Ion Density Holes observed by Cluster satellite: Electromagnetic PIC Simulation

In the upstream region of the bow shock, many transient structures have been found such as hot flow anomalies (HFAs), foreshock cavities (FCs), hot diamagnetic cavities (HDCs), and short- and large-amplitude magnetic structures. Density holes (DHs) are one of such transient phenomena with similar characteristics to those of HFAs, FCs, and HDCs: density depletion accompanied by the depression of magnetic field and "deflection of" flow velocity. While sometimes regarded as the early phase of HFA, DH has a lower magnetic shear and a smaller flow deviation than the HFA. However, the most significant difference between the two structures is the direction of motional electric field (convection electric field). The solar wind convection electric fields of DHs have an outward-component from the embedding IMF current sheets while HFAs usually have components directed inward on either or both of the edges. As the Cluster observations indicate the isolated DH structures generally accompany diffuse ion beams in the rotating magnetic fields, which can be interpreted as a current sheet or a solitary wave, we conjecture the ion-ion beam instability occurring around the current sheet to be the important factor of DHs structures and set up simulation models using a two-dimensional electromagnetic particle-in-cell (PIC) code. Here, we report the characteristics of DHs observed by Cluster and the progress of our simulation study.