High resolution observations with Artemis-IV and the NRH

I. Type IV associated narrow-band bursts

¹ Department of Physics, University of Athens, 15783 Athens, Greece
e-mail: kbouratz@phys.uoa.gr
² Department of Physics, University of Ioannina, 45110 Ioannina, Greece
³ Department of Informatics, University of Athens, 15783 Athens, Greece
⁴ Department of Electronics, Technological Educational Institute of Lamia, 35100 Lamia, Greece

Received: 22 August 2015
Accepted: 21 November 2015

Abstract

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 ~12 000 individual narrow-band bursts, groups, and chains. Spike sources were imaged with the Nançay radioheliograph (NRH) for the event of 21 April 2003.

Results. The mean duration of individual bursts at fixed frequency was ~100 ms, while the instantaneous relative bandwidth was ~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 ~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.