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| Titel |
Transitions between states of magnetotail–ionosphere coupling and the role of solar wind dynamic pressure: the 25 July 2004 interplanetary CME case |
| VerfasserIn |
P. E. Sandholt, C. J. Farrugia, W. F. Denig |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 33, no. 4 ; Nr. 33, no. 4 (2015-04-01), S.427-436 |
| Datensatznummer |
250121185
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| Publikation (Nr.) |
 copernicus.org/angeo-33-427-2015.pdf |
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| Zusammenfassung |
| In a case study, we investigate transitions between fundamental
magnetosphere–ionosphere (M-I) coupling modes during storm-time conditions
(SYM-H between −100 and −160 nT) driven by an interplanetary coronal mass
ejection (ICME). We combine observations from the near tail, at geostationary
altitude (GOES-10), and electrojet activities across the auroral oval at
postnoon-to-dusk and midnight. After an interval of strong westward
electrojet (WEJ) activity, a 3 h long state of attenuated/quenched WEJ
activity was initiated by abrupt drops in the solar wind density and dynamic
pressure. The attenuated substorm activity consisted of brief phases of
magnetic field perturbation and electron flux decrease at GOES-10 near
midnight and moderately strong conjugate events of WEJ enhancements at the
southern boundary of the oval, as well as a series of very strong eastward
electrojet (EEJ) events at dusk, during a phase of enhanced ring current
evolution, i.e., enhanced SYM-H deflection within −120 to −150 nT. Each
of these M-I coupling events was preceded by poleward boundary
intensifications and auroral streamers at higher oval latitudes. We identify
this mode of attenuated substorm activity as being due to a magnetotail state
characterized by bursty reconnection and bursty bulk flows/dipolarization
fronts (multiple current wedgelets) with associated injection dynamo in the
near tail, in their braking phase. The latter process is associated with
activations of the Bostrøm type II (meridional) current system. A
transition to the next state of M-I coupling, when a full substorm expansion
took place, was triggered by an abrupt increase of the ICME dynamic pressure
from 1 to 5 nPa. The brief field deflection events at GOES-10 were then
replaced by a 20 min long interval of extreme field stretching (Bz
approaching 5 nT and Bx ≈ 100 nT) followed by a major
dipolarization (Δ Bz ≈ 100 nT). In the ionosphere the
latter stage appeared as a "full-size" stepwise poleward expansion of the
WEJ. It thus appears that the ICME passage led to fundamentally different M-I
coupling states corresponding to different levels of dynamic pressure
(Pdyn) under otherwise very similar ICME conditions. Full WEJ
activity, covering a wide latitude range across the auroral oval in the
midnight sector, was attenuated by the abrupt dynamic pressure decrease and
resumed after the subsequent abrupt increase. |
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