Solar wind control of magnetospheric energy content: Substorm quenching and multiple onsets

In this paper we report coordinated multispacecraft and ground-based observ ations of a double substorm onset close to Scandinavia on November 17, 1996 . The Wind and the Geotail spacecraft, which were located in the solar wind and the subsolar magnetosheath, respectively, recorded two periods of sout hward directed interplanetary magnetic field (IMF). These periods were sepa rated by a short northward IMF excursion associated with a solar wind press ure pulse, which compressed the magnetosphere to such a degree that Geotail for a short period was located outside the bow shock. The first period of southward IMF initiated a substorm growth. phase, which was clearly detecte d by an array of ground-based instrumentation and by Interball in the north ern tail lobe. A first substorm onset occurred in close relation to the sol ar wind pressure pulse impinging on the magnetopause and almost simultaneou sly with the northward turning of the IMF. However, this substorm did not f ully develop. In clear association with the expansion of the magnetosphere at the end of the pressure pulse, the auroral expansion was stopped, and th e northern sky cleared. We will present evidence that the change in the sol ar wind dynamic pressure actively quenched the energy available for any fur ther substorm expansion. Directly after this period, the magnetometer netwo rk detected signatures of a renewed substorm growth phase, which was initia ted by the second southward turning of the IMF and which finally lead to a second, and this time complete, substorm intensification. We have used our multipoint observations in order to understand the solar wind control of th e substorm onset and substorm quenching. The relative timings between the o bservations on the various satellites and on the ground were used to infer a possible causal relationship between the solar wind pressure variations a nd consequent substorm development. Furthermore, using a relatively simple algorithm to model the tail lobe field and the total tail flux, we show tha t there indeed exists a close relationship between the relaxation of a sola r wind pressure pulse, the reduction of the tail lobe field, and the quench ing of the initial substorm.