Height of shock formation in the solar corona inferred from observations of type II radio bursts and coronal mass ejections

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.

The Relation Between Large-Scale Coronal Propagating Fronts and Type II Radio Bursts

“…Many start below the height (heliocentric) of 1.5 R , and a smaller number of type II bursts start around 2 R . This is basically consistent with the recent results by Gopalswamy et al (2013). The difference as reflected in the fits may partly come from their primary use of the wave diameter method and their inclusion of events with high starting frequencies; here we exclude the 13 February 2013 event, whose starting frequency was well above the RSTN range.…”

“…We may safely assume that the stand-off distance of the shock wave from the CME is still small at its genesis. Gopalswamy et al (2013) used two methods to study the height of shock formation, namely i) direct measurement of the CME viewed near the limb (to minimize projection effect), and ii) the so-called wave diameter method, where the outer edge of the LCPF on the disk was fitted to a circle and the radius of the circle was equated with the height of the CME shock, assuming spherical geometry. Here we exclusively use method i) as applied to data from STEREO (EUVI and COR-1), which observed the CMEs as limb events (see Figure 2).…”

“…Shock waves may be present at the flanks of the CMEs more likely in the early phase of LCPFs than later, when LCPFs would be identified with freely propagating MHD waves. Observations of LCPFs in the early phase, which usually coincides with the occurrence of the metric type II burst (Gopalswamy et al, 2009(Gopalswamy et al, , 2013, should give us a better insight into how shock waves are driven in the CME processes. Many LCPFs in this phase were found to be much faster than EIT waves, with an average speed of ≈ 600 km s −1 (Nitta et al, 2013a), although a small number of LCPFs were intrinsically slow (e.g.…”

“…This is intended as a first step toward determining when and where shock waves form in solar eruptions (e.g. Gopalswamy et al, 2013), which may have an important consequence in our understanding of the temporal and spatial properties of solar energetic particle events (e.g. Rouillard et al, 2012).…”

Short-term variability of the Sun-Earth system: an overview of progress made during the CAWSES-II period

“…They also estimated that the shock height is within approximately 40 Rs when the particle injection from the shock maximizes. Given the observation that CME-driven shocks form at a heliocentric distance of 1.5 to 5 Rs (Tsurutani et al 2003a;Gopalswamy et al 2013b), these findings indicate that observations close to the Sun are most important for large SEP events. Furthermore, the range of heliocentric distances matches with the frequency range of IP type II bursts (Gopalswamy et al 2012c).…”