EDP Sciences
Free access
Volume 408, Number 2, September III 2003
Page(s) 737 - 742
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20031020

A&A 408, 737-742 (2003)
DOI: 10.1051/0004-6361:20031020

On group velocity delays in microwave millisecond oscillating events and radio spikes

L. V. Yasnov1 and M. Karlický2

1  Radiophysical Research Institute of St.-Petersburg State University, St.-Petersburg, Russia
2  Astronomical Institute of the Academy of Sciences of the Czech Republic, 25165 Ondrejov, Czech Republic

(Received 17 February 2003 / Accepted 12 June 2003 )

Time delays between opposite polarizations, interpreted by a difference between the ordinary and extraordinary modes, are studied. Two different density and magnetic field models are used, and three emission mechanisms generating radio waves in double upper-hybrid, plasma, and gyro frequencies are considered. It is found that time delays and their spectra can reach various values and forms depending on plasma parameters in the radio wave emission and propagation. For the emission in double upper-hybrid frequency the considerable decrease of the time delay is due to an increase of the electron plasma density in the radio wave generation. In a dense flare plasma the power index a of the time delay spectrum is negative. On the other hand, in a diluted plasma a becomes positive, but in this case it is much lower than observed for dm-pulsations ( a=3). Furthermore model values are compared with the time delays presented by Fleishman et al. (2002), and it is shown that the best agreement is with the model assuming the emission on double gyro-frequency. But, in this case the gyro-resonant absorption limits the angle for escape of radio waves to $\Theta<3$-10 $^{\circ}$. Finally, an effect of the difference of the group and light velocities on the frequency drift is analyzed. It is shown that such an effect can be important for wave propagation along magnetic field lines at frequencies close to the plasma frequency.

Key words: plasmas -- Sun: flares -- Sun: radio radiation

Offprint request: L. V. Yasnov, karlicky@asu.cas.cz

© ESO 2003