High resolution observations of Intermediate Drift Bursts with the ARTEMIS-JLS Radio spectrograph and the Nançay Radioheliograph
Costas Bouratzis, University of Athens
We used high time resolution dynamic spectra from the ARTEMIS-JLS solar radiospectrograph for the study of intermediate drift bursts. We found average fiber exciter speed of 5900 2400 km s-1, corresponding to average magnetic field of 4.6 1.5G for whistler exciter and consistent with values estimated by other methods. A simplified model of magnetic field along the exciter trajectory, gave an average frequency scale length of 220Mm and a ratio of the whistler frequency to the electron cyclotron frequency in the range of 0.3 to 0.6. With the additional assumptions of a 4xNewkirk coronal density and a semi-circular loop, we estimated an average fiber onset at a height of ~46Mm, a vertical extent ~15 Mm, a magnetic field scale height ~240Mm and mirror ratio ~2.
We combined ARTEMIS-JLS and Nançay Radioheliograph observations for the July 14, 2000 large solar event Practically all fibers visible 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 post-flare loop arcade. We found that fibers appeared as a modulation of the continuum intensity with a root mean square value of the order of 10%. Both the fibers and the continuum were strongly circularly polarized in the ordinary mode sense, indicating plasma emission at the fundamental. We detected a number of discrete fiber emission sources along two ~300 Mm long parallel strands, apparently segments of large-scale loops encompassing both the EUV loops and the CME-associated flux rope. We found cases of multiple fiber emissions appearing 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 and 410.5 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. Finally we found that fibers in absorption and in emission are very similar, thus confirming that they are manifestations of the same wave train.