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.

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.

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.