The shock compression of the preexisting southward directed magnetic field can enhance a geomagnetic disturbance. A simple theoretical model is proposed to study the geoeffectiveness of a shock overtaking a preceding magnetic cloud. Our aim is to answer theoretically the question how deep the shock enters into the cloud when the event just reaches the maximum geoeffectiveness. The results suggest that the minimum value of Dst* decreases initially, then increases again while the shock propagates from the border to the center of the cloud. There is a position where the shock compression of the preceding cloud obtains the maximum geoeffectiveness. In different situations, the position is different. The higher the overtaking shock speed is, the deeper is this position, and the smaller is the corresponding Dst∗min. Some shortcomings of this theoretical model are also discussed.

An interplanetary magnetic cloud (MC) is usually considered the byproduct of a coronal mass ejection (CME). Due to the frequent occurrence of CMEs, multiple magnetic clouds (multi-MCs), in which one MC catches up with another, should be a relatively common phenomenon. A simple flux rope model is used to get the primary magnetic field features of multi-MCs. Results indicate that the magnetic field configuration of multi-MCs mainly depends on the magnetic field characteristics of each member of multi-MCs. It may be entirely different in another situation. Moreover, we fit the data from the Wind spacecraft by using this model. Comparing the model with the observations, we verify the existence of multi-MCs, and propose some suggestions for further work.

d in daily averages of Ap index for geomagnetic disturbances from the World Data Center (WDC) at the International Association for Geomagnetism and Aeronomy (IAGA) are also examined for the same four-year time span. By Fourier power spectral analyses, the CME data appears to contain significant power peaks at periods of ～358±38, ～272±26, ～196±13 days and so forth, while except for the ～259±24-day period, X-ray solar flares of class>～M5.0 show the familiar Rieger-type quasi-periods at ～157±11, ～122±5, ～98±3 days and shorter ones until ～34±0:5 days. In the data of daily averages of Ap index, the two significant peaks at periods ～273±26 and ～187±12 days (the latter is most prominent) could imply that CMEs (periods at ～272±26 and ～196±13 days) may be proportionally correlated with quasi-periodic geomagnetic storm disturbances; at the speculative level, the ～138 ±6-day period might imply that X-ray flares of class>M5.0 (period at ～157±11 days) may drive certain types of geomagnetic disturbances; and the ～28±0:2-day periodicity is most likely caused by recurrent high-speed solar winds at the Earth's magnetosphere. For the same three data sets, we further perform Morlet wavelet analysis to derive period-time contours and identify wavelet power peaks and timescales at the 99 percent confidence level for comparisons. Several conceptual aspects of possible equatorially trapped Rossby-type waves at and beneath the solar photosphere are discussed.

y than fast ones. This is consistent with the observational results obtained by Zhang et al. (2003), that all four Earth-encountered limb CMEs originated from the east. On the other hand, since the most of the EFHCMEs are fast events, the range of the longitude distribution given by the theoretical model is E40◦, W70◦, which is well consistent with the observational results (E40◦, W75◦)

A multiple-magnetic-cloud (Multi-MC) structure formed by the overtaking of two successive coronal mass ejections(CMEs) in the heliosphere is studied by using a 2.5-D MHD simulation. This simulation illustrates the process of the formationand propagation of two identical CMEs, which are ejected with speeds of 400km s−1 and 600 km s−1 respectively and initiallyseparated by 12h. The results show that it takes ～18h for the fast cloud to catch up with the preceding slow one, thenthe two clouds form a Multi-MC structure that arrives at 1 AU three days later. The fast cloud is slowed down significantlybecause of the blocking by the preceding slow one. This implies that the travel time of a Multi-MC structure is dominatedby the preceding slow cloud. Moreover, most primary observational characteristics of Multi-MC at 1 AU are well representedby the simulation. In addition, by combining observations, theoretical model and the simulation results, differences betweenMulti-MC and other types of in-situ observed double-flux-rope structure are addressed. A comparison of Multi-MC to coronalmass-ejection cannibalization near Sun is also given.

Based on the observations of the Sun and the interplanetary medium, a series of solaractivities in late October 2003 and their consequences are studied comprehensively. Thirteen X-rayflares with importance greater than M-class, six frontside halo coronal mass ejections (CMEs) withspan angle larger than 100◦ and three associated eruptions of filament materials are identified byexamining lots of solar observations from October 26 to 29. All these flares were associated with typeIII radio bursts, all the frontside halo CMEs were accompanied by type II or type II-like radio bursts.Particularly, among these activities, two major solar events caused two extraordinary enhancements(exceeding 1000 particles/(cm2 s−1 ster−1 Mev−1) of solar energetic particle (SEP) flux intensity nearthe Earth, two large ejecta with fast shocks preceding, and two great geomagnetic storms with Dstpeak value of −363 and −401 nT, respectively. By using a cross correlation technique and a forcefreecylindrical flux rope model, the October 29 magnetic cloud associated with the largest CME areanalyzed, including its orientation and the sign of its helicity. It is found that the helicity of the cloudis negative, contrary to the regular statistical pattern that negative- and positive-helical interplanetarymagnetic clouds would be expected to come from northern and southern solar hemisphere. Moreover, the relationship between the orientation of magnetic cloud and associated filament is discussed. Inaddition, some discussion concerning multiple-magnetic-cloud structures and SEP events is alsogiven.