During a unique solar radio event on March 2, 1970, among very diverse features, a very regular pattern was observed in the solar radio spectrographic record between 220 and 320 MHz. The proposed explanation is emission by slightly relativistic electrons at the sum frequency of the plasma frequency (~ 160 MHz) and the lower harmonics of the local gyrofrequency (n from 3 to 10), rather than emission at pure harmonics of the gyrofrequency (n from 15 to 20). The derived magnetic field strengths range from 3 to 8 G, and can be determined very accurately. From the simultaneously occurring pulsating structure in the type IV emission and the deduced magnetic field strength a typical size of the emitting region of 2-300 km is derived.
1. Observations
On March 2, 1970, between 1416 UT and 1420 UT a very complex solar radio event was observed at Nederhorst den Berg, with the 60 channel solar radiospectrograph, operating between 160 and 320 MHz (De Groot and Van Nieuwkoop, 1968;Van Nieuwkoop, 1971). A detailed description of the radio event on this date is given by Slottje (1972). The underlying large scale structure is a type IV continuum extending from 160 MHz to 200 MHz, falling off to higher frequencies and hardly detectable on the spectrograph records at 320 MHz (see Figure 1, lower half).
Among many other peculiar phenomena, superimposed on this continuum, we observed a structure which is periodic in frequency, with a period of approximately 15 MHz, and which extends from 220 to 300 MHz (Figure 1, upper half). Although it is not seen at lower frequencies, it might still be present there, but then it must certainly be much weaker. The insensitive (but absolute) recording would not show this structure, and in the sensitive (relative) recording these variations would be severely suppressed, due to the much higher background level of the type IV continuum at the lower frequencies. The structure seems to dissolve in chaos at frequencies higher than 300 MHz, but in some cases extends up to the 320 MHz limit of the spectrograph band. The structure appears to be a superposition of periodic patterns, resulting in a Moir6-1ike pattern. Occasionally one can clearly discern a simple periodic set of ridges drifting in frequency. At several instants it was possible to follow a set of ridges, drifting to higher frequencies, over several seconds of time. Although occasionally a simple set of ridges drifting to lower frequencies was observed, it was not possible to find a set which lasted long enough to make quantitiave measurements possible in this case.
Since there is a large variation in background emission both in time and in fre-