Theory of localized bipolar wave-structures and nonthermal particle distributions in the auroral ionosphere

Bipolar wave structures and nonthermal particle distributions measured by t he FAST satellite in regions of downward current are interpreted in terms o f the nonlinear evolution of a two-stream instability. The instability resu lts in holes, both in the electron distribution in phase space and in the e lectron density in real space. The wave potential energy, which traps the e lectrons, has a single minimum, and the associated electric field is bipola r. The early bipolar structures are coherent over hundreds of Debye lengths in the direction perpendicular to the magnetic field. After thousands of p lasma periods the perpendicular coherence is lost, the structures break up, and electrostatic whistlers begin to dominate. Simulations and preliminary analysis of this breakup and emission process are presented.