1. CLUSTER Electric Field Measurements in the Magnetotail O. Marghitu (1, 3), M. Hamrin (2), B.Klecker (3), M. André (4), L. Kistler (5), H. Vaith (3), H. Rème (6), and G. Paschmann (3) (1) Institute for Space Sciences, Bucharest, Romania (2) Physics Department, Umeå University, Umeå, Sweden (3) Max-Planck-Institut für extraterrestrische Physik, Garching, Germany (4) Swedish Institute of Space Physics, Uppsala, Sweden (5) Space Science Center, University of New Hampshire, Durham, USA (6) CESR - CNRS, Toulouse, France EGU 1st General Assembly, Nice, April 26, 2004

2. AMotivation BLimitations of the electric field measurements CEvents DSummary and prospects Outline

3. It is difficult to get quantitative estimates of the errors. If 2 (or 3) instruments show nearly identical values one can be more confident in these values. A Motivation A Computing the electric field in the tail is important for studies that investigate generator processes, requiring the evaluation of EJ, which is close to the error limit. FAST – CLUSTER conjunctions => 22 events of precipitating energetic electrons measured by FAST => 3 events presented in the poster session. The energy flux of a strong aurora, ~10 -2 W/m 2, maps to ~10 -5 W/m 2 in the tail (mapping factor ~1000). If the generator region extends 10 7 – 10 8 m along the magnetic field line, the power density is ~10 -13 – 10 -12 W/m 3.

4. 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 E x component –Cold plasma flow EDI: –Low magnetic field –The ‘noise’ from ambient keV electrons B Limitations B Electric field measurements are particularly difficult in the vicinity of the tail midplane, when CLUSTER is close to apogee. The full electric field vector is derived from EFW and EDI data by assuming EB=0 and from CIS data by E+v x B=0. The parallel electric field is neglected.

5. C Events: Sep. 20, 2001 C

6. C Events: Sep. 26, 2001 C

7. C Events: Apr. 23, 2002 C

8. Full, 3-component, electric field vector, for critical time intervals in the tail Allows the computation of JE and investigation of the generator region Cross-check of the electric field as obtained from CIS, EFW, and EDI improves the reliability D Summary D

9. Comparison of JE from CLUSTER data with the electron energy flux measured by FAST => “Energy transfer in the auroral magnetosphere as derived from CLUSTER and FAST data” – poster ST2 / P0580 Investigation of another 2 conjugated tail events in the fall of 2001 Detailed study of 5 conjugated morning events in the fall of 2002 D Prospects D