2.1. Radio spectrum and sites of type II emission
The type II burst starts at unusually high frequencies (near 500 MHz, Fig. 1.a). The spectrum does not extend below 7 MHz (WIND homepage, Kaiser 1999, pers. comm.), which means that the emission fades at about (5-10) above the photosphere. The type II emission is preceded by impulsive centimetric-to-decimetric and hard X-ray bursts. A 2B H flare starts at 1301 UT in active region NOAA 8113 near the north-eastern limb (N16 E63, Solar Geophys. Data 645 II) near the bright pre-flare loops seen in soft X-rays (Fig. 1).
The type II harmonic lane intersects all observing frequencies of the NRH. The insert in Fig. 1.a displays, on top of the Yohkoh SXT image taken 35 min before the type II burst, the iso-intensity contours of the harmonic type II source at 411 MHz and the centroids and half widths of the sources at 237 and 164 MHz during the time intervals indicated by horizontal bars at the appropriate frequency of the spectrogram (Fig. 1.a). The source centroids and half widths were determined by fitting gaussians to the scans taken with the east-west and north-south branches of the NRH. The type II emission starts south of the flaring active region. The sources are superposed upon each other; they are non-radially aligned along an axis close to the line of sight. We cannot analyze the relative source location in detail, since at the time of observation the accuracy of positioning is limited by the broad beam of the NRH and by ionospheric refraction.
2.2. Evolution of coronal structures and the origin of the type II shock
The type II burst occurs within an evolving environment, both within the underlying active region and on larger scales, as shown by the daily movies of the SoHO-LASCO coronograph (LASCO homepage, Schwenn 1998, pers. comm.). In this section we describe first the global coronal evolution above the eastern solar limb during 27 November 1997, and then present the relationship of the type II burst with the dynamics of flaring active region structures.
Two streamers lie above the eastern limb (Fig. 1.b). In the northern one, which overlies AR 8113, a CME is observed from 1155 UT. It is still in progress at 1330 UT, creating the wavy shape of the northern part of the structure. Both streamers are eventually disrupted by another CME that is visible from 1356 UT at heights 1 above the photosphere. The last pre-type II image taken by the C1 coronograph (1244 UT; height range 0.1-2 R above the photosphere) shows structural changes in the vicinity of the type II source (Schwenn, pers. comm.). The type II emission hence originates within a configuration that is being violently restructured on a very large spatial scale.
A sequence of Yohkoh SXT quarter resolution (9.84" pixel size) flare-mode, partial frame images of AR 8113 and its surroundings before and at the onset of the type II burst is displayed in Fig. 2. The first images show diffuse and highly inclined loops extending south-eastward from the inner active region whose emission saturates the detector. Their projected height is km. In the subsequent images a localized brightening seems to rise along the southern legs of these loops at a projected speed of km s-1. It is clearly visible at the border of the saturated region at 1313:42 UT, but shows up in other flare mode images from 1312:20 UT, near the time of an earlier rise of the hard X-ray emission than shown in Fig. 1.a. When this blob reaches the loop top (1315:56 UT), the loop expands and actually seems to be disrupted. A few minutes later, one of the loop legs is clearly visible and appears to straighten out. This straigthened feature persists for several minutes and then fades. Those soft X-ray emitting features from the top and upper parts of the rising loop appear to expand outwards from the Sun, fading as they do so, eventually moving out of the field of view. The first high-frequency signature of the type II burst becomes visible near the time of disruption, and just above the loop top (in projection). Two neighboring, but distinct sources are successively observed at 411 MHz, lasting 10 to 20 s each (cf. the superposition on the simultaneous SXT images in the two last panels of Fig. 2). They move eastward, i.e. in the direction of the straightening expanding loop leg, at a projected speed of 1400 km s-1.
The speed of the type II exciter can be derived from the drift rate in the dynamic spectrogram, by use of a model distribution of the thermal electron density along the trajectory. Given the observed non radial propagation, both the gravitational and the magnetic structuring of the coronal plasma must be considered in the density model. If the electron density decreases exponentially with scale height H along the trajectory, the exciter speed is , where is the frequency drift rate at frequency . This is broadly consistent with the estimated speed of the X-ray blob.
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999