Extended Fluid Simulations of Precursor Ion Acoustic Solitons

The topic of precursor nonlinear structures (solitons) excited by a charged object traveling through a plasma has attracted much recent attention due to its potential application in the detection and tracking of small sized space debris objects. The basic principle of such excitations in a plasma medium was expounded in a simple one dimensional fluid model calculation [1] leading to a paradigmatic nonlinear partial differential equation, the forced Korteweg de Vries (fKdV) equation, that yielded a variety of driven nonlinear solutions in the form of precursor solitons, “pinned” solitons, wakes and dispersive shocks. The scale sizes and amplitudes of these nonlinear structures for ionospheric conditions are within the detection capabilities of ground based radars or in-situ sensors. Hence they can provide an indirect means of detecting debris objects. Subsequent studies have established the existence of such solitons in a variety of theoretical models [2, 3] as well as in laboratory experiments in a dusty plasma medium [4], While the basic qualitative features of this novel phenomenon are similar to those appearing in analogous hydrodynamic excitations, there are important physical differences arising from plasma effects that are not well explored yet. In particular, the complex dynamics of the plasma sheath region surrounding the moving charged object that is responsible for the creation of the soliton needs to be better understood. In the present study we address this issue with the help of detailed fluid simulations of the sheath and presheath regions in front of the moving charged object. We investigate the evolution of driven nonlinear ion acoustic waves through numerical investigations of the full set of two fluid equations that go beyond the usual weak nonlinearity approximation of the fKdV or forced Kadomtsev-Petviashvilli (fKP) equations. The simulations are done for both planar and cylindrical objects in plasma conditions specific to the LEO and GEO regions of the ionosphere that are heavily populated by orbital debris. Realistic charging models are used to establish the surface potential and charge on the moving object and stochastic fluctuations of the charge are taken into account. The threshold condition for the excitation of the solitons is explored as a function of the ratio of the object speed to the ion thermal speed, the surface charge of the object, the shape of the object and the sheath parameters in order to identify the physical origins of the “pinned” and precursor solitons. It is found that the presheath region plays a dominant role in the excitation of the solitons and its dynamics needs to be investigated in greater depth. This can be done using a kinetic approach or particle-in-cell simulations both of which can benefit from the basic findings of the present extended fluid simulations.