A joint Cluster and ground-based instruments study of two magnetospheric substorm events on 1 September 2002
- 1Radio and Space Plasma Physics Group, Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
- 2Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
- 3Space Science and Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
- 4CESR/CNRS, BP 4346 9, Avenue Colonel Roche, 31028 Toulouse CEDEX 4, France
Abstract. We present a coordinated ground- and space-based 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 RE} distance of the spacecraft from the plasma sheet. The Cluster spacecraft then encounter an earthward-moving 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.
Key words. Magnetospheric physics (Magnetosphereionosphere interactions; Magnetic reconnection; Auroral phenomenon)