The Buneman instability is one of the most fundamental instabilities that arise along the magnetic field line, when thermal electrons are accelerated by an external electric field along the ambient magnetic field. Such electric fields are induced, when the magnetosphere undergoes dynamic changes of its structure such as magnetic reconnection or reconfiguration. We first assume a stable plasma with thermal electrons and ions at rest, where the thermal electrons and ions can have different temperatures as specified by the thermal velocities Ve and Vi, respectively. Under the electric field imposed along the magnetic field, the thermal electrons are accelerated to a finite drift velocity Vd relative to the thermal ions. When the Vd exceeds a critical value, the Buneman instability sets in to convert the drift kinetic energy to electrostatic field energy and thermal energies of electrons and ions. As the electrostatic field grows the electrons are rapidly diffused through nonlinear trapping motion [Omura et al., 2003].
We performed a series of electrostatic particle simulations [Omura and Matsumoto, 1993] starting with a finite drift velocity Vd and with various initial thermal velocities Ve/Vd and Vi/Vd. The nonlinear evolutions of the instability result in a wide variety of electrostatic potential structures along the magnetic field line such as isolated electron holes, isolated ion holes, double layers, and electron acoustic solitons. The final state of the nonlinear evolution also depends on whether the system is uniform with periodic boundaries or nonuniform with open boundaries.
In the present database of the SPACE-W project in Japan, we show a variety of nonlinear evolution processes of Buneman instability started with different sets of (Ve/Vd, Vi/Vd), presenting the animation of the phase space plots of ions and electrons in the (X, Vx) phase space, electric field Ex, and electrostatic potential Φ.