Magnetic cloud passage at Earth and associated substorm activity

An approach to the study of the solar wind-magnetosphere interaction by signal type, that is, by examining the effect in the magnetosphere of well defined interplanetary structures, is presented. Focus is on the response of the magnetosphere to interplanetary magnetic clouds. Among their properties are: the slow and smooth variation of the magnetic field vector, with fluctuation level well below common interplanetary values; the similarly well behaved bulk flow; the wide range of field and flow parameters; and the longevity of passage (1 to 2 days). If the magnetic cloud is oriented such that a long period of uninterruptedly northward pointing field is followed by a long interval of continuously southward pointing field, then the transition of the magnetosphere from a quiescent state (the 'ground state') to a very active state can be studied, the latter being sustained by continued forcing from the magnetic cloud. A synopsis of the main findings of a recent study in such an interaction is given, concentrating on the substorm activity attending the second part of cloud passage.