Astron. Astrophys. 323, 250-258 (1997)

5. Conclusions

A method to measure the delay between the arrival time of the two magnetoionic modes has been developed and tested. It has been successfully applied to weakly polarized narrowband spikes of solar flares at decimeter wavelengths. The measured delays are of the order of 100 µs.

In all four events the weaker mode was found to be delayed. The weakly polarized spikes thus appear to be predominantly ordinary mode at the origin of the polarization. One magnetoionic mode can couple to the other mode in a quasi-transverse region or be partially deflected into the other mode by low-frequency turbulence. Thus the source of emission may not coincide with the common starting point of the two observed modes.

The observed delays beteen 50 and 250 µs are extremely short. The difference in group velocities of the two modes is a possible cause of delays, but it causes considerably longer delays if the modes originate near the plasma frequency (assuming ) and typical coronal conditions are used. If the origin of polarization is in the source of emission, the proposed emission mechanisms require scale lengths cm. Alternatively, the radiation could be emitted fully polarized and be transformed to less polarization in a quasi-transverse region or by lower-hybrid waves. The delay would then be produced only during propagation from this site to Earth. The second scenario is consistent with spike emission models producing fully polarized radiation.

Ordinary mode emission is naturally predicted by spike theories based on plasma waves as the primary waves. They are preferentially transformed into ordinary mode radio waves. The observations thus support this if the mode is not changed in a quasi-transverse region (weak mode coupling, ). Alternatively, emission in extraordinary mode is possible if the coupling is strong. It would then agree with the leading spot rule. Although this rule is not well understood, it adds some credibility to the hypothesis of the extraordinary mode of emission.

Since there is no qualitative difference in metric and decimetric spike bursts, it has been proposed that they are produced by the same process (Csillaghy & Benz 1993). If metric and decimetric spikes have the same emission mechanism, the observations may be best interpreted by ordinary mode emission as in metric type III bursts, partial transformation into the other mode and a more complex field geometry in the decimetric sources inverting the leading spot rule in the majority of the cases. The confirmation of this scenario needs additional information on the flare geometry based on spatially resolved observations.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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