Abstract:
Employing coronagraphic and EUV observations close to the solar surface made by the Solar Terrestrial Relations Observatory
(STEREO) mission, we determined the heliocentric distance of coronal mass ejections (CMEs) at the starting time of associated metric
type II bursts. We used the wave diameter and leading edge methods and measured the CME heights for a set of 32 metric type II bursts
from solar cycle 24. We minimized the projection effects by making the measurements from a view that is roughly orthogonal to the
direction of the ejection. We also chose image frames close to the onset times of the type II bursts, so no extrapolation was necessary.
We found that the CMEs were located in the heliocentric distance range from 1.20 to 1.93 solar radii (Rs), with mean and median values
of 1.43 and 1.38 Rs, respectively. We conclusively find that the shock formation can occur at heights substantially below 1.5 Rs. In a few
cases, the CME height at type II onset was close to 2 Rs. In these cases, the starting frequency of the type II bursts was very low, in the
range 25–40 MHz, which confirms that the shock can also form at larger heights. The starting frequencies of metric type II bursts have a
weak correlation with the measured CME/shock heights and are consistent with the rapid decline of density with height in the inner
corona.
