Ya. G. Tsybko

Frequency and time splitting of decameter solar radio bursts: II: Chains

The paper deals with the observations of the fine structure of type III bursts in the 12.5–25 MHz band using the UTR-2 (IRE AN UkSSR, Kharkov) radio telescope. A fine structure arises in the form of chains of short-lived narrow-band bursts. The chains have a frequency drift analogous to type III bursts. Observations allow two different-type chains to be singled out. Ordinary stria-bursts, split-pairs and triplets belong to the first type chains. They may also involve the echo-type phenomena The second type chains (IIId) involve diffusive stria-bursts, diffusive split-pairs and triplets. The analysis of a harmonic structure of chains incidates that the first type chains are generated at the frequencies close to the local plasma electron frequency ωpe. The second type chains and, consequently, diffusive stria-bursts correspond to the second harmonic of the plasma frequency 2ωpe. Experimental data evidence that the type III bursts with a fine structure are excited by the faster particle streams than the ordinary type III bursts with a diffusive character both of the fundamental and the second harmonic.

Harmonic structure of type IIIb and III bursts

A decameter solar radio storm of type IIIb and III bursts has been analysed, using single frequency records at frequencies 12.5 and 25.0 MHz.Several kinds of burst associations are classified. As a result it is shown that in double oblique burst-traces of type IIIb + III on the frequency-time plane the type III burst is shifted by an octave above the type IIIb burst at any moment of the IIIb + III pairs lifetime. In particular, the harmonic structure of the spectrum is peculiar to the event of type IIIb + III in the initial and the final stages. This property of the pair is clear if the type IIIb and III radiations occur at the fundamental coronal plasma frequency and its harmonic respectively. On the other hand, if it is assumed that a type IIIb burst is the precursor of a type III one, there is no reason why the two bursts should be harmonically related.

Radio echo and sporadic radiation scattering in the solar corona

Some properties of solar radio bursts observed at the Earth are mainly due to propagation effects in the corona. A radio echo of short-time narrow-band bursts is observed by a decameter radioheliograph on the basis of UTR-2 antenna. Propagation effects are manifested in the marked regular change of the burst intensity-time profile at 25 MHz during a half-rotation of the Sun. A displacement of limb diffuse bursts deep into the solar atmosphere of 1.5 ∼- 2R⊙ has been also found during the burst lifetime.

Multiple radio echoes in the solar corona

Echo-type solar radio bursts are associated with preceding short-lived bursts in double events. The peculiar and rather rare decameter echoes are observed with a UTR-2 antenna. The initial narrow-band burst is followed, some 5 to 10 s later, by an echo-like burst at the same frequency. The observational data obtained for decameter echo evens are, on the whole, consistent with the model of a pulsed source emitting radio signals at the plasma-frequency harmonic, which is placed in a non-uniform corona and rotates together with the Sun.Intensity-time profiles of 25 MHz echo bursts are of an unusual shape, featuring an extended leading edge and an abrupt decay at the trailing edge and also showing some fine structures in the form of an additional, weakly pronounced maximum or a ‘step’ at the final stage of the burst. Time parameters characterizing the profiles are evaluated. The ‘step’ is delayed with respect to the main pulse at about two times longer than the principal echo maximum. At the same time, the time delays depend essentially on the heliolongitude of the active region and achieve their maximum values at the meridian. The ‘step’ height does not exceed 0.5 of the echo maximum. At this level, the echo-decay time almost coincides with the initial burst duration but is about 1.7 times less than the echo-rise time. The feature at the echo tail can be interpreted as a result of a repeated reflection of the burst from the source region. The causes and conditions for the formation of multiple echoes are discussed. The extended leading edge of the echo permits us to assume a quasi-radial fibrous structure of the corona, capable of back-scattering the incident radiation.

