Centrifugally Stimulated Exospheric Ion Escape at Mercury

We investigate the escape of exospheric ions such as Na+ into the Hermean magnetosphere. We demonstrate that, because of small spatial scales, the centrifugal effect due to curvature of the ExB drift paths is enhanced at Mercury and that it can lead to significant (up to several hundreds of eV) Na+ energization in the parallel direction. We demonstrate that, because of this centrifugal effect, ions with initial speed well below the escape speed such as those produced via thermal desorption can overcome gravity and gain access to the magnetotail. We show that the escape route of this low-energy exospheric material spreads over a narrower range of altitudes when the convection rate increases and that bulk transport occurs within a limited region of space. These results suggest that, via release of planetary material that would otherwise be gravitationally trapped, the ExB related centrifugal acceleration is an important mechanism for the net supply of plasma to Mercury's magnetosphere.