17th cluster workshop 12-15 may 2009 r. maggiolo 1, m. echim 1,2, m. roth 1, j. de keyser 1 1...

17th Cluster Workshop 12-15 May 2009

R. Maggiolo1, M. Echim1,2, M. Roth1, J. De Keyser1

1 BIRA-IASB Brussels, Belgium2 ISS Bucharest, Romania

Quasi-stationary acceleration of auroral Quasi-stationary acceleration of auroral electrons and ions from conjugated Cluster-electrons and ions from conjugated Cluster-DMSP observations and a magnetosphere-DMSP observations and a magnetosphere-

ionosphere coupling modelionosphere coupling model

OUTLINE

Introduction

Auroral structures and magnetospheric discontinuities

Model

Self consistent modeling of the coupling of a magnetospheric plasma discontinuity with the auroral ionosphere

Cluster-DMSP conjunction

Comparison between data and model


H+,O+, e-

EE

E

Plasma discontinuity

Ionospheric O+, H+

21 H+,O+, e-

J//

INTRODUCTION

Modeling

• Magnetospheric Tangential Discontinuities (TD) can generate convergent electric fields with scales typical for discrete aurora (Roth et al. 1993)

• Coupling TD model with ionosphere: generation of current system and field aligned potential drop consistent with discrete auroral arcs (Echim et al. 2007, 2008)

Observations

• Observation of intense auroral electric fields and auroral current system at magnetospheric boundaries with Cluster(Vaivads et al. 2003 ; Marklund et al. 2004, 2007; Johansson et al. 2006, 2007)


e-

MODEL

H+,O+, e-

IONOSPHERIC LOAD

EE

E


21

MAGNETOSPHERIC GENERATOR

H+,O+, e-

J//

Coupling of a magnetospheric discontinuity with the auroral ionosphere

Echim et al. 2007

Kinetic model of TD

Current continuity on topside ionosphere

Adiabatic current-voltage relation


MODEL

TD

Kinetic model of TD (Roth et al. 1996) MAGNETOSPHERIC GENERATOR

1 2

ZM

B

XM

Planar TD surface in the X-Z plane,

Time stationarity, BN=0

Gradients of plasma bulk properties along XM, normal to the TD plane.

Vlasov and Maxwell equations to describe the transition between 2 asymptotic states

Asymptotic populations: piecewise Maxwellian VDFs (Roth et al., 1996)

The Vlasov solution depends on the constant of motion

The moments of the distribution function are computed analytically

They are used to compute the charge density and currents in the Maxwell equations

Poisson’s equation replaced by quasineutrality equation

Z


MODEL

Conservation of the magnetic moment Conservation of the total energy Monotonic variation of the potential Magnetospheric and ionospheric populations taken separately

Electrons (Knight 1973; Lemaire&Scherer 1973)

Ions (Lemaire & Scherer 1971)

Kinetic model of TD (Roth et al. 1996)

Jpedersen

J//


IONOSPHERIC LOAD

ZI

B

XI


TD1 2

ZM XM

Z



MODEL


Kinetic model of TD (Roth et al. 1996)

)(1605.0 IMp (Harel et al. 1977)

)())()(,(//I

Ip

IIIIMI xxxxzj

Uniform ionospheric densities and temperatures: e-, H+, O+, He+

Ionospheric feedback: energy dependent conductance

Jpedersen

J//


IONOSPHERIC LOAD

ZI

B

XI


TD1 2

ZM XM

Z


MODEL

Magnetospheric generator input parameters

Asymptotic plasma parameters at each side of the discontinuity

BM, NM, TM, VM

Ionospheric load input parameters

Ionospheric parameters

NI, TI, P

Tangential Discontinuity modelAdiabatic current-voltage relationshipCurrent continuity on topside ionosphere

Output inside the magnetospheric discontinuity

M(xM), N(xM), J//(xM)

Output at ionospheric altitude

I(xI), J//(xI), (xI)


Cluster-DMSP conjunction (Vaivads et al. 2003 GRL)

Southern Hemisphere, 20H MLT

Altitudes: Cluster 3.7 RE DMSP 850 km

CLUSTER DMSP CONJUNCTION: in situ data VS model


19h17:03

19h18:10

19h15:09

19h16:54

19h18:10


Upgoing O+ ~3 keV

Bipolar electric field structure

Potential drop ~3.5 kV V=E.dl



Precipitating electrons ~100 eV (not shown)

H+

O+


Structure stationary at a time scale of a few minutes

1- CLUSTER data

SC1

SC2

SC3

SC4


CLUSTER DMSP CONJUNCTION : in situ data VS model

In the region I current

Thickness ~ 30 km

Precipitating electrons: ~3.5 keV

2- DMSP data



- H+ and e- on both sides of the discontinuity

- center : upgoing O+ and downgoing e-

1- Cluster measurements as input parameters for the TD model

DMSP (850 km)

H+, e-

e-

IONOSPHERIC LOAD

EE

E


Ionospheric O+

21


CLUSTER (4,7 RE)

H+, e-



The spatial scale of the structure (200 km)The field-aligned potential drop (3.5 kV)

An upward current sheet (0.3 µA/m²)

The density overshoot due to both: - upgoing O+ ions - interpenetration of magnetospheric plasma from the left and right sides of the TD

2- Comparison between the TD model profiles and Cluster data inside the magnetospheric discontinuity

The model reproduces:



The spatial scale of the structureModel ~20-25 km DMSP ~30 km

The field aligned potential dropModel 3.2-3.9 kV DMSP ~3.5 kV

The field aligned current Model 4-6 µA/m² DMSP ~6 µA/m

The peak electron energy fluxModel 12-26 mW/m² DMSP 20 mW/m²

3- Comparison between DMSP data and the model predictions at ionospheric altitude

The model reproduces:


CONCLUSION

Cluster and DMSP data during a conjunction above the Southern auroral oval are well reproduced by a quasi-static model coupling a magnetospheric plasma discontinuity with the ionosphere.

The convergent electric field formed inside a magnetospheric plasma discontinuity can sustain the auroral current system associated with discrete auroral arcs This mechanism generates a quasi-static electric field/acceleration

The electromotive force is due to:

i) thermoelectric effects due to kinetic pressure gradientsii) convective effects due to shears of plasma bulk velocity


SC3

E B



Field aligned O+ ~2 keV

Bipolar electric field structure

Potential drop ~2kV

J//=1/0 rot(B)

V=E.dl

Upward field aligned current


17th cluster workshop 12-15 may 2009 r. maggiolo 1, m. echim 1,2, m. roth 1, j. de keyser 1 1...

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