V. O. Rapoport

A dynamic theory of type III solar radio bursts

Numerous experimental data indicate that type III solar radio bursts are generated by the streams of fast electrons in the corona. The process of the electron acceleration in the flare region is, in general, of the character of a short time local explosion. As a result, a spatially limited stream with inhomogeneous front and back is formed in the corona. The present paper shows that such a spatial structure of fast electrons radically changes the dynamics of the stream instability development. In particular, for example, despite strong quasilinear relaxation, in this case the electron stream can generate plasma waves in the corona for a long time due to faster electrons escaping out of the front of the stream. The extension of the stream in the outer corona where the collisions are negligible is of similar character. The maximum of the energy density in the packet of excited plasma waves travels in the corona with constant mean velocity which is defined by the fast-electron energy at the moment of their formation in the flare region. Therefore, in spite of the considerable influence of quasilinear effects on the stream motion, the velocity of type III sources found in terms of the drift velocity remains unchanged. This creates the illusion of stream stabilization. The energy dissipation of plasma waves for the low frequency type III bursts is fully determined by Landau damping in the tail of the stream. Because of this, the temperature estimates of the outer corona from time profiles of type III bursts are incorrect. The theoretical curves of the time variation of radiation at different frequencies agree well with the experimental data in the hectometer wave range under the assumption that the electromagnetic wave generation takes place at the second harmonic of the plasma frequency. In this case it is necessary to decrease the electron density in the solar corona at distances of about 5 to 30 R⊙ 4 times as compared with the previous densities previously derived from type III data.

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.

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.

On the origin of continuum emission during decametric solar noise storms

The stationary current of diffusely-distributed super-thermal electrons along a weakly inhomogeneous coronal magnetic field is considered as a possible model of the noise storm continuum source in decametric wavelengths. It is shown that the realization of such a streaming leads to a considerably increased level of plasma noise in the diffuse component region and then to enhanced radio emission from this region.

THE ROLE OF ATMOSPHERIC TEMPERATURE GRADIENTS AND WINDS FOR ESTIMATING THE ENERGY POTENTIAL OF RADIO-ACOUSTIC SOUNDING SYSTEMS

We present the theory of radio-acoustic sounding (RAS) of the atmosphere allowing for the dependence on temperature, altitude, and wind. For the case of a linear temperature profile, we obtained expressions for the received signal power. Since the acoustic wave front differs from the sphere if the temperature gradient is allowed for, we can introduce the notion of projector (Fresnel) and Fraunhofer regions. In the Fresnel region, the size of the diffraction spot on the earth is determined by radar antenna dimensions and the received signal power is proportional to z−2. In the Fraunhofer region, the spot size is greater than the antenna dimensions and the power is proportional to z−6. The existing RAS facilities work in the projector region. This can be used as a basis for developing a new method of diagnostics of a large-scale inhomogeneous atmospheric structure, including wave disturbances.