• PHYSICAL PROBLEM: What is the origin of the localized electron excesses (or "electrostatic bipolar structures") observed at high altitudes in the auroral zones?

 
• OBSERVATIONS: in the polar auroral zones by FAST, FREJA (spacecrafts on terrestrial orbits measuring waveforms for the electric potential); similar wave forms are observed in the solar wind (here the Wind mission)

 
• EQUATIONS: Vlasov equation for a strongly magnetized plasma (particles move along the magnetic field; the electic field depends on two spatial coordinates, parallel and perpendicular to the field)

 
• NUMERICAL EXPERIMENTS: One starts from a plasma with uniform electron density but made of two electron populations with opposite velocities (mimicking the auroral population). The boundaries of the domain are open.

 
• CONCLUSION: the two populations are unstable, and the density does not remain uniform; electron packets are generated corresponding to the observed "bipolar structures".

 
• REFERENCE: work in progress...

The FILM (mov, 4 Mb; gif, 2,9 Mb) shows the plasma evolution starting from two electron populations with opposite velocities but with uniform density.

The initial homogeneous state changes rapidly: fluctuations of the electronic density appear.

Two phases show up:
a) The linear phase, during which the fluctuations grow (oblique strips which then undergo fragmentation)
b) The saturation phase, during which sporadic, intermittent electron blobs appear.