PIC simulation study of the effect of ion to electron mass ratio on Ion beam driven instability and generation of ion holes

Solitary wave structures with a lifetime of a few 'ms' and a scale width of 10~50 electron debye length have been observed in the magnetosphere. These relatively slow solitary waves have been considered as ion holes, which are suggested to be generated by ion beam driven instability. Previously, some particle simulations have shown that the ion holes are generated by ion beam driven instability. However, these simulations posed a problem that they used reduced ion to electron mass ratios to save computation time. It was assumed that ion and electron dynamics are sufficiently separated with the reduced mass ratios, but it was not clearly verified. In this study, we examine the effect of ion to electron mass ratios on the generation of ion holes by ion beam driven instability using one-dimensional electrostatic particle-in-cell simulation. Using the same plasma condition with one of the previous simulations we obtained almost same results when the same reduced ion to electron mass ratio was used. However, with the real ion to electron mass ratio ion holes were not generated. This implies that the previously suggested ion beam driven instability is strongly affected by the coupling between ions and electrons and the ion to electron mass ratio is important on the development of the instability. Also, this result suggests that the ion beam driven instability may not be the generation mechanism of the ion holes. We will discuss in detail the physical differences in the ion beam driven instability arising from the different ion to electron mass ratios.