Type IV solar radio emission

Conclusion

We have got a reasonably clear idea of the various forms under which the type IV continuum emission may appear. Also we can imagine what kind of processes come into play during a type IV event. But the insight gained so far applies to the general case. Individual cases are widely different, and we are still far from understanding why a given event behaves as it does. For instance, why are metric responses lacking at a certain big microwave outburst, or why is the decimetric component particularly strong or prolonged on certain occasions? One can imagine that such questions would receive an answer if one were allowed to “see” the configuration of magnetic lines of force above the activity region !

Does the type IV event tell us a fine story of the interplay of energetic particles and streams of particles with coronal magnetic fields ? Maybe the story would be a fine one if the language could be understood. At present we know only a few words of it; for this reason to us the story is very fragmentary. First of all, however, the message should be recorded far more completely than has been done so far. The number of observations that should be made of one and the same event is tremendous; the program comprises:

1) spectral observations from 1000 Mc/s down to the lowest frequencies; 2) single frequency observations at a great many wavelengths covering the whole radio spectrum; 3) measurements of polarization and 4) determinations of position and angular extent in at least every octave of the whole radio spectrum.

Especially as regards the latter two points, the present situation is still very unsatisfactory, though good work has been done already in Japan. The realization of a complete recording of phenomena during a type IV event calls for a combined effort of several observatories.

Very encouraging are the established relations between solar type IV events and terrestrial phenomena. From an analysis of solar cosmic ray events as recorded on several places on the earth, interesting inferences have been drawn regarding the travelling conditions of particles in interplanetary space (cf. Carmichael, 1962). Likewise, one may expect interesting information on the behaviour of interplanetary particle clouds of solar origin from (interferometric) observations of decametric radio emission on the occasion of type IV events.

The occurrence of a major type IV event enables forecasters to predict successfully geomagnetic and ionospheric storms. Type IV events will determine at what times certain space research experiments will be launched in the next solar cycle. One should like to be able to indicate the probability for the occurrence of type IV solar radio flares themselves. It is known that these flares generally occur in complex sunspot groups; but a complex sunspot group does not of necessity imply the occurrence of a type IV flare. Observations of coronal condensations at microwave frequencies with a high resolution interferometer may help sorting out those centres of activity that are most likely to produce type IV flares.