated by mildly-relativistic electrons trapped in the poloidal magnetic field of the ring current.Part of the research reported here was carried out while the author was at the Division of Radiophysics, C.S.I.R.O., Sydney, Australia.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Spotless flares and type II radio bursts

Flares accompanied by type II meter radio bursts that occurred in plages with no visible spots are examined in this paper. There have been found 12 such spotless flares observed in the period of January 1981–August, 1990. Six out of all the flares may be said to have not been associated with any filament activation or disruption. A few of these flares have shown features of major events. The study suggests that a filament activation seems not to be the crucial factor for the occurrence of major flares in regions with no visible spots.     

The third harmonic of type III solar radio bursts

The harmonic ratios of a large sample of inverted-U bursts are found to be smaller at the turning frequency than at the starting frequency. Ratios <2.0 are explained by postulating that the lowest fundamental frequencies emitted are prevented from escaping from the corona by an evanescent region between the source and the observer. This concept is used to construct a source model for inverted-U bursts where the density is lower inside a magnetic flux tube than it is outside.     

Pulsations of type IV radio bursts as an indicator of protonarility of solar flares

The analysis of observational data has shown that the duration of a pulse train in type IV radio bursts decreases with increasing hardness of the spectrum of high-energy protons and increases with decreasing proton fluxes from the Sun. It is shown that such a correlation corresponds to a magnetohydrodynamic (MHD) model of pulsations and is inexplicacable within the framework of a nonlinear periodical regime of plasma instabilities. The pulse train duration is determined by proton pitch-angle diffusion caused by Alfvén waves in coronal magnetic loops. A method of predicting solar proton hardness and proton fluxes using type IV radio burst pulsations is proposed.     

Circulation changes in the free atmosphere during proton events associated with type IV radio bursts

New evidence of statistically significant circulation changes in the lower stratosphere and in the middle and upper troposphere following Type IV Proton and Ground Level Events confirms some earlier findings of a relationship between solar activity and weather. The maximum 24-hr circulation changes appear to occur in the North Pacific-Alaskan area. A comparison between the effects that followed the flare sample used in this study with earlier results reported for less intense solar particle emission flare samples, implies that if these particles are responsible for triggering the atmospheric circulation in times of enhanced solar activity, their atmospheric effect is enhanced as we move to the more energetic solar proton events. The physical explanation of the results may possibly be found in numerical-experimental models. Some possible causes of the results are discussed.     

Polarization of fundamental type III radio bursts

The fundamental of type III bursts is only partially polarized, yet all theory for emission near the plasma frequency predicts pure o-mode emission. I argue depolarization is inherent in the burst itself. The o-mode radiation is intensely scattered and mode-converted when it temporarily falls behind its own source and finds itself in the medium that is already disturbed by the electron beam. In particular, mode conversion is very efficient and yet causes only modest angular scattering at the height were _p + 0.5.The predicted minimum polarization nearly equals the polarization of the harmonic, as observed. Spike polarization is naturally explained by the earlier arrival of the scattered o-mode. Additional residual polarization depends on the refraction at the site of emission; larger beam velocities imply higher polarization, as observed, because a larger fraction of the radiation escapes without mode-conversion. The polarization at the frequencies where U-bursts reverse is of particular interest.Support is acknowledged from the NSF Solar-Terrestrial Research Program.     

Observation of the magnetic structure of a type IV solar radio outburst

A continuous record of the 80 MHz image and polarization of a type IV solar outburst has been made with the Culgoora radioheliograph from which the magnetic structure of the event can be directly inferred. The first (‘moving’) part of the event appears beyond the limb as an expanding magnetic arch along which three concentrated sources develop: one unpolarized source near the peak, attributed to synchrotron radiation; and two polarized sources of opposite polarity near the feet, attributed to plasma radiation. The radio-emitting arch appears to lie above an eruptive prominence seen in Hα. The second (‘stationary’) part is seen later as a separate highly polarized source on the disk above the projected position of the flare that had previously triggered the prominence activity.     

Polarization features of type IV bursts

Using both the polarization records of our institute at seven different frequencies and polarization records from other stations, the spectral diagrams of some important type IV bursts are completed by polarization diagrams. Combining both types of diagrams and adding the results of optical observations and X-ray data it is possible to come to a deeper understanding of the processes taking place during strong solar radio bursts.     

Homologous type II radio bursts and coronal transients

The homology of seven successive type II solar radio bursts, which occurred at the times of flares from an active region near the solar west limb on 1980, July 27–29, is described, together with evidence for coronal mass outflows accompanying these bursts. It is argued that homologous type II bursts imply that the corona is restructured in a similar manner by successive coronal transients.