Generation and motion of pickup ions in the upstream regionof Mars
Abstract
Mars does not have a global intrinsic dipole magnetic field, and it has a weak gravitational field, both of which allow the neutral exosphere to extend far beyond the Martian bow shock. Therefore, the solar wind interacts directly with the Martian exosphere, and the neutral particles in the atmosphere can be photoionized owing to their interactions with solar ultraviolet rays or interact with the solar wind to produce newborn ions through electron impact ionization and charge exchange. The newborn ions are subjected to the electromagnetic field of the solar wind. These so-called pickup ions are then accelerated by the solar wind convective electric field, and they simultaneously gyrate around the magnetic field line due to Lorentz forces, forming a cycloidal trajectory in real space. This corresponds to a ring-beam distribution in velocity space, which is unstable and can generate electromagnetic waves due to cyclotron resonant instability. Furthermore, the pickup ions may experience strong acceleration and reflection during the ion pickup process. In this brief review, we introduced the research progress of the pickup ions upstream of the Martian bow shock, focusing on the generation mechanism, trajectory, reflection, and acceleration of pickup ions.
