Abstract
Abstract. In this work we examine simultaneous observations from the two geosynchronous satellites GOES-5 and GOES-6 located at 282°E and 265°E respectively, and from middle and low latitude ground observatories located within 250°E and 294°E geographic longitude, during isolated substorms of moderate activity. The spatial distribution of our observation points allows us to make a detailed study of the azimuthal expansion of the substorm current wedge. The data analysis shows evidence that the substorm initiation and development mechanism include the cross-tail current diversion/ disruption, the substorm current wedge formation and the azimuthal expansion of the inner plasma sheet. The triggering mechanism is initially confined in a longitudinally narrow sector, estimated to be less than 15° and located very close to local midnight to the east or to the west. The current disruption region expands both eastward and westward in the magnetotail, so that the location of major field-aligned currents flowing into the ionosphere shifts successively eastward, and the location of the currents flowing out of the ionosphere shifts successively westward. Evidence was found that the perturbation travels toward the west with velocities greater than those expanding the wedge eastward. The drastic decrease of the velocity with the azimuthal distance from the location of the disturbance initiation, i.e., the onset sector, indicates that the energy release is a very localized phenomenon. Finally, the transient D perturbation observed by the geosynchronous satellites suggests that the field-aligned currents forming the wedge have a longitudunally limited extent.Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions)
Highlights
Quiet time magnetospheric models suggest that the magnetic ®eld con®guration near the geostationary orbit (6.6 Re) should be nearly that of a dipole
In this work we examine simultaneous observations from the two geosynchronous satellites GOES-5 and GOES-6 located at 282°E and 265°E respectively, and from middle and low latitude ground observatories located within 250°E and 294°E geographic longitude, during isolated substorms of moderate activity
To have a global view of the disturbances taking place in the magnetosphere, we present in Fig. 5 the north to south component of the interplanetary magnetic ®eld from IMP8, in 15.5 s resolution, the energy consumption rate in the ionosphere, UI, and the magnetic energy density calculated at GOES-5 and GOES-6 position
Summary
Quiet time magnetospheric models suggest that the magnetic ®eld con®guration near the geostationary orbit (6.6 Re) should be nearly that of a dipole. From this description of the research progress done on this subject, it is evident that the manner in which the SCW initially forms and spreads longitudinally (Kokubun and McPherron, 1981) and radially down the tail (Jacquey et al, 1991) is still the subject of active research and conicting theoretical models (Hesse and Birn, 1991; Lui, 1996; Rostoker, 1996) The purpose of this contribution is to determine the time sequence of the events that make up a magnetospheric substorm phenomenon at geosynchronous orbit (6.6 Re radial distance), and to estimate the velocity of the eastward and westward expansion of the substorm current wedge. Most of the events are consistent with the general results of this work, we present two in particular as the most characteristic events of our sample, showing clearly the azimuthal expansion of the SCW
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