Tangential discontinuities as “roots” of auroral

arcs: an electrostatic magnetosphere-ionosphere

coupling mode

M. Echim (1,2), M. Roth (1), J.de Keyser (1)

Institute d’Aeronomie Spatiale de Belgique, Bruxelles, Belgique

Institute for Space Sciences, Bucharest, Romania

OUTLINE

Electrostatic couplingAuroral current – voltage relationshipLyons modelIonospheric population: parallel fluxTangential discontinuities: kinetic solutionsSolutions for the current continuity Conclusions, future work

Electrostatic coupling

A parallel electric field (distributed or confined) accelerates precipitating particles producing auroral emissions6 out of 12 auroral acceleration mechanisms are stationary (Borovski, 1993)Lyons (1980, 1981): divergence of the electric field as source of auroral arcs Roth, Evans, Lemaire (1993): magnetospheric boundary layers as source of auroral arcs

Current-voltage relationship

•Parallel particle flux as moment of the velocity distribution function

•Two plasma populations connected by geomagnetic field lines; assumed monotonic profile of the potential (Lemaire and Scherer, 1971,1973; Knight, 1973)

)(

3|||| )( vdfvqj

•Domain of integration determined by conservation of the total energy and magnetic moment

Lyons model (1)

•Mapping along auroral field lines (cilindrical, dipolar)•Current continuity on top of the ionosphere:

P

i

ii

ii dx

d

dx

djj )()(0 ||

•Two-point boundary condition:

,0)(|| xV )()(|| imii xxV

Lyon’s model (2)

•Magnetospheric boundary condition: 0 E•Pedersen conductivity (Harel et al., 1977):

P ||2106.15.0

•Energy flux of precipitating electrons: (Lundin and Sandahl, 1978)

2|||| V

•Current – voltage relationship: downgoing electrons (Knight, 1973)

SSe

SSe TeVgKT

eV

m

KTeBNj ,exp

2 ||||

||

•Inverted-V (102 km) scale at ionospheric altitudes for simple, step jump, profiles

•Smaller scale size (101 km) for “nested-V” profiles

Lyon’s model (3)

Parameters: Bm/Bi,m , KTm, nm, x, cut-off

Ionospheric population: parallel flux

•Knight (1973) model:

Ee

EEe TeVg

m

KTeBNj ,

2 ||||

No ion parallel flux

•Lemaire and Scherer (1971): By neglecting gravitational potential, the ions have negative potential energy (i > m)

i

m

i

EiEii B

B

m

KTBNqj

2||

Tangential discontinuities: kinetic solutions• One-dimensional models: Sestero(1964,1966), Lemaire and

Burlaga (1976), Roth(1984), Roth et al.(1996)

•Parameters: n1,2, T1,2, V1,2, m1,2, BC for elmag field

Solutions for current continuity: Upgoing and downgoing electrons

“Lyons” m

Multiple auroral FA current curtains (Echim et al.,1997)

Solutions for current continuity: Upgoing and downgoing electrons m from TD models (hot dense plasma through cold rarefied)

CONCLUSIONS, FUTURE WORK

The ionospheric latitudinal scaling is mainly determined by the structure of the TD generator: proton (101 km) and electron (102 m) Larmor scales may be interpenetratingA return current is obtained from the current continuity when the ionospheric electron population is addedThe return current density is more sensitive to the temperature than the density of the ionospheric electronsIonospheric ions do not contribute significantly to the upward currentFuture development: investigation of the location of the generator; towards a 2D model

Solutions for current continuity: Upgoing and downgoing electrons m from TD models (cold dense plasma through thin rarefied)