Radio evidence on shock wave formation in the solar corona

In order to investigate the formation of radio emitting shock waves above f laring active regions, we combine spectral and imaging observations of type II radio events with X-ray imaging and full-Sun observations and, in one c ase, with the extrapolated magnetic field configuration in the corona. We c onfirm and extend earlier findings that type II bursts are emitted above ac tive region loops seen in soft X-ray images. Sources at successively lower frequencies are non-radially displaced from the axis of the active region l oops. Two new radio features identified in high resolution spectrograms est ablish a possible link between the type II emission and the preceding activ ity in the underlying corona: 1. Groups of fast drift bursts or pulsations with a restricted bandwidth ar e observed in coronal loops from the impulsive flare phase until the onset of the type II emission. These groups or their high frequency cut-off are f ound in the spectral range around the backward extrapolated type II lanes. Envelope features of the group (starting frequency and/or cut-off frequency ) drift gradually to lower frequencies, at a normalized drift rate similar to the following type II lanes. The sources are located between the sites o f Her emission and of the type II emission. We refer to this burst group an d its envelope features as a type II precursor. 2. Immediately before the type LI emission a short(less than or equal to 1 min duration) series of narrow-band bursts occurs at frequencies between th e split bands of the type II lanes. As a whole, the burst sequence has an i nverted U-shaped spectral envelope. We therefore call it an are. It has fun damental-harmonic structure as the subsequent type II, burst, but no band s plit. The source is located near or above the summits of the coronal loops where the precursor emission occurred before, and close to the site where t he type II emission starts. The ai-e feature occurs especially prior to hig h frequency type II bursts, i.e. type IT shocks formed at comparatively low coronal height. It is concluded that the type II burst is related to a plasma jet or a blas t wave that originates in closed magnetic structures in the active region, and is first recognized during the early impulsive phase (between the onset and main maximum of the hard X-ray emission). This disturbance becomes or launches a shock wave when it attains the summit of these structures. The s hock is refracted into overlying field regions, where it becomes visible th rough the type II emission.