High-resolution observations with ARTEMIS-JLS and the NRH. III. Spectroscopy and imaging of fiber bursts

Aims: We study the characteristics of intermediate drift bursts (fibers) embedded in the July 14, 2000 large solar event. Methods: We used dynamic spectra from the acousto-optic analyzer (SAO) of the ARTEMIS-JLS solar radiospectrograph, in conjunction with high time resolution images from the Nanc cay Radioheliograph (NRH) and EUV images from TRACE . We computed both 2-dimensional and 1-dimensional images and applied high pass time filtering to the images and the dynamic spectrum in order to enhance the fibers. For the study of the background continuum emission we used images averaged over several seconds. Results: All fibers in the SAO dynamic spectra are identifiable in the NRH images. Fibers were first detected after the primary energy release in a moving type IV, probably associated with the rapid eastward expansion of the flare and the postflare loop arcade. They appeared as a 10% modulation of the continuum intensity. The fibers and the continuum were strongly circularly polarized in the ordinary mode sense, indicating fundamental plasma emission. We detected discrete fiber sources along two ~300, Mm long parallel stripes, apparently segments of large scale loops encompassing both the EUV loops and the flux rope. We found multiple fiber emissions at slightly different positions and times; their consecutive appearance can give the impression of apparent motion with supra-luminal velocities. Images of individual fibers were very similar at 432.0 and 327.0 MHz. From the position shift of the sources and the time delays at low and high frequencies we estimated, for a well observed group of fibers, the exciter speed and the frequency scale length along the loops; we obtained consistent values from imaging and spectral data, supporting the whistler origin of the fiber emission. The fibers in absorption and in emission are very similar as manifestations of the same wave train.