Estimating the Height of CMEs at the Onset of Metric Type II Bursts
Abstract
The onset of type II radio bursts indicates the moment when a shock front ahead of a coronal mass ejection (CME) forms and starts to accelerate electrons in the solar corona. Therefore, the onset of metric type II bursts, which can be observed remotely by radio instruments, provides us the first indication of the CME-driven shock front propagating in the solar corona. Several methods have been suggested to estimate the height of the associated CME. Here we estimate the CME height by assuming that CMEs with an initial height of 1.25 Rs accelerate from rest to final speed (the measured linear speed of the CME) during a period starting at the flare onset time and ending at the flare peak time, after which they propagate with a final speed. Using this method Gopalswamy et al. (2012) found the CME height at metric type II burst onset with mean and median values of 1.53 Rs and 1.47 Rs, respectively for the cycle 23 ground level enhancement (GLE) events. We have expanded this study to include all major non-GLE solar energetic particle (SEP) events. We find that the average and median height of non-GLE CMEs at the onset of the metric type II bursts to be 1.73 Rs and 1.61 Rs, respectively. For cycle 23 (cycle 24) CMEs the average height is 1.78 Rs (1.59 Rs) and the median height 1.61 Rs (1.45 Rs), respectively. Because Alfven speed is proportional to magnetic field, weaker solar magnetic fields during cycle 24 result in lower Alfven speeds on average and hence shocks could form earlier, if we assume that CME speed distributions are similar in cycle 23 and cycle 24.
- Publication:
-
Solar Heliospheric and INterplanetary Environment (SHINE 2014)
- Pub Date:
- June 2014
- Bibcode:
- 2014shin.confE.103M