Abstract.An isolated plasma sheet flow burst took place at 22:02 UT, 1 September 2002, when the Cluster footpoint was located within the area covered by the Magnetometers-Ionospheric Radars-All-sky Cameras Large Experiment (MIRACLE). The event was associated with a clear but weak ionospheric disturbance and took place during a steady southward IMF interval, about 1 h preceding a major substorm onset. Multipoint observations, both in space and from the ground, allow us to discuss the temporal and spatial scale of the disturbance both in the magnetosphere and ionosphere. Based on measurements from four Cluster spacecraft it is inferred that Cluster observed the dusk side part of a localized flow channel in the plasma sheet with a flow shear at the front, suggesting a field-aligned current out from the ionosphere. In the ionosphere the equivalent current pattern and possible field-aligned current location show a pattern similar to the auroral streamers previously obtained during an active period, except for its spatial scale and amplitude. It is inferred that the footpoint of Cluster was located in the region of an upward field-aligned current, consistent with the magnetospheric observations. The entire disturbance in the ionosphere lasted about 10 min, consistent with the time scale of the current sheet disturbance in the magnetosphere. The plasma sheet bulk flow, on the other hand, had a time scale of about 2 min, corresponding to the time scale of an equatorward excursion of the enhanced electrojet. These observations confirm that localized enhanced convection in the magnetosphere and associated changes in the current sheet structure produce a signature with consistent temporal and spatial scale at the conjugate ionosphere.

Abstract.This paper provides a brief review of the understanding of the coupled solar windmagnetosphere-ionosphere system that can be attained from observations of the size of the ionospheric polar caps from space and the ground. These measurements allow the occurrence and rate of dayside and nightside reconnection to be deduced. The former can be correlated with upstream interplanetary magnetic field observations to give an estimate of the effective length of the dayside reconnection X-line. The latter allows a preliminary statistical study of flux closure during substorms and other, smaller nightside reconnection events.

Abstract. We present data from both ground-and spacebased instruments for a substorm event which occurred on 5 October 2002, with an expansion phase onset time of 02:50 UT determined from the ground magnetometer data. During this substorm, the Cluster spacecraft were located around 15 R E downtail, 8 R E from midnight in the premidnight sector and just 2 R E above the equatorial plane (in GSM coordinates). At expansion phase onset the Cluster spacecraft were located in the plasma sheet, tailward of a near-Earth neutral line and detected a significant time delay of 6 min between the tail field B z component becoming negative and the subsequent detection of Earthward flows. This is explained by the formation of a tailward-directed travelling compression region initially Earthward of the spacecraft; 7 min later the Cluster spacecraft entered the plasma sheet boundary layer; they remained in and close to the plasma sheet boundary layer for around 15 min before exiting to the lobe. The spacecraft then re-entered the plasma sheet 30 min after onset. Earthward then tailward directed currents detected in the plasma sheet boundary layer after onset indicate that the Cluster spacecraft encountered the dawnward and duskward portions of the reconnection flow associated current system with Region 1 sense, respectively. The reconnection site and current system were initially skewed towards the pre-midnight sector, consistent with previous observations that found the majority of substorm onsets locatedCorrespondence to: N. Draper (email: ncd8@ion.le.ac.uk) in this sector. At later times the reconnection site and current system had moved towards dawn, to be located more centrally in the midnight sector.

Abstract. We present a coordinated ground-and spacebased multi-instrument study of two magnetospheric substorm events that occurred on 1 September 2002, during the interval from 18:00 UT to 24:00 UT. Data from the Cluster and Polar spacecraft are considered in combination with ground-based magnetometer and HF radar data. During the first substorm event the Cluster spacecraft, which were in the Northern Hemisphere lobe, are to the west of the main region affected by the expansion phase. Nevertheless, substorm signatures are seen by Cluster at 18:25 UT (just after the expansion phase onset as seen on the ground at 18:23 UT), despite the ∼5 R E distance of the spacecraft from the plasma sheet. The Cluster spacecraft then encounter an earthwardmoving diamagnetic cavity at 19:10 UT, having just entered the plasma sheet boundary layer. The second substorm expansion phase is preceded by pseudobreakups at 22:40 and 22:56 UT, at which time thinning of the near-Earth, L=6.6, plasma sheet occurs. The expansion phase onset at 23:05 UT is seen simultaneously in the ground magnetic field, in the magnetotail and at Polar's near-Earth position. The response in the ionospheric flows occurs one minute later. The second substorm better fits the near-Earth neutral line model for substorm onset than the cross-field current instability model.

During the recovery phase of a substorm occurring on 1 September 2002 the four Cluster spacecraft crossed from the northern tail lobe into the plasma sheet. While the spacecraft were located in the plasma sheet boundary layer, the magnetic field data from the four spacecraft detected a cavity of close to zero magnetic field. The plasma in this cavity had characteristics similar to that of the central plasma sheet, possibly implying that the central plasma sheet had expanded over and subsequently receded back over the spacecraft. However, the unique four-spacecraft tetrahedral configuration of the Cluster spacecraft shows that this is not a valid scenario as the cavity passed over the four spacecraft, travelling continuously equatorwards and Earthwards. Our analysis is based on data from the Fluxgate Magnetometer, Cluster Ion Spectrometer, Plasma Electron and Current Experiment, and Research with Adaptive Particle Imaging Detectors instruments on board the Cluster spacecraft.