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

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.