Interference origin of the fine structure in the dynamic spectra of solar radio bursts

The multibeam propagation of radio waves in the solar plasma is analyzed, because the emission from a solar flare passes through an inhomogeneous solar atmosphere on its way to the observer. A formula (a mathematical model) for calculating the structure of the dynamic spectrum for flare radio bursts has been obtained. Comparison of the calculated spectra with the observed ones shows that the results of interference explain the formation of a zebra structure and the separation of its stripes into individual spikes, describe the time profile of the spikes, and explain the properties of fibers, ropes of fibers, and chains of “point” bursts. The similarity of the dynamic spectra testifies that the fine structure of the spectra is formed not in the emission source but as a result of the propagation of waves through the solar corona and interplanetary space.     

Frequency separation in structure of solar continuum radio bursts

A plot of frequency separation in fine structure in solar continuum radio bursts against emission frequency indicates that the frequency structure cannot represent local proton plasma frequency modulation. However, the observations are consistent with the interpretation of the frequency structure as harmonics of the local electron cyclotron frequency and lead to reasonable estimates of the ratio between magnetic and kinetic pressures in stable coronal magnetic field configurations producing continuum radio sources.     

Gyro-resonance absorption of plasma waves in the corona and the fine structure of solar radio bursts

We consider the important part played by plasma wave absorption at the higher harmonics of the electron gyrofrequency (under conditions of the so-called double plasma resonance) in the solar corona. An explanation is proposed for the fine structure of Type III bursts on the basis of this effect.     

Interpretation of fast ripple structure in solar impulsive bursts

The Helios spacecraft zodiacal light photometers are used to observe the earthward-directed solar mass ejection transient of 27 November, 1979 described by Howard et al. (1982) that completely circles the Sun in coronagraph observations. At this time, Helios B was situated 30° east of the Sun-Earth line at 0.5 AU. The brightness increase moved outward directly along the Sun-Earth line over a period of approximately 24 hr, indicating a strong collimation of the ejection. The outward motion and mass estimates of the ejected material from the photometers compared with near-Earth observations from IMP spacecraft show that at least a portion of the density increase observed at Earth on 29 and 30 November was associated with this ejection.     

Time structure of solar decametre type III radio bursts

The time structure of solar radio decametre Type III bursts occurring during the periods of enhanced emission is investigated. It is found that the time profiles can take a variety of forms of which three distinct types are the following: (1) profiles where the intensity rises to a small but steady value before the onset of the main burst, (2) the intensity of the main burst reduces to a finite level and remains steady before it decays to the base level, (3) the steady state is present during the rise as well as the decay phase of the main burst. It is shown that these profiles are not due to random superposition of bursts with varying amplitudes. They are also probably not manifestations of fundamental-harmonic pairs. Some of the observed time profiles can be due to superposition ot bursts caused by ordered electron beams ejected with a constant time delay at the base of the corona.     

Flux-rms relation in solar radio bursts

One recent discovery that provides a strong constraint on the mechanisms of astrophysical activities is the correlation between the flux and the root-mean-squared (rms) variability of X-ray emission. In this work we study the flux-rms relation of solar radio bursts. Four flares observed by the Solar Radio Broadband Spectrometer (SRBS) of China are analyzed. In these flares, fine structures (FSs) emerge at least in one frequency band of SRBS. We find that the flux-rms relation consists of two components. One relates to the non-FS emission and the other to the FS emission. The flux-rms relationship for the non-FS part of the radio bursts is clearly different from that for the FS part. The former shows a curve-like behavior, while the latter shows a dramatic variation. We propose a model to describe the flux-rms relation of the non-FS part. Our results imply that the non-FS part emission could be triggered by some multiplicative processes. On the contrary, multiplicative mechanisms should be excluded from the explanations of FSs in the radio bursts.     

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.     

Plasma emission and the decimetre portion of type-IV solar bursts

An attempt is made to account for the decimetre portion of the Type-IV solar radio bursts by plasma emission. Non-thermal electrons (E ～ 500 keV) trapped in a magnetic mirror (IV_dm, burst source) having loss-cone gap distribution excite plasma waves which are transformed into transverse waves through non-linear scattering by ions. A good agreement was reached between the calculated spectrum and the observed fluxes for the event of 1972 August 2. A distribution of the number of non-thermal electrons with height, and a total number of 10³², were obtained. Also it was found that the Langmuir waves can accelerate some background thermal electrons to the MeV range.     

Observations of fine time structure in solar flare hard X-ray bursts

Hard X-ray (?100 keV) time histories of solar flares which occurred on 1978 December 4 and 1979 February 18 are presented. The first flare was observed by 3 identical instruments from near-earth orbit (Prognoz 7) and interplanetary space (Venera 11 and 12). Fine time structure is present down to the 55 ms level for the e-folding rise and fall times. These data may be used to localize the emission region by the method of arrival time analysis.     

