O. Marghitu (1, 2), M. Hamrin (3), B.Klecker (2), K. R ö nnmark (3)

1. Experimental Evidence for Concentrated Generator Regions in the Nightside Auroral Magnetosphere by Cluster / FAST Conjunctions O. Marghitu (1, 2), M. Hamrin (3), B.Klecker (2), K. Rönnmark (3) (1) Institute for Space Sciences, Bucharest, Romania (2) Max-Planck-Institut für extraterrestrische Physik, Garching, Germany (3) Department of Physics, Umeå University, Umeå, Sweden CIS Team Meeting, Paris July 6-8, 2005

2. Preamble • This talk => follow up ofInvestigation of the energy conversion in the auroral magnetosphere with conjugated Cluster and FAST data, CIS meeting, June 2004 • Since then => two companion papers submitted to Annales Geophysicae: • Experimental investigation of auroral generator regions with conjugated Cluster and FAST data • Observations of concentrated generator regions in the nightside magnetosphere by Cluster / FAST conjunctions • The results were also presented at the EGU General Assembly in Vienna. • In the following => an overview of the two papers, which concentrate on 5h of Cluster data, from 22 UT on Septemeber 20, 2001, to 03 UT on Sepetember 21. During this time there were 3 conunctions with FAST.

3. Outline • Background • Conjunction geometry • Conjunction 3 • Data overview; Signature; Timing; Generator ingredients • Conjunctions 1 and 2 • CGR physics • Summary and prospects

4. A Background A • To our knowledge, the experimental investigations of the generator region are missing, as far as the evaluation of E•Jand Sis concerned: • The one s/c missions before Cluster could not fully resolve J • Both J and (mainly) E are close to the instrumental detection limit • Recent experimental studies on Alfven waves: • Polar data, near the PSBL – Wygant et al., 2000, 2002; Keiling et al., 2000, 2001. • Polar vs. Geotail data, in the PS – Angelopoulos et al., 2002. • There is a significant number of theoretical 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

5. The generator region (E·J<0) in the magnetosphere powers the loads (E·J>0) in the auroral acceleration region and ionosphere. • Cluster is in the southern plasma sheet, at 18 RE. FAST passes below the auroral acceleration region, at 0.6 RE. A Background A

6. The energy flux of a moderate 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 (1.5 – 15 RE) along the field line, the power density is ~10-13 – 10-12 W/m3. A Background A

7. A Background – Precautions... A • ... Choice of the reference system. • ... Derivation of the electric field by using EFW, CIS/CODIF and CIS/HIA data. • ... Evaluation of the current density from FGM data, via the Curlometer method.

8. B Conjunction GeometryB No ground optical data. No optical data from IMAGE or Polar.

9. C Conjunction 3– Data OverviewC 1 mW/m2 16 mW/m2

10. C Conjunction 3 – Generator SignatureC E•J  5 •10 -13 • Left: Average on the available spacecraft. Right: All the available instruments and spacecraft. • Both CODIF and HIA agree on E·J < 0, but not EFW. However, ASPOC is off on SC1 and SC2!! • The main contribution to E·J < 0 comes from the Y direction, on SC1 and SC3. This can be understood by checking the conjunction timing.

11. C Conjunction 3 – TimingC • Tail footprint of FAST and projections of Cluster spacecraft. • The conjunction is indicated with the horizontal dashed line. • The shaded yellow area near Cluster shows roughly the CGR projection near Cluster. • The detection of the generator signature just on SC1 and SC3 suggests that the CGR varies both in space and in time. • We estimate a CGR extension along the field line of up to a few 1000km.

12. C Conjunction 3 – Generator IngredientsC • The CGR signatures (a) are detected when Jy is large and positive, while Ey is negative (b) • Jy is large at those times when Cluster probes the gradients in the thermal pressure (panel c, yellow) close to the PSB => J diamagnetic, as expected. • The diamagnetic current is of the same order with what we get from the Curlom. • Sometimes the relative orientation of the Cluster tetrahedron with respect to the PSB is such that no pressure gradient is seen (magenta), and no current. • The total pressure (d) is constant. This suggests a complicated, 3D wavy structure, of the PSB.

13. C Conjunction 3 – Generator IngredientsC • Vz < 0 and Vy > 0 (g, h) on SC1 and SC3, close to the PSB, support the 3D wavy structure of the PSBL. • The plasma velocity agrees also with the CGR orientation inferred from the timing analysis.

14. D Conjunctions 1 & 2: Overview FAST D Earthward energy flux 14 mW/m2 mapped to ionos. Earthward energy flux 10 mW/m2 mapped to ionos. • The electron energy flux is of the same order as the mapped Poynting flux. • The small scale structure in the inverted-V is of the same order as the mapped extension of the CGRs.

15. CODIF proton and FGM, Sep. 19 – 20, 2001. (a) Energy spectrogram. (b) Density and temperature. (c) Velocity (GSE). (d) Magnetic field (GSE). Magenta = Conjunctions. Yellow = Concentrated generator regions (CGRs). D Conjunctions 1 & 2: Overview ClusterD

16.  CGR2 CGR3 CGR4 CGR1 • The dot product of J (a) and E (c) is negative within the CGRs, E·J<0 (d), which shows as sharp gradients in the cumulative sum (e). • During the CGRs 1, 2, and 4 the Poyning flux (d) is directed to the Earth. • The divergence of B is small (b) => we trust the Curlometer. D Conjunctions 1 & 2: Cluster Data – E ·J, S D

17. Cumulative sum of E·J, with J from the Curlometer and E from CODIF, HIA, and EFW. The CODIF and HIA E computed as –v x B. • Quite good agreement between CODIF and EFW. • E·J≈10-12 W/m3, consistent with the estimate. • Most of E·J from the Y direction. D Conjunctions 1 & 2: Cluster Data - CGR s D

18. 1. WK , panel (c) , is the work done by the thermal pressure forces on the volume element (VE). 2. WK > 0 during CGR1: • part of WK serves to increase the internal energy of the VE (proportional to PK) => panel (a) • part of WK is spent to push the plasma against the Lorentz force => conversion of mechanical energy into electromag. energy, E·J<0, panel (c). – 4 – 2 CGR1 E CGR Physics – Work E

19. 3. The Poynting theorem: div S = – ∂PB/ ∂t – E·J Panel (b) => – ∂PB/ ∂t >0. Both terms on the right side are positive => electromagnetic energy is carried away from the CGR. – 4 – 2 CGR1 E CGR Physics – Poynting flux E

20. F Summary F • Cluster data provide the first in-situ experimental evidence for the crossing of generator regions in the magnetosphere. • The CGRs are located near the PSB, where there are strong gradients in the plasma pressure. • The associated diamagnetic current, Jy, together with a negative Ey cause the main contribution to E·J. • The identified CGRs correlate with auroral electron precipitation observed by FAST. • There is a net elmag. energy flux leaving the CGRs, which could contribute to power the aurora near the polar cap boundary.

21. More detailed discussion on the energy conservation => upcomming paper (?). • The 3D structure of the CGRs. • Potential for a statistical investigation of several events in September – October 2001. • The coupling between CGRs and Alfvén waves. • Potential for application to other generator regions (e. g. LLBL). F Prospects F