Astron. Astrophys. 333, 1034-1042 (1998)

4. Conclusions

New observations allow to investigate IMD structures up to 2550 MHz and to analyze them with greatly enhanced precision. These broadband observations show them generally located in the high-frequency part of the type IV continuum and in its later phase.

The quantitative results on the first derivative (drift rate) and second derivative have been used to test the two existing models. At the newly accessible high frequencies, the Alfvén soliton model by Treumann et al. (1990) predicts very high magnetic fields that are inconsistent with plasma emission. The model may be modified by proposing electron maser emission in evacuated flux tubes. The k -values are then derived to be in a small range between 1.5 and 2.0, contrary to the observed range of 0.42 to 4.19.

The other model, suggesting whistler waves (e.g. Mann et al. 1987), predicts relations of and with frequency dependances that contradict the observations.

The discrepancies may be resolved for both models by allowing more parameteres to vary systematically with height, for instance and . This investigation then exhibits a general problem facing complex models with many free parameters: They can often be adapted to fit the observations. Thus it is not possible to decide definitively between the models.

As an example, the following parameters are derived from the observations at 2 GHz: The whistler model yields an electron density of cm^-3 and a magnetic field of 212 G; the soliton model yields cm^-3 and 143 G.

The adaption of the Alfvén soliton model to maser emission has the effect to diminish the discrepancies in the determination of the magnetic field strength between the two models. They differ by less than a factor of 2.5 in the range from 200 to 2000 MHz (10 G to 200 G), and in the restricted range of 1 - 2 GHz, relevant for these observations, the difference is less than a factor of 1.5. It may make IMD bursts attractive to measure magnetic fields in post-flare loops in spite of the uncertain physics.

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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