Homologous Flare-CME Events with and without a metric type II radio burst

Active region NOAA 11158 produced many flares and CMEs during its disk passage, out of which at least two can be considered as homologous. The events are the C6.6 flare at 06:51 UT and C9.4 flare at 12:41 UT on February 14, 2011. Both flares occurred at the same location (eastern edge of the active region) and have a similar decay of the GOES soft X-ray light curve. The associated CMEs were slow (319 km/s and 337 km/s) and of similar widths (63 and 64 degree), but they had different radio signature. The second event was associated with a metric type II burst while the first one was not. The COR1 coronagraphs on board the STEREO spacecraft clearly show that the second CME propagated into the aftermath a preceding CME. These observations suggest that CME-CME interaction might be a key process in exciting the type II radio emission by slow CMEs.