Experimental Evidence for Concentrated Experimental Evidence for Concentrated Generator Regions in the Nightside Auroral Generator Regions in the Nightside Auroral

Magnetosphere by Cluster / FAST ConjunctionsMagnetosphere by Cluster / FAST Conjunctions

O. Marghitu (1, 2), M. Hamrin (3), B.Klecker (2), K. RO. Marghitu (1, 2), M. Hamrin (3), B.Klecker (2), K. Röönnmark (3)nnmark (3)

(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

CIS CIS Team Team Meeting, ParisMeeting, Paris

July 6-8, 2005July 6-8, 2005

This talk => follow up of Investigation 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.

Preamble

A. Background

B. Conjunction geometry

C. Conjunction 3

Data overview; Signature; Timing; Generator ingredients

D. Conjunctions 1 and 2

E. CGR physics

F. 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 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.

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 Conjunction 3 – Data Overview C

16 mW/m2 1 mW/m2

C Conjunction 3 – Generator Signature C

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.

E•J 5 •10 -13

C Conjunction 3 – Timing C

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.

C Conjunction 3 – Generator Ingredients C

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.

C Conjunction 3 – Generator Ingredients C

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.

D Conjunctions 1 & 2: Overview FAST D

• 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 flux14 mW/m2 mapped to ionos.

Earthward energy flux10 mW/m2 mapped to ionos.

D Conjunctions 1 & 2: Overview Cluster D

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: Cluster Data – E ·J, S D

• 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 CGR2 CGR3 CGR4

D Conjunctions 1 & 2: Cluster Data - CGR s D

• 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.

E CGR Physics – Work E

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 – Poynting flux E

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

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.

F Summary F

F Prospects F

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).