Experimental Experimental Investigation of the Auroral Investigation of the Auroral GeneratorGenerator with Conjugatedwith Conjugated Cluster Cluster andand FAST FAST Data Data

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

J. McFaddenJ. McFadden ( (44)), M. Andr, M. Andréé (5), S. Buchert (5), and H. R (5), S. Buchert (5), and H. Rèème (6)me (6)

(1) Institute for Space Sciences, Bucharest, Romania(1) Institute for Space Sciences, Bucharest, Romania

(2) Max-Planck-Institut f(2) Max-Planck-Institut fürür extraterrestrische Physik, Garching, Germany extraterrestrische Physik, Garching, Germany

(3) Department of Physics, Umeå University, Umeå, Sweden(3) Department of Physics, Umeå University, Umeå, Sweden

(4) Space Sciences Lab., University of California at Berkeley, USA(4) Space Sciences Lab., University of California at Berkeley, USA

(5) Swedish Institute of Space Physics, Uppsala, Sweden(5) Swedish Institute of Space Physics, Uppsala, Sweden

(6) CESR – CNRS, Toulouse, France(6) CESR – CNRS, Toulouse, France

IAGA 10IAGA 10thth Scientific Assembly Scientific Assembly,, Toulouse Toulouse, , July 28, 2005July 28, 2005

A. Background

B. Conjunction geometry

C. Data overview

D. Generator signature

E. Conjunction timing

F. Generator ingredients

G. 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, 1996

A Background A

To our knowledge, the experimental investigations of the generator region are missing, as far as the evaluation of E•J and S 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

Recent experimental studies on Alfvén 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.

A Background A

• 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

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.

... 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.

A Background – Precautions... A

B Conjunction Geometry B

No ground optical data.No optical data from IMAGE or Polar.

C Data Overview C

16 mW/m2 1 mW/m2

D Generator Signature D

Left: Average over 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.

E•J 5 •10 -13

E Conjunction Timing E

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. With a wavy PSB, a similar structure might be located at the FAST footprint.

Tail footprint of FAST, at 18 and 20 RE; Cluster track, XGSM19RE. The conjunction is indicated with the horizontal dashed line. The shaded yellow area shows roughly the CGR projection near Cluster.

CGR

F Generator Ingredients F

CGRs (a) when Jy>>0 and Ey<0 (b).

Pkin0 (c) near PSB =>

Jdiamag=BPkin/B2 JCurlometer

Sometimes no visible Pkin (c), and no

current, due to the tetrahedron orientation.

F Generator Ingredients F

Ptotconst. for all SC, also when Pkin

varies between SC (d) => Pkin depends on

the distance to the PSB. Complicated 3D structure of the PSB

• Alternation of intervals where Pkin

is similar / not similar between SC.

• Vz < 0, Vy > 0 (g, h) on SC 1 & 3.

• Consistent with the timing analysis. CGR partially coloc. with an Earthward

Flow Region (f) => relation to BBFs ?

CGR

F Generator Ingredients F

The Poynting theorem:div S = – ∂W/ ∂t – E·J (1)

with WWB=B2/20 PB

∂ / ∂t d / dt in the satellite system,

because Vsat << Vplasma.

In panel (c) => regions where dPkin/ dt

>0. Since Ptot const. => – dPB/ dt >0. Both terms on r.h.s. of (1) positive => elmag. energy carried away from CGR. A very clean example => presented in the poster C119.

CGR

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 CGR correlates with auroral electron precipitation

observed by FAST.

At times, Cluster data indicate a net electromagnetic energy flux

leaving the CGR, which could contribute to power the aurora

near the polar cap boundary.

G Summary G

G Prospects G

Additional CGR examples => poster C119, Concentrated Concentrated

generator regions in the nightside auroral magnetosphere.generator regions in the nightside auroral magnetosphere.

More detailed discussion on the energy conservation.

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).