Mercury's extended sodium exosphere: pickup ion observations in the solar wind by MESSENGER

We present the first observations of sodium pick-up ions upstream of Mercury's magnetosphere. This is an important result as it shows that Mercury's exosphere can at times extend beyond the bow shock. The MESSENGER spacecraft's Fast Imaging Plasma Spectrometer observed multiple events of high sodium counts in the solar wind upstream of Mercury. The pick-up ions displayed a ring velocity distribution, which is the characteristic signature of freshly picked up ions in the solar wind. From these observations we calculate ion densities and estimate neutral densities of Mercury's sodium exosphere. These values are also compared to the UVVS density estimations for such large altitudes.