Download A magnetospheric dynamo to explain quasi

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