Energy of solar noise-storm radio bursts

Solar noise storms (NS) are analyzed by an algorithm which separates a random signal into pulses. The burst duration distribution is shown to be inversely proportional to the squared duration of bursts. The distribution ordinates are proportional to the average pulse repetition frequency, and the distribution maximum corresponds to the limiting pulse duration equal to 0.4–0.6 s. The aggregate lifetime of all short-lasting bursts is approximately equal to the aggregate lifetime of bursts of any other duration. The energy of short-lasting bursts with a duration of 0.2–0.4 s is five times smaller than the energy of longer bursts, and it constitutes only 2–5 percent of the energy of the NS burst component. The power of bursts increases as their duration changes from 0.2 to 1.2 s until it reaches some limit at a duration of 1.2–1.4 s. The power of longer bursts remains almost unchanged up to the end of the investigated duration interval (up to durations of 300 s). Solar burst chains can be some superposition of short-lasting bursts on one longer burst. Thus, the burst energy measurements do not support the widespread point of view that solar noise storms consist of short-lasting type I bursts.     

Interplanetary phenomena and solar radio bursts

The relationship between solar radio emissions and transient interplanetary phenomena is reviewed. It is believed that the most significant advance in recent years has come from coordinated studies of coronal mass ejections and moving type IV bursts, where the evidence appears to favour the Langmuir wave hypothesis as the emission mechanism. Type II bursts are not generally a signature of the main energetic particle acceleration in flares. They do, however, occasionally propagate to 1 AU, and beyond, where they are normally accompanied by protons in the 20 MeV region. Apart from the impulsive microwave burst, there is no reliable radio signature associated with energetic particle acceleration in flares, although many phenomena have high correlations with radio emissions. The exceptions suggest that such correlations may be incidental. Therefore, it is concluded that attention should also be given to events with a positive absence of radio emission in order to make progress in understanding solar processes.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

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.     

The efficiency of electron acceleration in solar type-IV radio pulsations with a zebra pattern

Based on a comprehensive analysis of the October 25, 1994 event, we consider the balance of energetic particles in a type-IV solar radio emission source with a zebra-type fine structure (in a coronal magnetic loop). The zebra pattern is formed through the injection of fast electrons into a trap and the formation of a ring-type nonequilibrium electron distribution function. We estimated the characteristic zebra-pattern lifetime, which is determined by the escape of fast particles from the trap into the loss cone. In addition, we determined the number of fast particles that must be injected into the trap to provide the observed radio brightness temperature in zebra-pattern stripes by analyzing the plasma emission mechanism responsible for the zebra-pattern generation. As a result, we estimated the efficiency of the electron acceleration mechanism in coronal magnetic loops at the post-flare evolutionary phase of an active region.     

Harmonic components of decametric solar radio bursts

Type-IIIb, IIId, and III solar decametric radio bursts, being distinguished by the typical negative drift rate of their dynamic spectra, are compared. Observational data were obtained with a UTR-2 antenna during the period 1973–1982. During the analysis of the bursts of all these spectral varieties, the frequency drift time (drift delay) was measured in the ranges 25 to 12.5 MHz, 25 to 20 MHz, and 12.5 to 10 MHz. Durations of type-III bursts were determined at the harmonically-related frequencies of 25 and 12.5 MHz; radio source locations were also used.It is shown that these decametric bursts are distinctly divided into two groups: (1)type-IIIb chains of simple stria bursts and also normal type-III storm bursts observed at central regions constitute a group of events with a fast drifting spectrum; (2) type-III bursts from type-IIIb-III pairs and the limb variant of normal III bursts, as well as peculiar type-IIId chains of diffuse striae and related chains with an echo component, constitute a second group of events with comparatively slow drift rates.The first group of the phenomena is associated with the fundamental F frequency and the second one, with the harmonic H of the coronal plasma frequency. The results of the present investigation agree well with earlier conclusions on the harmonic origin of decametric chains and type-III bursts. Measurements of drift delays in narrow frequency ranges, an octave apart, as well as type-III burst durations at harmonically-related frequencies confirm the existence of both F and H components in the solar radiation. The essential result of 10 years of decametric observations is that the frequency drift rates and durations are rather stable parameters for the various type-III bursts and stria-burst chains. The stability characterizes some unspecified conditions of burst generation in the middle corona.     

The high-frequency characteristics of solar radio bursts

We compare the millimeter, microwave, and soft X-ray emission from a number of solar flares in order to determine the properties of the high-frequency radio emission of flares. The millimeter observations use a sensitive interferometer at 86 GHz which offers much better sensitivity and spatial resolution than most previous high-frequency observations. We find a number of important results for these flares: (i) the 86 GHz emission onset appears often to be delayed with respect to the microwave onset; (ii) even in large flares the millimeter-wavelength emission can arise in sources of only a few arc sec dimension; (iii) the millimeter emission in the impulsive phase does not correlate with the soft X-ray emission, and thus is unlikely to contain any significant thermal bremsstrahlung component; and (iv) the electron energy distributions implied by the millimeter observations are much flatter (spectral indices of 2.5 to 3.6) than is usual for microwave or hard X-ray observations.     

