Interplanetary origin of geomagnetic storms

Around solar maximum, the dominant interplanetary phenomena causing intense magnetic storms (Dst <-100 nT) are the interplanetary manifestations of fa st coronal mass ejections (CMEs). Two interplanetary structures are importa nt for the development of storms, involving intense southward IMFs: the she ath region just behind the forward shock, and the CME ejecta itself. Wherea s the initial phase of a storm is caused by the increase in plasma ram pres sure associated with the increase in density and speed at and behind the sh ock (accompanied by a sudden impulse [SI] at Earth), the storm main phase i s due to southward IMFs. If the fields are southward in both of the sheath and solar ejecta, two-step main phase storms can result and the storm inten sity can be higher. The storm recovery phase begins when the IMF turns less southward, with delays of approximate to 1-2 hours, and has typically a de cay time of 10 hours. For CMEs involving clouds the intensity of the core m agnetic field and the amplitude of the speed of the cloud seems to be relat ed, with a tendency that clouds which move at higher speeds also posses hig her core magnetic field strengths, thus both contributing to the developmen t of intense storms since those two parameters are important factors in gen ering the solar wind-magnetosphere coupling via the reconnection process. During solar minimum, high speed streams from coronal holes dominate the in terplanetary medium activity. The high-density, low-speed streams associate d with the heliospheric current sheet (HCS) plasma impinging upon the Earth 's magnetosphere cause positive Dst values (storm initial phases if followe d by main phases). In the absence of shocks, SIs are infrequent during this phase of the solar cycle. High-field regions called Corotating Interaction Regions (CIRs) are mainly created by the fast stream (emanating from a cor onal hole) interaction with the HCS plasma sheet. However, because the B-z component is typically highly fluctuating within the CIRs, the main phases of the resultant magnetic storms typically have highly irregular profiles a nd are weaker. Storm recovery phases during this phase of the solar cycle a re also quite different in that they can last from many days to weeks. The southward magnetic field (B-s) component of Alfven waves in the high speed stream proper cause intermittent reconnection, intermittent substorm activi ty, and sporadic injections of plasma sheet energy into the outer portion o f the ring current, prolonging its final decay to quiet day values. This co ntinuous auroral activity is called High Intensity Long Duration Continuous AE Activity (HILDCAAs). Possible interplanetary mechanisms for the creation of very intense magneti c storms are discussed. We examine the effects of a combination of a long-d uration southward sheath magnetic field, followed by a magnetic cloud B-s e vent. We also consider the effects of interplanetary shock events on the sh eath plasma. Examination of profiles of very intense storms from 1957 to th e present indicate that double, and sometimes triple, IMF B-s events are im portant causes of such events. We also discuss evidence that magnetic cloud s with very intense core magnetic fields tend to have large velocities, thu s implying large amplitude interplanetary electric fields that can drive ve ry intense storms. Finally, we argue that a combination of complex interpla netary structures, involving in rare occasions the interplanetary manifesta tions of subsequent CMEs, can lead to extremely intense storms.