Sodium-ion pickup observed above the magnetopause during MESSENGER's first Mercury flyby: Constraints on neutral exospheric models



[1] Single-particle tracings of sodium pickup ions launched upstream of Mercury's magnetopause are used to investigate the access of these ions to the magnetosphere and set limits on the distribution of neutral sodium about the planet during the first MESSENGER flyby. The transport of pickup ions is modeled using flow velocity and magnetic fields from a three-dimensional magnetohydrodynamic simulation. Extensive penetration of pickup ions into the magnetosphere is found in the post-noon and dusk local time sectors due to the northward interplanetary magnetic field at the time of MESSENGER's first flyby. It is concluded that: (1) pickup of magnetosheath photoions may be an important source of hot planetary ions within the magnetosphere; and (2) the sodium ions in the magnetosheath observed by MESSENGER must originate from an extended neutral exosphere due to ion sputtering and/or to a partially escaping distribution generated by photon-stimulated desorption with yields modified by regolith trapping.