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In a case study, we investigate transitions between fundamental\udmagnetosphere–ionosphere (M-I) coupling modes during storm-time conditions\ud(SYM-H between −100 and −160 nT) driven by an interplanetary coronal mass\udejection (ICME). We combine observations from the near tail, at geostationary\udaltitude (GOES-10), and electrojet activities across the auroral oval at\udpostnoon-to-dusk and midnight. After an interval of strong westward\udelectrojet (WEJ) activity, a 3 h long state of attenuated/quenched WEJ\udactivity was initiated by abrupt drops in the solar wind density and dynamic\udpressure. The attenuated substorm activity consisted of brief phases of\udmagnetic field perturbation and electron flux decrease at GOES-10 near\udmidnight and moderately strong conjugate events of WEJ enhancements at the\udsouthern boundary of the oval, as well as a series of very strong eastward\udelectrojet (EEJ) events at dusk, during a phase of enhanced ring current\udevolution, i.e., enhanced SYM-H deflection within −120 to −150 nT. Each\udof these M-I coupling events was preceded by poleward boundary\udintensifications and auroral streamers at higher oval latitudes. We identify\udthis mode of attenuated substorm activity as being due to a magnetotail state\udcharacterized by bursty reconnection and bursty bulk flows/dipolarization\udfronts (multiple current wedgelets) with associated injection dynamo in the\udnear tail, in their braking phase. The latter process is associated with\udactivations of the Bostrøm type II (meridional) current system. A\udtransition to the next state of M-I coupling, when a full substorm expansion\udtook place, was triggered by an abrupt increase of the ICME dynamic pressure\udfrom 1 to 5 nPa. The brief field deflection events at GOES-10 were then\udreplaced by a 20 min long interval of extreme field stretching (Bz\udapproaching 5 nT and Bx ≈ 100 nT) followed by a major\uddipolarization (Δ Bz ≈ 100 nT). In the ionosphere the\udlatter stage appeared as a "full-size" stepwise poleward expansion of the\udWEJ. It thus appears that the ICME passage led to fundamentally different M-I\udcoupling states corresponding to different levels of dynamic pressure\ud(Pdyn) under otherwise very similar ICME conditions. Full WEJ\udactivity, covering a wide latitude range across the auroral oval in the\udmidnight sector, was attenuated by the abrupt dynamic pressure decrease and\udresumed after the subsequent abrupt increase

Transitions between states of magnetotail–ionosphere coupling and the role of solar wind dynamic pressure: the 25 July 2004 interplanetary CME case