O. Marghitu (1, 3), M. Hamrin (2), B.Klecker (3)

1. Investigation of the energy conversion in the auroral magnetosphere with conjugated CLUSTER and FAST data O. Marghitu (1, 3), M. Hamrin (2), B.Klecker (3) (1) Institute for Space Sciences, Bucharest, Romania (2) Physics Department, Umeå University, Umeå, Sweden (3) Max-Planck-Institut für extraterrestrische Physik, Garching, Germany CIS Meeting, Paris June9-11, 2003

2. Preamble Presentation at the CIS Meeting in September: FAST – CLUSTER conjunctions above the auroral region In the Summary and Conclusions section: • FAST electron and CLUSTER ion data examined for 7 conjunctions, 3 on the evening and 4 on the morning side, during relatively quiet or moderately disturbed conditions • On the evening side it is tempting to consider that CLUSTER crosses the arc generator region • Since September a collaboration developed with the Physics Department of the • Umeå University, Sweden, focusing on potential nightside generator regions. • First results communicated at EGU04, Nice: • O. Marghitu, M. Hamrin, B.Klecker, M. André, L. Kistler, H. Vaith, H. Rème, and G. Paschmann, CLUSTER electric field measurements in the magnetotail – presentation • M. Hamrin, O. Marghitu, B.Klecker, M. André, J. McFadden, S. Buchert, and H. Rème, Energy transfer in the auroral magnetosphere as derived from CLUSTER and FAST data – poster

3. Outline • Motivation • Electric field measurements in the tail: limitations • Event study: September 20, 2001 • Electric field: CODIF vs. EFW • Computation of E•J • Cross-check of E: CODIF@2 energies & CODIF vs. HIA • Extension of the time interval • Summary and prospects

4. The energy flux of a strong aurora, ~10-2 W/m2, maps to ~10-5 W/m2 in the tail (mapping factor ~1000). If the generator region extends 107 – 108 m along the magnetic field line, the power density is ~10-13 – 10-12 W/m3. A Motivation A • There is a significant number of studies on the auroral generator region: • Analytical => e.g. Rostoker and Boström, 1976 • Semi-analytical => e.g. Lysak, 1985, Vogt et al., 1999 • Numerical simulations => e.g. Birn et al, 1996, Birn and Hesse, 1996 • But, to our knowledge, the experimental investigations of the generator region are missing, as far as the evaluation of E•J is concerned.

5. B Tail-E Limitations B The full electric field vector is derived from EFW and EDI data by assuming E•B=0 and from CIS data by E+v x B=0. The parallel electric field is neglected. • CIS: • Low count statistics • Particle distributions close to isotropic • Other drifts, in addition to E x B • EFW: • Small angle between B and the s/c spin plane • Sun offset in the Ex component • Cold plasma flow • EDI: • Low magnetic field • The ‘noise’ from ambient keV electrons Electric field measurements are particularly difficult in the vicinity of the tail midplane, when CLUSTER is close to apogee.

6. C Sep. 20, 2001: CODIF vs. EFW C

7. C Sep. 20, 2001: Computation of E•J C

8. C Sep. 20, 2001: Cross-checks of E C

9. C Sep. 20, 2001: Extended time C

10. D Summary and Prospects D • A method for the experimental investigation of the auroral generator region close to being born. • Signatures of the generator – extended (require more checking), and concentrated (more reliable) – identified. • Two papers in status nascendi. • Potential for investigation of several events in September – October 2001. • Potential for application to other generator regions, like the dusk LLBL => several conjunctions in December 2002.