Abstract
Abstract. The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase. In addition, we suggest that the IMF Bz component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.
Highlights
If the nightside reconnection rate is not increased to balance the dayside merging, the nightside near-Earth magnetic field lines start to stretch and the polar cap area expands. This leads to a substorm growth phase and one could expect correlation between Vday, the polar cap boundary (PCB) latitude and the near-Earth magnetic topology
The results have been compared to a global GUMICS-4 MHD simulation
The magnetic signature of the convection reversal boundary (MCRB), which is the latitude where the equivalent east-west electrojets change direction according to the MIRACLE magnetometer analysis, followed the temporal variations seen in the EISCAT PCB location
Summary
The time interval studied is on 18 February 2004 from 15:00 to 24:00 UT (∼ 17:30–02:30 MLT in the Scandinavian sector). The expansion phase starts when the AE index starts a rapid rise and it ends when the index has reached a maximum (McPherron, 1995). In this case, the AE index is almost entirely determined by the AL index: AU is small and slowly varying throughout the time interval studied (data not shown). Pi2 magnetic pulsations recorded by the NUR station (56.8◦ cgmLat) in the Scandinavian sector are shown in the bottom panel. Pseudobreakups are smaller scale events than substorms with shorter duration and associated with only small or no poleward expansion. Another small steplike increase in pressure occurs at about 19:41 UT Neither of these coincides with substorm onsets. There is no indication of an external trigger of substorms
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