TYPE-III RADIO-BURST PRODUCTIVITY OF SOLAR-FLARES .2. MAGNETIC GEOMETRY ABOVE ACTIVE REGIONS

We study the occurrence probability of type III radio bursts during fl ares as a function of the flare position on the Sun. We find that this probability peaks around 30 degrees east of the central meridian, whi ch points to a reciprocal tilt of the average radiation pattern of typ e IIIs. We argue that anisotropic scattering of the radiation by overd ense coronal fibers parallel to the magnetic field is the dominant fac tor determining the orientation of radiation patterns. It follows that the average magnetic field appears to be tilted 30 degrees west from the vertical. We also find that within a given active region, the aver age type III production rate of flares peaks approximate to 1 degrees west of the center of gravity of all the flares of this active region. We infer that the coronal magnetic field above active regions present s a strong east-west asymmetry, resulting from the well known asymmetr y at the photospheric level. As the west side of an active region cove rs a smaller area with stronger magnetic field than the east side, wes tern flares are generally closer to open field lines than eastern flar es. As a consequence, accelerated particles on the trailing (east) sid e of active regions usually stay trapped in magnetic loops, while on t he leading (west) side they are more likely to escape along open lines into interplanetary space. As a result of the initial westward tilt o f these open lines, we estimate that the corresponding Archimedean spi ral is on average (apparently) rooted 15 degrees west of the flare.