High resolution observations with Artemis-IV and the NRH

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.

Detection of spike-like structures near the front of type-II bursts

“…In Table 1 we summarize the average parameters of the type IIassociated spike-like structures. These parameters are very close to the corresponding parameters of type IV-associated spikes from Bouratzis et al (2016, their Table 1), which are also included in the table for comparison.…”

“…Type_II_FS_Revision02corr Table 1. Parameters of type II-associated spike-like bursts and type IV-associated spikes from Bouratzis et al (2016 the differential spectrum. They are nearly vertical in the dynamic spectrum, hence they look more like pulsations rather than herringbones.…”

“…The duration-frequency and bandwidth-frequency dependence of the spikes (narrowband structures) are usually expressed by phenomenological power laws of the form δτ ∝ f −a , (a ≈ 1.32) (Guedel & Benz 1990;Rozhansky et al 2008;Sirenko & Fleishman 2009), and δ f ∝ 0.66 f 0.42 by (Csillaghy & Benz 1993), respectively (see plots in Fig. 6 of Bouratzis et. al., 2016).…”

The Low-Frequency Solar Corona in Circular Polarization

“…A few studies have ulilized the polarimetric imaging capabilities of the NRH to examine spatial variation in radio bursts. For example, Mercier (1990) showed that Type III bursts have different spatial characteristics in circular polarization compared to the total intensity, and Bouratzis et al (2016) investigated similar differences in spike bursts as a function of time. Several others have examined source positions and structures in total intensity NRH observations, while using the polarization information to help discriminate between emission mechanisms (e.g.…”

High-resolution observations with ARTEMIS-JLS and the NRH

“…From the original NRH visibilities, we computed 2D images with a resolution of 1.04 by 1.52 at 432 MHz. In a previous work on spike bursts (Bouratzis et al 2016) we also computed 1D images using visibilities from the east-west (EW) and northsouth (NS) arrays only; this improved the resolution by a factor of two, due to the fact that the extension antennas make a very small contribution to the 2D images. In the present case these 1D images were relatively noisy, whereas the fiber sources were well resolved and thus we preferred to compute 1D images by integrating the 2D images along the EW and NS directions.…”