.~~urnrrlojAtmusphrri~ und TrrresrririlPh~sic~.Vol. 53.No. 1lrlLpp. IQYI-1098. IYYI. 0021-9169'91 $3.00+ .?+I Printed in Great Bntam. Pergamon Press plc Ionospheric con~ibu~on to the cross-tail current enhancement during the substorm growth phase I. A. DAGLIS* Max-Planck-Institut ftir Aeronomie. D-341 1 Katlenburg-Lindau. Federal Republic of Germany E. T. SARRIS Section of Telecommunications and Space Science, Demokritos University of Thrace, GR-67100 Xanthi. Greece and G. KREMSER~ Department of Physics, University of Oulu, SF-90570 Oulu, Finland Abstract-We present a growth phase case study in the near-Earth nightside magnetosphere with AMPTE/CCE data. The observations indicate an increase of the difference between parallel and transverse pressures of the particle distribution, which can lead to or favour the enhancement of the near-Earth cross tail current, necessary for the magnetic field configuration changes during the growth phase. The highest relative pressure enhancement is observed for the ionospheric origin O+ ions, indicating an active par- ticipation of the ionosphere in the substorm growth phase. There is experimental evidence that drastic changes of the magnetic field configuration take place in the near nightside magnetosphere during the growth phase of a substorm (MCPHERRON, 1972; KOKUBUN and MCPHERRON, 1981; BAKER, 1984: and others). A transition from the originally dipole-like con- figuration of the field to a tail-like configuration is observed (demonstrated in the decrease of the field inclination 0, i.e. the angle between the field direction and the equatorial plane) at relatively small geocentric distances (e.g. LIN and BARFIELD, 1984). In a recent simulation study KAUFMANN (1987) showed that the measured magnetic field changes require the formation of a current sheet at R - 6- 10 RE, with a strength exceeding the normal values by more than a factor of 10. To explain the observed evolution he proposed the idea of a positive feedback between the curvature drift current and the degree of the magnetic field line stretching. --.._-- *On leave from Demokritos University of Thrace, Xanthi, Greece. t On leave from Max-P~anck-Inst~tut fur Aeronomie, Katlenburg-Lindau, F.R. Germany. As an alternative TSYGANENKO (1989) proposed a self-consistent response of adiabatic particles to the development of a tail-like field configuration (initially resulting from increased dayside reconnection), lead- ing to an increase of the difference between the parallel and transverse pressures and to an enhanced cross- tail current. In this work we present CCE particle and field observations during the growth phase of the 30 April 1985 substorm, which show that: (a) the particle dis- tribution can account for a cross-tail current needed to support the observed magnetic field ; and (b) iono- spheric ions contribute to the cross-tail current enhancement. 2. OBSERVATIONSAND DISCUSSIONS Our data comes from the Charge Composition Explorer (CCE), one of the three AMPTE spacecraft, launched in August 1984; the CCE has an almost equatorial orbit, with an apogee at -8.8 zyxwvutsrqponmlk Rp We use particle data from the Charge-Energy-Mass (CHEM) spectrometer (GLOECKLER et al., 1985) and magnetic field data from the magnetic field experiment (POTEMRA et al., 1985) also onboard CCE. The CHEM instrument uses a combination of electrostatic analysis, post acceleration and time-of-fight and I091