Hybrid simulations of solar wind interaction with magnetized asteroids: Comparison with Galileo observations near Gaspra and Ida

[1] We use a two-dimensional hybrid code (fluid electrons and particle ions) to simulate the interaction of the solar wind with a magnetized asteroid. Understanding the nature of this interaction and of the signatures such as whistler waves that can be generated in the perturbed region can help us to establish the strength of magnetization of the asteroid. We model the asteroid magnetic field as a dipole with different strengths and consider different geometries between the interplanetary magnetic field (IMF) and the solar wind flow. We find that the characteristics of the wake change dramatically with asteroidal magnetic moment. When the magnetic moment is small a whistler wake is generated. Stronger dipolar moments generate a wake with magnetosonic characteristics. Galileo data showed the existence of magnetic perturbations near asteroids Gaspra and Ida. In contrast, no magnetic signature was observed by the NEAR-Shoemaker spacecraft in the vicinity of asteroid 433 Eros. Previous works have compared Galileo observations with Hall-MHD simulations. In this work we compare the results of hybrid simulations with Galileo observations near Gaspra and Ida. Our simulations show that when the IMF is perpendicular to the solar wind flow, the interaction of the plasma with a magnetized asteroid generates a perturbation downstream of the asteroid that is formed by whistler and magnetosonic waves. This perturbation resembles in some ways the signature observed by Galileo in the vicinity of Gaspra. However, the discrepancies found between observations and simulated signatures lead us to conclude that the perturbation near Gaspra was not generated by the interaction of the solar wind with a magnetized asteroid. In the case of Ida the IMF is at 45degrees to the flow, and our simulation results differ from observations. The wave wake forms upstream of the asteroid at a distance that is small compared with observations and has a small size. Therefore we believe that the signature observed near Ida was not generated by the interaction of the solar wind with the asteroid. In both cases the observed signatures are linearly polarized resembling the magnetic discontinuities that are commonly found in the solar wind and in contrast to the nearly circularly polarized signatures of the simulations.