Fine structure of decametric type II radio bursts

Abstract

We have performed a comparative analysis of the fine structure of two decametric type II bursts observed on July 17 and August 16, 2002, with the 1024-channel spectrograph of the UTR-2 radio telescope in the frequency range 18.5-29.5 MHz and with the IZMIRAN spectrograph in the frequency range 25-270 MHz. The August 16 burst was weak, ∼ 2-5 s.f.u., but exhibited an unusual fine structure in the form of broadband fibers (∆fe > 250-500 kHz) that drifted at a rate characteristic of type II bursts and consisted of regular narrow-band fibers (∆fe > 50-90 kHz at 24 MHz) resembling a fiber bundle. The July 17 burst was three orders of magnitude more intense (up to 4500 s.f.u. at 20 MHz) and included a similar fiber structure. The narrow fibers were irregular and shorter in duration. They differed from an ordinary fiber bundle by the absence of absorption from the low-frequency edge and by slow frequency drift (slower than that of a type II burst). Both type II bursts were also observed in interplanetary space in the WIND/WAVES RAD2 spectra, but without any direct continuation. Analysis of the corresponding coronal mass ejections (CMEs) based on SOHO/LASCO C2 data has shown that the radio source of the type II burst detected on August 16 with UTR-2 was located between the narrow CME and the shock front trailing behind that was catching up with the CME. The July 17 type II fiber burst also occurred at the time when the shock front was catching up with the CME. Under such conditions, it would be natural to assume that the emission from large fibers is related to the passage of the shock front through narrow inhomogeneities in the CME tail. Resonant transition radiation may be the main radio emission mechanism. Both events are characterized by the possible generation of whistlers between the leading CME edge and the shock front. The whistlers excited at shock fronts manifest themselves only against the background of enhanced emission from large fibers (similar to the continuum modulation in type IV bursts). The reduction in whistler group velocity inside inhomogeneities to 760 km s −1 may be responsible for the unusually low drift rate of the narrow fibers. The magnetic field inside inhomogeneities determined from fiber parameters at 24 MHz is ∼0.9 G, while the density should be increased by at least a factor of 2.