Nonlinear stabilization of the Weibel instability driven by counterstreaming ion beams

Summary form only given. Counterstreaming ion beams are a common feature of the Earth´s magnetosphere. Such ion beams occur in the outer regions of the plasmasphere, the plasma sheet and the lobes. Weibel showed that when fast ion beams counterstream they drive purely growing modes. We have studied the excitation and nonlinear evolution of the ion-beam driven instabilities by means of a 2.5-D particle-in-cell code. It is shown that in the electrostatic regime, the instability is not effective in slowing down the beams; it is stabilized by perpendicular heating of the beam ions. The driven waves interact with the beam ions through the anomalous cyclotron resonance, which converts a small fraction of the parallel drift energy of the beam ions into their perpendicular energy. As the ions´ perpendicular energy increases, the increase in the ion Larmor radius pi makes J/sub n/(k/sub /spl perp///spl rho//sub i/)/spl rarr/0, where J/sub n/ is the Bessel function of order n, k/sub /spl perp// is the wavelength perpendicular to the ambient magnetic filed. This stops the perpendicular ion energization and the instability is quenched. Since for the electrostatic ion cyclotron modes k/sub /spl perp///spl rho//sub i//spl sim/1; the perpendicular ion acceleration by the electrostatic Weibel instability is only modest. The perpendicular acceleration of ions leads to change in the ions´ pitch angle. This leads to the trapping of the ions in a magnetic mirror. Incorporation of these results into a large-scale refilling model of the plasmasphere will be discussed.