Quasiperiodic structure of solar type IIId radio bursts with echo components

Abstract

By solar observations with an UTR 2 antenna, we detected a new variety of type IIId radiation in which the narrow-band spectral elements have an explicit quiasiperiodic structure and contain an additional component in the form of a triple echo. According to amplitude records at 25 MHz, the initial short radio burst looks like a weak precursor of a more powerful echo-like burst accompanied by a pair of final pulses which exceed slightly the background. Nine events of this kind were registered in August 19, 1992 during a type IIId noise storm related to the active region in the central heliolongitude sector. In each event, all the three time intervals between successive peaks of the complex amplitude-time profile were almost identical and were equal to 4.7, 5.1, and 5.7 s on the average. They were somewhat (by 1 or 2 s) shorter than the maximum delay in the known events with a two-hump amplitude profile and time-split narrow-band spectral elements in which the delay t2−t1 and relative intensity (I2/I1≲1) of echo-like bursts depend on the heliolongitude of the type IIId sources. The discovery of a new complex variety of decameter type IIId radiation confirms the hypothesis of multiple quiasiperiodic echoes after short radio bursts at the second harmonic of the local plasma frequency in the solar atmosphere.