Coronal transients and metric type II radio bursts

II. Accelerations at low coronal heights

INAF, Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy e-mail: mancuso@oato.inaf.it

Received: 23 December 2005
Accepted: 24 October 2006

Abstract

In the companion Paper I, Mancuso & Raymond (2004) investigated the relationship between type ii and coronal mass ejection (CME) activity for a sample of twenty-nine CME/shock events that occurred between March and December 1999. Most of the events appeared to lead the type ii emission locations by several minutes and the two sets of speeds were not well-correlated, in apparent disagreement with a CME-driven origin interpretation of the coronal shocks. The above discrepancies were attributed to an artifact effect due to geometry, favoring emission at the flanks of the CME leading edges in correspondence with denser low-Alfvén-speed coronal structures, where shock strengths are enhanced. An important caveat in the analysis carried out in Paper I is that the conclusions that supported the CME-driven shock front-flank scenario were based on sunward-extrapolated CME trajectories deduced from the analysis of Large Angle and Spectrometric Coronagraph (SOHO/LASCO) observations in the outer corona that might not be accurate at low coronal heights where significant accelerations should be present before the CMEs acquire the speeds inferred by coronagraphic images. In the present paper, we re-examine the above relationship for a subset of ten events by integrating the LASCO measurements with ancillary CME observations taken by other instruments (MLSO/Mk4, SOHO/EIT, and UVCS) at coronal heights comparable to the typical type ii radio emissions (1.2–2.5 ). We investigate the bias introduced in the sunward extrapolation of LASCO data and discuss whether the presence of previously undetected CME accelerations at low coronal heights might have affected the conclusions put forth in the CME-driven shock front-flank scenario proposed in Paper I. We show that the new set of observations neither solve the problem of the timing between CMEs and shocks nor improve the correlation between CME and shock speeds, although acceleration effects are found to be important at the typical metric type ii heights and must be taken into account for a proper analysis of the CME/shock relationship.