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Quasi-stationary acceleration of auroral electrons and ions from conjugated ClusterDMSP observations and a magnetosphereionosphere coupling model R. Maggiolo1, M. Echim1,2, M. Roth1, J. De Keyser1 1 BIRA-IASB Brussels, Belgium 2 ISS Bucharest, Romania 17th Cluster Workshop 12-15 May 2009 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 17th Cluster Workshop 12-15 May 2009 INTRODUCTION Modeling • Magnetospheric Tangential Discontinuities (TD) can generate convergent electric fields with scales typical for discrete aurora (Roth et al. 1993) Plasma discontinuity 2 1 H+,O+, e- • 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) 17th Cluster Workshop 12-15 May 2009 H+,O+, e- eE E Ionospheric O+, H+ J// E MODEL Coupling of a magnetospheric discontinuity with the auroral ionosphere MAGNETOSPHERIC GENERATOR Plasma discontinuity Echim et al. 2007 1 H+,O+, e- 2 H+,O+, e- E E Kinetic model of TD J// Adiabatic current-voltage relation E Current continuity on topside ionosphere IONOSPHERIC LOAD 17th Cluster Workshop 12-15 May 2009 MODEL Kinetic model of TD (Roth et al. 1996) Planar TD surface in the X-Z plane, Time stationarity, BN=0 MAGNETOSPHERIC GENERATOR Z 1 TD 2 ZM XM 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 17th Cluster Workshop 12-15 May 2009 B MODEL Kinetic model of TD (Roth et al. 1996) MAGNETOSPHERIC GENERATOR Z 1 TD 2 ZM XM Adiabatic current-voltage relation 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) Current continuity on topside ionosphere J// B IONOSPHERIC LOAD ZI Jpedersen 17th Cluster Workshop 12-15 May 2009 XI MODEL Kinetic model of TD (Roth et al. 1996) MAGNETOSPHERIC GENERATOR Z 1 TD 2 ZM XM Adiabatic current-voltage relation J// B IONOSPHERIC LOAD Current continuity on topside ionosphere j// ( z I , M ( xI ) I ( xI )) ZI Jpedersen XI ( p I ) xI xI Uniform ionospheric densities and temperatures: e-, H+, O+, He+ Ionospheric feedback: energy dependent conductance p 0.5 160 ( M I ) 17th Cluster Workshop 12-15 May 2009 (Harel et al. 1977) MODEL Magnetospheric generator input parameters Asymptotic plasma parameters at each side of the discontinuity BM, NM, TM, VM Tangential Discontinuity model Adiabatic current-voltage relationship Current continuity on topside ionosphere Output inside the magnetospheric discontinuity M(xM), N(xM), J//(xM) Output at ionospheric altitude I(xI), J//(xI), (xI) Ionospheric load input parameters Ionospheric parameters NI, TI, P 17th Cluster Workshop 12-15 May 2009 CLUSTER DMSP CONJUNCTION: in situ data VS model Cluster-DMSP conjunction (Vaivads et al. 2003 GRL) Southern Hemisphere, 20H MLT Altitudes: Cluster 3.7 RE DMSP 850 km 19h18:10 19h15:09 19h18:10 19h16:54 19h17:03 17th Cluster Workshop 12-15 May 2009 CLUSTER DMSP CONJUNCTION: in situ data VS model H+ Plasma discontinuity Upgoing O+ ~3 keV O+ Bipolar electric field structure V=E.dl Potential drop ~3.5 kV Precipitating electrons ~100 eV (not shown) 17th Cluster Workshop 12-15 May 2009 CLUSTER DMSP CONJUNCTION: in situ data VS model 1- CLUSTER data SC1 SC2 Structure stationary at a time scale of a few minutes SC3 SC4 17th Cluster Workshop 12-15 May 2009 CLUSTER DMSP CONJUNCTION : in situ data VS model 2- DMSP data In the region I current Thickness ~ 30 km Precipitating electrons: ~3.5 keV 17th Cluster Workshop 12-15 May 2009 CLUSTER DMSP CONJUNCTION : in situ data VS model 1- Cluster measurements as input parameters for the TD model MAGNETOSPHERIC GENERATOR Plasma discontinuity - H+ and e- on both sides of the discontinuity - center : upgoing O+ and downgoing e- 1 2 H+, e- H+, e- CLUSTER (4,7 RE) E E + Ionospheric O E eDMSP (850 km) IONOSPHERIC LOAD 17th Cluster Workshop 12-15 May 2009 CLUSTER DMSP CONJUNCTION : in situ data VS model 2- Comparison between the TD model profiles and Cluster data inside the magnetospheric discontinuity The model reproduces: 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 17th Cluster Workshop 12-15 May 2009 CLUSTER DMSP CONJUNCTION : in situ data VS model 3- Comparison between DMSP data and the model predictions at ionospheric altitude The model reproduces: The spatial scale of the structure Model ~20-25 km DMSP ~30 km The field aligned potential drop Model 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 flux Model 12-26 mW/m² DMSP 20 mW/m² 17th Cluster Workshop 12-15 May 2009 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 gradients ii) convective effects due to shears of plasma bulk velocity 17th Cluster Workshop 12-15 May 2009 17th Cluster Workshop 12-15 May 2009 17th Cluster Workshop 12-15 May 2009 SC3 E B 17th Cluster Workshop 12-15 May 2009 Plasma discontinuity Field aligned O+ ~2 keV Bipolar electric field structure Potential drop ~2kV V=E.dl Upward field aligned current J//=1/0 rot(B) 17th Cluster Workshop 12-15 May 2009 17th Cluster Workshop 12-15 May 2009