Advances in modeling and understanding in-situ observations and coupling of the
 magnetosphere and the auroral ionosphere during Cluster-DMSP conjunctions

Marius Echim(1,2), Romain Maggiolo(1), Johan De Keyser(1), Michel Roth(1)

(1)Belgian Institute for Space Aeronomy, Brussels, Belgium
(2)Institute for Space Sciences, Magurele, Romania

Conjugated observations by Cluster and DMSP connected by the same auroral
 geomagnetic field lines provide physical insight on the processes actively
 coupling the magnetosphere and the auroral ionosphere. During the first years on
 orbit the Cluster spacecraft traversed, close to their perigee, the upper segment
 of the auroral acceleration region; the DMSP spacecraft survey the auroral
 ionosphere probing in situ the accelerated downgoing electrons connecting the
 ionospheric load with the magnetospheric generator. With a quasistationary model
 developed to describe the coupling between the high-altitude magnetosphere and the
 auroral ionosphere, we can estimate the state of the magnetospheric generator as
 well as the electrodynamic properties of the associated auroral arc. The model
 consists of two modules: (1) the magnetospheric module computes the generator
 plasma and field parameters (the electric potential, the plasma density and bulk
 velocity, the current density, the parallel flux of electrons and ions), to be
 compared with data - from Cluster - at high altitude magnetospheric interfaces and
 discontinuities; (2)  the ionospheric module computes a height integrated solution
 of the current continuity at the topside ionosphere considering  the coupling (via
 a Knight-type current-voltage relationship) with the generator described by the
 magnetospheric module. The ionospheric module provides the electrodynamic
 parameters of the discrete auroral arc (ionospheric potential, field-aligned
 potential drop, field-aligned current density, flux of precipitating energy,
 Pedersen conductance enhancement, spatial scale size of the inverted-V and of the
 luminous emission) to be compared with DMSP measurements. We illustrate how the
 model results compare with data collected during the Cluster-DMSP conjunction on
 April 28, 2001.  Both modeling and experimental results provide evidence for the
 quasi-static acceleration of upgoing ions and downgoing electrons by a parallel
 potential drop of the order of 3 kiloVolt, corresponding to a discrete auroral arc
 15 kilometers wide and generated by a magnetospheric plasma interface with a scale
 length of the order of 500 kilometers at 4.5 Earth radii, in the dusk flank.