Towards a theory for type III solar radio bursts

The possibilities for type III burst excitors are reviewed and it is concluded that particle streams are the most likely excitor. Possible methods of resolving the apparent discrepancy between the number of particle events observed in interplanetary space in the vicinity of the earth and the number of type III bursts are indicated. Observations relevant to the excitor are reviewed and translated into requirements for a theory of the exciting stream. Possibilities for an electron stream excitor are considered and it is concluded that, while such an excitor cannot be eliminated at the present time, there are definitely theoretical difficulties with it which can be overcome only by seemingly ad hoc and improbable assumptions. Possibilities for a proton stream excitor are examined and it is found that all theoretical difficulties can be overcome in a natural manner. The number of 50 MeV protons required to explain a strong type III burst is estimated conservatively as 3 × 10²⁵ which, after diffusion in interplanetary space, would be undetectable by the instruments flown thus far. This number is consistent with some theoretical ideas about the flare mechanism and also with present observational data.This paper concerns major type III bursts that have a measurable effect at low frequencies ( 10 MHz). The author is aware of the existence of different kinds of fast drift bursts which are fainter and mostly limited to the m-wave region (de Groot, 1970). These may be due to different kinds of excitors.Postdoctoral Fellow on the U.S.-U.S.S.R. Cultural Exchange Program.     

Towards a theory for type III solar radio bursts

The mechanisms for the transformation of plasma waves into radiation near the fundamental and second harmonic of the plasma frequency are reviewed and equations are given for both the emission and absorption coefficients for these mechanisms. Near the fundamental the process is the scattering of plasma waves on the polarization clouds of ions and the absorption coefficient can be negative, i.e. the radiation can be amplified. Near the second harmonic the process is the combination of two excited plasma waves for which the absorption coefficient can only be positive. These results are applied to construct models of the radiation source for type III solar radio bursts both at high frequencies where the fundamental is dominant and at low frequencies where the second harmonic is dominant using two model plasma wave spectra, one being one-dimensional, the other isotropic. At high frequencies second harmonic radiation is used to determine the source area for a given energy density in plasma waves W _p . The source size and W _p are detrmined uniquely for a given plasma wave spectrum by tracing rays in a model source taking into account amplification of the fundamental. The results for a strong source at the 80 MHz plasma level with a ratio of emissivities of the fundamental to second harmonic P(ω _p )/P(2ω _p ) ≈ 10 are that the source with a one-dimensional plasma wave spectrum is about 14000 km in diameter and W _p = 10^?6.52 erg cm^?3, and the source with an isotropic distribution of plasma waves is about 200 km in diameter and W _p = 10^?6.3 erg cm^?3. It is shown that at low frequencies, where amplification of the fundamental is no longer possible, second harmonic radiation must be dominant and thus very little information about the source can obtained from the radiation.     

Towards a theory for type III solar radio bursts

The procedure developed in Smith (1974) to model the radiation source for type III bursts is modified to include scattering of radiation in the source itself. Since the inhomogeneities in the source must have the same statistical properties as the inhomogeneities used in tracing radiation from the source to the observer, these two parts of the type III problem are no longer uncoupled. Thus we use inhomogeneities consistent with the scattering inhomogeneities of Steinberg et al. (1971) and Riddle (1974) and apply the procedure to an archetype ‘fundamental-harmonic’ pair observed at Culgoora on 28 September, 1973 at 0319 UT. We find that it is impossible to model this burst with a source which is homogeneous in the sense that every part of the source has the same energy density in plasma waves. The density inhomogeneities in the source severely hamper amplification of the supposed fundamental. Possible ways out of this dilemma are discussed, including second harmonic pairs and a source with an inhomogeneous distribution of plasma waves. It is concluded that none of the possibilities are completely satisfactory to explain present observations and suggested that critical observations are missing.     

Wide-band average spectra of solar radio bursts

Peak flux spectra of solar radio bursts in a wide frequency band have been statistically determined for different morphological types of bursts, for various ranges of magnetic field of the burst-associated sunspots and also for the bursts occurring in the central and limb region of the solar disk. Important results obtained are: (i) The generalised spectra have two peaks, one near to meter-wave and the other in the centimeter-wave region, the former peak being more pronounced than the latter; (ii) identical spectral shape is observed for the great and impulsive types and also for GRF and PBI types of bursts; (iii) the radio emission intensity is relatively higher in the central part than that in the limb part of the solar disk for frequencies 1–10 GHz, while the reverse is true for frequencies 0.245–1 GHz and 10–35 GHz; (iv) the optical depth of the absorbing layer above the source of a burst is found to be the same for meter to centimeter-wavelength bursts, implying that the radio sources in this wide band have uniform characteristics with respect to optical thickness; (v) in case of simultaneous emission in the dekameter to X-ray band, most of the decimetric bursts are seen to be very prompt and coincident with the associated flare's starting time. The interpretations of the obtained spectra give an insight into the possible generation mechanisms, pointing to the location of the source region in the solar atmosphere.