1. 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

2. OUTLINE Electrostatic coupling Auroral current – voltage relationship Lyons model Ionospheric population: parallel flux Tangential discontinuities: kinetic solutions Solutions for the current continuity Conclusions, future work

3. Electrostatic coupling A parallel electric field (distributed or confined) accelerates precipitating particles producing auroral emissions 6 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

4. 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) Domain of integration determined by conservation of the total energy and magnetic moment

5. Lyons model (1) Mapping along auroral field lines (cilindrical, dipolar) Current continuity on top of the ionosphere : Two-point boundary condition :

6. Lyon’s model (2) Magnetospheric boundary condition: Pedersen conductivity (Harel et al., 1977): Energy flux of precipitating electrons: (Lundin and Sandahl, 1978) Current – voltage relationship: downgoing electrons (Knight, 1973) Inverted-V (10 2 km) scale at ionospheric altitudes for simple, step jump, profiles Smaller scale size (10 1 km) for “nested-V” profiles

7. Lyon’s model (3) Parameters: B m /B i,  m, KT  m, n  m,  x,  cut-off

8. Ionospheric population: parallel flux Knight (1973) model: No ion parallel flux Lemaire and Scherer (1971): By neglecting gravitational potential, the ions have negative potential energy (  i >  m )

9. Tangential discontinuities: kinetic solutions One-dimensional models: Sestero(1964,1966), Lemaire and Burlaga (1976), Roth(1984), Roth et al.(1996) Parameters: n  1,2, T  1,2, V  1,2, m  1,2, BC for elmag field

10. Solutions for current continuity: Upgoing and downgoing electrons “Lyons”  m

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

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

13. CONCLUSIONS, FUTURE WORK The ionospheric latitudinal scaling is mainly determined by the structure of the TD generator: proton (10 1 km) and electron (10 2 m) Larmor scales may be interpenetrating A return current is obtained from the current continuity when the ionospheric electron population is added The return current density is more sensitive to the temperature than the density of the ionospheric electrons Ionospheric ions do not contribute significantly to the upward current Future development: investigation of the location of the generator; towards a 2D model

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