Observations of Coronal Mass Ejections with Gauribidanur Radioheliograph

Studies of the influence of solar activity on our terrestrial environment has taken on increasing importance in recent years, as has been the realization of how damaging space influences can be. The state of near-Earth space environment is significantly controlled by CMEs, the most geo-effective manifestation of solar activity. Though CMEs are primarily observed with whitelight coronagraphs onboard space missions, one needs non-coronographic data to obtain information on the early evolution of CMEs, in particular for those directed along the Sun-Earth axis that lies far from the plane of the sky.

The latter class of events have a large angular span (close to 360 degrees) and appear as a 'halo' of enhanced emission encircling the Sun. But not all 'halo' CMEs are Earth-directed and there are several false alarms. One reason for this is that their source region could be on the back side of the visible solar disk. Since the occulting disk of a coronagraph covers the solar disk, it will be difficult to establish the source region of a CME using a whitelight coronagram alone. Radio observations play an useful role in this regard since there is no occulter here. By noticing the presence/absence of transient activity in the visible solar disk, one can establish whether a 'halo' CME is Earth-directed or not. The frequency coverage of Gauribidanur Radioheliograph (GRH) is well-suited to study CMEs in the 1-2 Roradial distance range, above the solar surface. The coronagraphs that are in operation at present do not cover the above region due to the large size of their occulting disk. We present here the observation of the halo CME of April 10, 2006, a multi-wavelength study using GRH and SOHO/LASCO data. It was established that the source region of the CME was located on the back side of the solar disk. The low corona kinematics of the event derived using GRH data were also in support of the above. ( R. Ramesh )