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Astron. Astrophys. 346, L53-L56 (1999)

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1. Introduction

Type II bursts at decimetric to dekametric wavelengths are radio signatures of extended shock waves in the solar corona (cf. reviews by Nelson & Melrose 1985; Mann 1995; Aurass 1997). Their origin may be either the sustained propagation at super-magnetosonic speeds of coronal material or a blast wave, i.e. a fast MHD shock generated by short localized energy release during a flare (e.g. Dryer 1981; Hundhausen 1985; Steinolfson 1985). Observations suggest that the type II shock in the low and middle corona results from processes during a flare, rather than being the bow shock of a coronal mass ejection (CME). Direct evidence for the flare association is given by the recent finding that type II precursor signatures occur from the onset of the impulsive flare in active region loops (Klassen et al. 1999). Arguments against the idea that decimetric-to-dekametric type II emission comes from the bow shock of a CME are provided both by statistical analyses and by studies of individual events: The few available imaging observations show that generally the type II source does not have the appropriate position or speed consistent with the bow shock of the white-light CME (Wagner & MacQueen 1983, Gopalswamy & Kundu 1995, and references therein; Klein et al. 1997; cf. Dryer 1982 for the discussion of counterexamples). Furthermore, a systematic study from metric to hectometric wavelengths during a period of weak activity (Gopalswamy et al. 1998) showed that type II bursts originating in the low corona fade before reaching interplanetary space. Arguing that at least some of the type II bursts should continue to long wavelengths if CMEs were the drivers, these authors attributed low (i.e., near surface) coronal type II emission to a blast wave in the flaring active region, on much smaller spatial scales than the CME.

Although the arguments that the CME is not the driver of the type II shock are to a certain extent circumstantial, the prevailing view is presently that type II emission in the low and middle corona is related to the flare, not to the CME. The question is then which specific process during a flare generates the type II shock. The identification of the region where the type II emission first arises and its relationship with the flaring active region structures is fundamental. This information is difficult to obtain: on the one hand the reliable identification of the shock wave requires spectrography and imaging in the same radio waveband. On the other hand, X-ray imaging at a cadence of a few tens of seconds is necessary to track the evolution of active region structures which may reveal the driver. Such structures have typical heights of (104-105) km, and in order to generate fast shocks, speeds of several (102-103) km s-1 must be reached. These combined observational requirements have not been met before.

On 27 November 1997 a bright type II burst was observed from decimetric to dekametric waves by the Tremsdorf spectrograph (Mann et al. 1992) and the Nançay Radioheliograph (henceforth NRH; Kerdraon & Delouis 1997), and soft and hard X-ray observations were available from Yohkoh (Tsuneta et al. 1991; Kosugi et al. 1991). We use these data sets to put new constraints on the dynamics of the coronal features that lead to type II shock formation, and on the shock's propagation through the corona.

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999