Variations of type III burst parameters during a decametric solar storm

An analysis has been made of type III bursts recorded during a decametric solar storm observed from July 29 to August 16, 1975 with the UTR-2 antenna (Kharkov, IRE Acad. Sci. Ukr. SSR). The bursts were recorded with a dynamic spectrograph and radiometers at 25.0, 20.0, 16.7, and 12.5 MHz. Daily observations have yielded histograms of the type III burst distribution with respect to the frequency drift rate in three subbands between 25.0 and 12.5 MHz. During the middle stage of the storm the drift rate was about twice as high as at the onset and the final stage of the storm. Abrupt changes in the mean frequency drift rate were registered some two to three days after the active region McMath 13790 had come onto the limb and also before it disappeared behind the solar disk. Sudden changes in the drift rates of the type III bursts were accompanied by sudden changes of their mean duration. The rather long burst durations observed at 25.0 MHz at the beginning and the end of the radio storm coincided with such at the twice lower frequency, i.e. 12.5 MHz, during the period when an increased drift rate was observed.Similar variations of type III burst parameters can be interpreted in the framework of the plasma mechanism of burst generation in the corona, assuming that at the middle stage of the storm the bursts observed in the 25.0–12.5 MHz range were emitted at the fundamental whereas when the emitting region was near the limb the bursts received corresponded to the second harmonic of the Langmuir oscillations in the range of 12.5 to 6.25 MHz excited at greater heights.

Angular sizes of stria-burst sources in the range 24–26 MHz

The UTR-2 antenna has been used to measure angular sizes of sources of narrow-band short-lived solar stria-bursts at frequencies 24–26 MHz. The majority of these sources have apparent diameters between 20 and 40′. According to this parameter they do not differ noticeably from that of type III bursts at the same frequency. The short duration of the stria-bursts prevents explanation of the large diameter by scattering in the solar corona.

Harmonic relation of type IIIb-III solar radio bursts in 6.25, 12.5, and 25.0 MHz octaves

This study of type IIIb-III evenmts strongly supports their interpretation as fundamental-harmonic burst pairs. Type IIIb chains and the related type III bursts drift from 12.5 to 6.25 MHz and from 25.0 to 12.5 MHz, respectively, during similar time intervals of 11.1 and 11.0 s, on the average. This harmonic similarity is emphasized by the fact that the drift times of type IIIb chains across the upper octave are significantly less than those of type III bursts in the lower octave, the values being around 6 and 19 s.

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.

COLLISIONLESS DECELERATION OF FAST ELECTRON STREAMS IN THE SOLAR CORONAL PLASMA

The collisionless deceleration of electron streams responsible for type IIIb bursts has been investigated. For this the difference between the mean velocities of electron streams at plasma levels corresponding to 25 and 12.5 MHz, on one hand, at 12.5 and 6.25 MHz, on the other hand, is estimated. The mean velocity of electron streams between these levels is determined by the time delay in the moments of arrival of radio bursts from these levels. The distance between plasma levels is determined under the assumption that the (statistical) mean velocity of sources of the diffusive type III bursts is constant and equal toc/3 at all considered levels of the solar corona.It is shown that under this assumption the electron streams with the initial velocities of the order of 0.4–0.8c undergo a sufficient deceleration which is characterized by a decrease in their mean velocity by 15–17% between plasma levels at 25 to 6.25 MHz. The stream deceleration becomes more essential with the growth of the initial velocity of the stream. On the other hand, the deceleration disappears when the initial velocity of the stream is of the order of 0.35c. This critical velocityVs* ∼- 0.35c is assumed to define a boundary between two different expansion regimes of fast electrons moving in the solar corona. In the first regime (Vs >Vs*) the induced scattering of plasma waves produces energy losses of the streams. A decrease in the velocities of streams up to the value of the order of 0.35c is due to these losses. In the second regime (Vs ∼-Vs*) a quasilinear expansion of streams is realized. In this case the energy losses of the streams are almost absent.

Preliminary data on decameter type IIId bursts with echo components obtained from a two-dimensional radio heliograph

AbstractWe present for the first time a set of intensity-time records for one solar type IIId radio burst with an echo component obtained as a result of angular selection of the radiation observed using an UTR 2 antenna operating in the mode of a two-dimensional heliograph at f=25 MHz. It is found that in the case of such events, which occur only in the central sector of heliolongitudes |l|≤50°, the apparent pulsating source of narrow-band (∼0.1 MHz) radiation at the second harmonic (f≈2fp) can have a complex, constantly varying angular structure. The coronal source positions determined at the time of the main peak of a two-hump burst and 6.5 s later at the time of its second, relatively low, maximum did not coincide and their divergence was ≈12′ in hour angle and ≈0′ in declination. During the first pulse (having a steep front) the source did not remain immobile and travelled for 3 s, gradually decaying, predominantly to the west at an average velocity close to the velocity of light. The amazing fact of initial nonstationarity and other properties of the pulsing type IIId source show that a fast imaginary source occurred in the solar corona at the site of short-time (< 1 s) radio wave generation, i.e., well above the normal level of plasma frequency