Particle-in-cell simulations with full dynamics ions and drift kinetic electrons are essential tools to model kinetic instabilities in strongly magnetized plasmas. In these guiding center models, parallel acceleration of the electrons is retained while their perpendicular velocity is given to lowest order by the ${\bf E} \times{\bf B}$ drift. These simulations are applied to a study of the electrostatic stability of energetic velocity-space ion shells observed in the Earth's plasma sheet boundary layer by the Cluster satellites.