Solar eruptions and long wavelength radio bursts: The 1997 May 12 event Original Research Article

We report on the cause of the 1997 May 12 type II bursts observed by ground based and space-based radio instruments. We estimate the fast mode speed in the corona as a function of heliocentric distance to identify the regions where fast mode shocks can be driven by CMEs. We find that both the coronal and the interplanetary type II bursts can be explained by shocks driven by the same CME at two different spatial domains. The fast mode speed in the corona has a peak at a heliocentric distance of ∼ 3 R⊙ which does not allow the coronal shock wave to propagate beyond this distance. When the CME continues to travel beyond the fast mode peak, another shock forms in the interplanetary medium where the fast mode speed falls sufficiently. From the radio observations we can infer that the plane of the sky speed of the CME is smaller than the space speed by at least a factor of 2, consistent with the location of the eruption at N21 W08. The inferred CME speed is also consistent with previous deprojected speed estimates.