Solar and interplanetary causes of very intense geomagnetic storms

The dominant interplanetary phenomena causing intense magnetic storms are t he interplanetary manifestations of fast coronal mass ejections (CMEs). Two interplanetary structures are important for the development of such class of storms, involving an intense and long duration B-s component of the IMF: the sheath region just behind the forward shock, and the CME ejecta itself . Frequently, these structures lead to the development of intense storms wi th two-step growth in their main phases. These structures also lead sometim es to the development of very intense storms, especially when an additional interplanetary shock is found in the sheath plasma of the primary structur e accompanying another stream. The second stream can also compress the prim ary cloud, intensifying the B-s field, and bringing with it an additional B -s structure. Thus, at times very intense storms are associated with three or more B-s structures. We also discuss evidence that magnetic clouds with very intense core magnetic fields tend to have large velocities, thus imply ing large amplitude interplanetary electric fields that can drive very inte nse storms. (C) 2001 Elsevier Science Ltd. All rights reserved.