Height of Shock Formation in the Solar Corona Inferred from Observations of Type II Radio Bursts and Coronal Mass Ejections

TL;DR

This study uses coronagraphic and EUV observations from STEREO to determine the heights at which shocks form in the solar corona, revealing they can occur below 1.5 solar radii, with implications for understanding solar eruptions.

Contribution

First direct measurements of CME heights at type II burst onset using orthogonal viewing angles, minimizing projection effects and providing new insights into shock formation heights.

Findings

Shock formation occurs below 1.5 Rs in most cases.

Type II burst starting frequencies weakly correlate with CME heights.

Shock formation can also occur near 2 Rs with low-frequency bursts.

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 to 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.