Concentrated Generator Regions in the Auroral Magnetosphere as Derived from Conjugated Cluster and FAST Data M. Hamrin (1),O. Marghitu (2, 3), B.Klecker (3), M. Hamrin (1), O. Marghitu (2, 3), B.Klecker (3), A. Vaivads (4), L.M. Kistler (5), M. André (4), S. Buchert (4), J. McFadden (6) and Y. Khotyaintsev (4) S. Buchert (4), J. McFadden (6) and Y. Khotyaintsev (4) (1) Physics Department, Umeå University, Umeå, Sweden (2) Institute for Space Sciences, Bucharest, Romania (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) Space Sciences Lab., University of California at Berkeley, USA EGU General Assembly, Vienna, April 28, 2005

1.Introduction 2.Conjunction geometry 3.Data: Cluster and FAST 4.CGR physics 5.Summary and prospects Outline

 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, Introduction 1  However, to our knowledge, the experimental investigations of the generator region are missing, as far as the evaluation of EJ is 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

1 Introduction 1 The generator region (E·J 0) in the auroral acceleration region and ionosphere. Cluster is in the southern plasma sheet, at 18 R E. FAST passes below the auroral acceleration region, at 0.6 R E.

1 Introduction 1 The energy flux of a moderate 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 (1.5 – 15 R E ) along the field line, the power density is ~ – W/m 3.

2 Conjunction Geometry 2 Cluster 1 and FAST ionospheric footprints (110km). Conjunctions at 22:23 and 00:29 UT. T96 model used for mapping. Rather low magnetospheric activity, Kp=1.

... 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.  More discussion in the poster session, ST8 / X Cluster Data – Precautions... 3

3 Cluster Data – Overview 3 CODIF proton and FGM, Sep. 19 – 20, (a) Energy spectrogram. (b) Density and temperature. (c) Velocity (GSE). (d) Magnetic field (GSE). Magenta = Conjunctions. Yellow = Concentrated generator regions (CGRs).

3 Cluster Data – E · J, S 3 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.   CGR1 CGR2CGR3CGR4

3 Cluster Data - CGR s 3 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 ≈ W/m 3, consistent with the estimate. Most of E·J from the Y direction.

3 FAST Data 3 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. Earthward energy flux 14 mW/m 2 mapped to ionos.

4 CGR Physics – Work 4 1. W K, panel (c), is the work done by the thermal pressure forces on the plasma element (PE). 2. W K > 0 during CGR1: part of W K serves to increase the internal energy of the PE (proportional to P K ) => panel (a) part of W K 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

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

 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, J y, together with a negative E y 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 CGR1, which could contribute to power the aurora near the polar cap boundary. 5 Summary 5

5 Prospects 5  More discussion on auroral generator issues in the poster ST8 / X552, “The auroral generator: A case study using conjugated Cluster and FAST data“.  Potential for a statistical investigation of several events in September – October  The 3D structure of the CGRs.  The coupling between CGRs and Alfvén waves.  Potential for application to other generator regions (e. g. LLBL).