High resolution observations with Artemis-IV and the NRH. I. Type IV associated narrow-band bursts

Context. Narrow-band bursts appear on dynamic spectra from microwave to decametric frequencies as fine structures with very small duration and bandwidth. They are believed to be manifestations of small scale energy release through magnetic reconnection. Aims. We analyzed 27 metric type IV events with embedded narrow-band bursts, which were observed by the ARTEMIS-IV radio spectrograph from 30 June 1999 to 1 August 2010. We examined the morphological characteristics of isolated narrow-band structures (mostly spikes) and groups or chains of structures. Methods. The events were recorded with the SAO high resolution (10 ms cadence) receiver of ARTEMIS-IV in the 270-450 MHz range. We measured the duration, spectral width, and frequency drift of similar to 12 000 individual narrow-band bursts, groups, and chains. Spike sources were imaged with the Nancay radioheliograph (NRH) for the event of 21 April 2003. Results. The mean duration of individual bursts at fixed frequency was similar to 100 ms, while the instantaneous relative bandwidth was similar to 2%. Some bursts had measurable frequency drift, either positive or negative. Quite often spikes appeared in chains, which were closely spaced in time (column chains) or in frequency (row chains). Column chains had frequency drifts similar to type-IIId bursts, while most of the row chains exhibited negative frequently drifts with a rate close to that of fiber bursts. From the analysis of NRH data, we found that spikes were superimposed on a larger, slowly varying, background component. They were polarized in the same sense as the background source, with a slightly higher degree of polarization of similar to 65%, and their size was about 60% of their size in total intensity. Conclusions. The duration and bandwidth distributions did not show any clear separation in groups. Some chains tended to assume the form of zebra, lace stripes, fiber bursts, or bursts of the type-III family, suggesting that such bursts might be resolved in spikes when viewed with high resolution. The NRH data indicate that the spikes are not fluctuations of the background, but represent additional emission such as what would be expected from small-scale reconnection.