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 pair's 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.

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.

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 fre~luency c,'~pe. The second type chains and, consequently, diffusive stria-bursts correspond to the second harmonic of the plasma frequency 2c,)pe. 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.

To locate two-dimensional positions of the solar decametric radio bursts a heliograph was developed on the basis of the UTR-2 radiotelescope (Khar'kov) operated in the range 10-26 MHz. The beamwidth of the heliograph rapid-scanning pencil-beam is 25 arc min at 25 MHz, and its field of view is about 3.5 ~ (E-W) • 2.0 ~ (N-S). The instrument yields rapidly forty records of the radio brightness of all (8 x 5) elementary parts (each 25 arc min in diameter) of the investigated sky area during every period of i s. Both coordinates of a burst center are measured with an accuracy 5 arc min. The bandwidth of the receiving system is 10 kHz. The heliograph operates in conjunction with a radiospectrograph connected to the output of a N-S arm of the UTR-2 array. The data observations with the UTR-2 correspond only to one linear polarized component.The ionospheric distortion of the test records of the radio source Cassiopeia-A that occurred sometimes is illustrated.First results of 25 MHz observations of the solar radio storm in August, 1976 with the heliograph are presented here. This storm is accompanied by the compact sunspot group travelling all over the optical disk. The type III and stria bursts were predominant during the storm. On the given day the scattering regions of their apparent centers were overlapped and the sizes of these regions were usually not more than 5 arc min. On some days there occurred additional burst sources displaced in position from the persistent storm region. It was found out that, as a rule, 25 MHz stria-bursts from the type IIIb chain coincided in position with the following type III burst at the same frequency. The difference of the daily averaged coordinates of both stria and type III bursts was considerably smaller than the mean diameter of their sources.The radial distance of the 25 MHz storm region from the solar center was calculated by using the three methods. The storm height was estimated as 1.8Ro from the rotation rate close to the central meridian of the storm center. Definite association of the spots with the storm near the limb allowed to determine the average value 2.1R| for the height. The limb measurements give the mean height of 2.3R|The center-to-limb variation of the storm source height is a known fact in the meter-wavelength range. This is the evidence of the propagation effects in the solar corona being essential to interpret the results of the radio source location.

The paper deals with the stria-burst observations by the UTR-2 radio telescope (IRE AN UkSSR, Kharkov) at the frequencies of 12.5-25 MHz.Two varieties of these bursts are pointed out: ordinary stria-bursts and diffusive stria-bursts. The ordinary stria-bursts may occur isolated, as split-pairs and spirited triple bursts (triplets). At the frequency 25 MHz, the ordinary stria-bursts are characterized by the average duration about 1 s, the band width 0.07 MHz and the frequency drift 0.07 MHz s -I. A scale of the frequency splitting in pairs and triplets is 0.1 MHz. The diffusive stria-bursts have essentially larger duration (of the order of 10 s) and no visible frequency drift. Like the ordinary stria-bursts, the diffusive ones may appear isolated and in the form of split-pairs and triplets. The occurrence of echoes is typical of both types of stria-bursts which are a succession of two or several (multiple echoes) similar bursts separated by the time interval 1.5-2.5 s for the ordinary and by 3-10 s for the diffusive stria-bursts. Sometimes the bursts with two scales of the frequency splitting are observed.