ESTIMATING THE HEIGHT OF CMEs ASSOCIATED WITH A MAJOR SEP EVENT AT THE ONSET OF THE METRIC TYPE II RADIO BURST DURING SOLAR CYCLES 23 AND 24

ABSTRACT We studied the coronal mass ejection (CME) height at the onset of 59 metric type II radio bursts associated with major solar energetic particle (SEP) events, excluding ground level enhancements (GLEs), during solar cycles 23 and 24. We calculated CME heights using a simple flare-onset method used by Gopalswamy et al. to estimate CME heights at the metric type II onset for cycle 23 GLEs. We found the mean CME height for non-GLE events (1.72 R☉) to be ∼12% greater than that (1.53 R☉) for cycle 23 GLEs. The difference could be caused by more impulsive acceleration of the GLE-associated CMEs. For cycle 24 non-GLE events, we compared the CME heights obtained using the flare-onset method and the three-dimensional spherical-shock fitting method and found the correlation to be good (CC = 0.68). We found the mean CME height for cycle 23 non-GLE events (1.79 R☉) to be greater than that for cycle 24 non-GLE events (1.58 R☉), but statistical tests do not definitely reject the possibility of coincidence. We suggest that the lower formation height of the shocks during cycle 24 indicates a change in the Alfvén speed profile because solar magnetic fields are weaker and plasma density levels are closer to the surface than usual during cycle 24. We also found that complex type III bursts showing diminution of type III emission in the 7-14 MHz frequency range are more likely associated with events with a CME height at the type II onset above 2 R☉, supporting suggestions that the CME/shock structure causes the feature.