Authors: H.V. Cane; W.C. Erickson
Affiliation: NASA Goddard Space Flight Center; BIRS
It is well established that type II radio bursts are caused by shocks that are associated with flares and coronal mass ejections (CMEs). However the detailed relationship between "metric" type II bursts observed at frequencies above about 10 MHz from the ground and the shocks expected to exist ahead of fast (speeds ~>800 km/s) CMEs is a topic of continued debate. We have examined radio data from the Waves experiment on the Wind spacecraft in conjunction with ground--based data in order to investigate this relationship. We find many examples in the Waves 1-14 MHz data in which there are two shock-like phenomena that occur simultaneously at different frequencies with differing morphology. Radio emissions from strong interplanetary (IP) shocks (which are the bow shocks of fast, large CMEs) often consist of single broad bands starting below ~4 MHz; such emissions were previously called IP type II events. In contrast, metric type II bursts are usually narrow banded and comprise two harmonically related bands. The probable existence of two separate shocks has ramifications for several aspects of Sun-Earth connections. One is that using the drift rates of type II bursts observed in the 10-100 MHz range to predict arrival times of IP shocks at 1 AU is highly questionable. Another is that shock acceleration upstream of CMEs, and relatively high in the corona, may not be the most important process in the early stages of energetic particle events.