Kilometric type III burst enhancements associated with interplanetary shocks

Abstract

Type III bursts have been observed in the 30-100kHz range which show localized enhancements in their spectra. The enhancements exhibited by a series of type III bursts extending over a period of several hours show a consistent decrease in frequency. The rate of this decrease is consistent with the movement of an interplanetary shock. The passage of such a shock by the Earth is suggested by the occurrence of sudden commencements at approximately the expected time.Examination of solar type III bursts at kilometric wavelengths shows that the intensity of emissions generally decreases with decreasing frequency. In contrast, occasional bursts are observed which display band-limited enhancements of their spectra. Two such bursts are discussed below.The observations reported here were made by the Stanford University vlf experiment on the OGO-3 spacecraft. This experiment measured the characteristics of radio waves in the 0.2-100 kHz range; only the data from a receiver sweeping the range 10-100 kHz each 2.3 min are shown here. Further details concerning this experiment are given by Dunckel et al. (1972) andDunckel (1974). The digital output from this receiver has been converted into the 'ampligram' shown in Figure 1. The height of an individual trace is proportional to the logarithm of the receiver output; the overlap produced by strong emissions causes the large white areas. The narrow spectral lines of invariant frequency, as at 1415 UT at 93 kHz, are due to spacecraft interference. Figure 1 shows a sequence of three type III bursts that exhibit large enhancements in their spectra near 70 kHz. These enhancements have power spectral densities about ten times greater than that at 100 kHz. The frequency of these enhancements decreases from 72 kHz in the first event at 12 UT to 64 kHz in the third event at 19 UT. The enhancements do not appear to be related to the plasma immediately surrounding the Earth, since the low-energy electron detector on board the OGO-3 spacecraft indicates that it was outside the bow shock during ~he period shown in Figure 1 except for 1203-1243UT and 1250-1311UT (personal communication, V. Vasyliunas). Neither do the enhancements appear to result from the effects of the local interplanetary medium, since Vela 3 measurements at 18-21 UT on this day indicate a plasma frequency of 28 kHz (Bame et al., 1970). The second event is associated with a flare at W51 commencing at 1414 UT and reaching maximum at 1418 UT. The third event is associated with a flare in the same sunspot region commencing at 1753 and reaching maximum at 1806 UT. No known flares are associated with the first