3.10 Kilometric type II radio bursts

There is another type of solar radio bursts, called the kilometric type II radio bursts. They also drift in frequency, from a few 100 kHz to some tens of kHz over time scales of several days. Cane (1985) had established that the kilometric radio bursts are due to electrons accelerated at interplanetary shock waves on their way from the Sun out into space. In that respect, these kilometric radio burst are similar to the metric radio bursts. In some cases, the shock motion can be well traced from the Sun to the Earth orbit and often far beyond (see, e.g. Reiner et al., 1998Jump To The Next Citation Point2001Jump To The Next Citation Point), as is illustrated in Figure 20View Image.
View Image

Figure 20: Dynamic spectra of the WIND-WAVES radio data from January 6 to 11, 1997 in the frequency range from 23 to 245 kHz. The ordinate scale is the inverse of the observing frequency. The observed type II radio emissions for this event are bracketed by the upper two lines that originate from the CME lift-off time at 15:00 UT on January 6. For a more detailed description see Reiner et al. (1998).

There is still a controversial discussion going on about the nature of the respective shocks causing the radio bursts. Cliver and some colleagues (Cliver et al., 19992004Cliver, 1999) argue that both the metric and the kilometric burst are produced by CME-driven shocks. The opposing position is held, e.g., by Cane and Reames (1988); Gopalswamy et al. (1998); Cane and Erickson (2005Jump To The Next Citation Point). They argue that the metric radio bursts stem from coronal shock waves driven by flares as blast waves, as suggested before by, e.g., Wagner and MacQueen (1983) or Sheeley Jr et al. (1984). In fact, there is never a continuous spectrum seen connecting the metric and the kilometric radio ranges. This gap had often been assigned to instrumental shortcomings. Only recently, the whole spectral range could be covered, but the gap in the spectrum usually remains. Most metric type II bursts appear to die out within 2 Rs (from Sun center) since their frequency rarely drops further than 5 MHz. This suggests that they are driven by shock waves that die out soon. On the other hand, a CME driven shock cannot be formed unless the CME speed exceeds the local fast magnetosonic speed. For a majority of the slower CMEs this stringent condition is not fulfilled before the CMEs reach a distance of several Rs (Mann et al., 2003Jump To The Next Citation Point). That would explain why the kilometric radio bursts rarely have frequencies above 1 MHz. In some cases, both radiation types are seen simultaneously at different frequencies. That does not necessarily prove the simultaneous existence of 2 shocks. There might as well be one large shock front that manages to accelerate electrons at very different distances from the Sun.

The controversy is still open. The interested reader may wish to look up, e.g., Kahler (1992Jump To The Next Citation Point); Gopalswamy et al. (2005a); Cane and Erickson (2005); Reiner et al. (2005) and other articles by these authors and others such as Mann, Klassen, Reames, Kaiser, Webb, to be found using the NASA External LinkADS.

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