Superconcentration of photospheric sources of the large-scale open solar magnetic field in the main zone of active longitudes,blocking of differential rotation,and origination of the four-sector structure: 2. Geoeffectiveness

The effectiveness of the series of powerful heliospheric storms, originated during the decline phase of cycle 23 owing to the superconcentration of the open field photospheric sources in the main zone of active longitudes, has been studied. The geoeffectiveness of the storm of July 16—27, 2004, was closely related to the origination of the four-sector structure and depended on the destabilization of two activity centers weakly and strongly geoeffective with ARs 10649 and 10652. The first center was localized in one of the new sectors; the second center, in the western surroundings of the zone of active longitudes. The departure of coronal mass ejections from AR 10649 was substantially hindered: they were completely absent after the series of powerful X-ray flares, and a rare phenomenon of “sunquake” was observed: shock waves did not reach the Earth in spite of its favorable position. The Earth was strongly shielded by new sector boundaries from coronal ejections from AR 10652 with a gradual weakening and disappearance of this region, as a result of which the cascade of three near-Earth storms with an increasing power and Ap, indices of 52, 154, and 186 originated. Rare phenomena in AR 10649 and the cascade of solar—terrestrial heliospheric storms made the storm of July 16— 27 a unique phenomenon in cycle 23, and a short-term prediction of its geoeffectiveness was impossible.