Characteristics of coronal mass ejections associated with long‐wavelength type II radio bursts

Abstract

We investigated the characteristics of coronal mass ejections (CMEs) associated with long‐wavelength type II radio bursts in the near‐Sun interplanetary medium. Type II radio bursts in the decameter‐hectometric (DH) wavelengths indicate powerful MHD shocks leaving the inner solar corona and entering the interplanetary medium. Almost all of these bursts are associated with wider and faster than average CMEs. A large fraction of these radio‐rich CMEs were found to decelerate in the coronagraph field of view, in contrast to the prevailing view that most CMEs display either constant acceleration or constant speed. We found a similar deceleration for the fast CMEs (speed > 900 km s−1) in general. We suggest that the coronal drag could be responsible for the deceleration, based on the result that the deceleration has a quadratic dependence on the CME speed. About 60% of the fast CMEs were not associated with DH type II bursts, suggesting that some additional condition needs to be satisfied to be radio‐rich. The average width (66°) of the radio‐poor, fast CMEs is much smaller than that (102°) of the radio‐rich CMEs, suggesting that the CME width plays an important role. The special characteristics of the radio‐rich CMEs suggest that the detection of DH radio bursts may provide a useful tool in identifying the population of geoeffective CMEs.