Are Small Asteroids Magnetic?

The possible presence of significant magnetic remanence on asteroids is important because a significant magnetic moment would both shield the surface from the solar wind flux and cause a disturbance in the solar wind flow. This possibility has been raised for the two small (approx. 30 km) asteroids Gaspra and Ida. Galileo data showed magnetic perturbations near these S-type asteroids that have been interpreted as signatures of the solar wind interaction with an asteroidal magnetic field. The size of the observed disturbance suggested that both Gaspra and Ida could have a strong magnetic field. Interplanetary magnetic perturbations are common in the solar wind. To determine if the signatures observed by Galileo are asteroid related or if simply solar wind in origin we compare Galileo observations with the signature generated near a magnetized asteroid using 2-D hybrid simulations (fluid electrons, kinetic ions). We find that when the IMF is perpendicular to the 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 near Gaspra. However, sufficient discrepancies are found between observations and simulated signatures so that we conclude that the perturbation near Gaspra was not generated by the interaction of the solar wind with a magnetized asteroid. For Ida, the IMF is at 45^o to the flow, and our simulations differ even more 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 conclude that the signature observed near Ida was not generated by the solar wind interaction with the asteroid. Furthermore, 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 whistler-mode signatures of the simulations. Our results suggest that it is unlikely that Gaspra and Ida have a magnetic field able to shield them from the space environment.