High resolution observations with Artemis--JLS, (II) Type IV associated intermediate drift bursts

Aims. We examined the characteristics of isolated intermediate drift bursts (IMDs) and their morphologies on dynamic spectra, in particular the positioning of emission and absorption ridges and the repetition rate of fiber groups. These were compared with a model in order to determine the conditions under which the IMDs appear and exhibit the above characteristics. Methods. We analyzed sixteen metric type IV events with embedded IMDs. The events were recorded with the Artemis-JLS/SAO high resolution (10 ms cadence) receiver in the 270-450MHz range with a frequency resolution of 1.4 MHz. We developed cross- and autocorrelation techniques to measure the duration, spectral width, and frequency drift of fiber bursts in 47 IMD groups. We also developed a semi-automatic algorithm to track fibers on dynamic spectra. Results. We distinguish six morphological groups of fibers, based on the relative position of the emission and absorption ridges. These included fibers with emission or absorption ridges only, fibers with the absorption ridge at lower or higher frequency than the emission, or with two absorption ridges above and below the emission or with two emission ridges were separated by an absorption ridge. Some borderline cases of IMDs with very high drift rate (~0.30 s-1) or very narrow total bandwidth (~8 MHz) were recorded; among them a group of rope-like IMDs with fast repetition rate and relatively narrow total frequency extent. The whistler hypothesis leads to reasonable magnetic field (~4.6 G), but the Alfven-wave hypothesis requires much higher field. We estimated the ratio of the whistler to the cyclotron frequency, x, to be ~ 0.3 to 0.6 and the average frequency scale along the loop of ~220Mm. We present empirical relations between fiber burst parameters and discuss their possible origin.
Comment: Accepted for publication in Astronomy & Astrophysics