Multiple magnetic clouds: Several examples during March–April 2001

Abstract

Multiple magnetic cloud (Multi‐MC), which is formed by the overtaking of successive coronal mass ejections (CMEs), is a kind of complex structure in interplanetary space. Multi‐MC is worthy of notice due to its special properties and potential geoeffectiveness. Using the data from the ACE spacecraft, we identify the three cases of Multi‐MC in the period from March to April 2001. Some observational signatures of Multi‐MC are concluded: (1) Multi‐MC only consists of several magnetic clouds and interacting regions between them; (2) each subcloud in Multi‐MC is primarily satisfied with the criteria of isolated magnetic cloud, except that the proton temperature is not as low as that in typical magnetic cloud due to the compression between the subclouds; (3) the speed of solar wind at the rear part of the front subcloud does not continuously decrease, rather increases because of the overtaking of the following subcloud; (4) inside the interacting region between the subclouds, the magnetic field becomes less regular and its strength decreases obviously, and (5) β value increases to a high level in the interacting region. We find out that two of three Multi‐MCs are associated with the great geomagnetic storms (Dst ≤ −200 nT), which indicate a close relationship between the Multi‐MCs and some intense geomagnetic storms. The observational results imply that the Multi‐MC is possibly another type of the interplanetary origin of the large geomagnetic storm, though not all of them have geoeffectiveness. Based on the observations from Solar and Heliospheric Observatory (SOHO) and GOES, the solar sources (CMEs) of these Multi‐MCs are identified. We suggest that such successive halo CMEs are not required to be originated from a single solar region. Furthermore, the relationship between Multi‐MC and complex ejecta is analyzed, and some similarities and differences between them are discussed.