Ion density and phase space density distribution of planetary ions Na⁺, O⁺ and He⁺ in Mercury's magnetosphere

Photo-ionization of Mercury's tenuous exosphere contributes to the heavy ion population in the Hermean environment. Observations with the MESSENGER Fast Imaging Plasma Spectrometer (FIPS) have revealed the ion density and spatial distribution of the three most abundant planetary ions or ion groups around Mercury: The Na⁺-group (mass-per-charge ratio m/q=21–30amu/e), O⁺-group (m/q=16–20amu/e) and He⁺. We developed a test-particle model coupled to a neutral exosphere model and two different models of the magnetosphere to simulate the ion density distribution of Na⁺, He⁺ and O⁺. We compare the modeled ion density distribution at aphelion for northward interplanetary magnetic field (IMF) with FIPS observations from the entire orbital phase (23 March 2011 to 30 April 2015). Our model reproduces several observed features but the average ion density is up to 18x too high. However, we find that the discrepancy is less than 3x for other solar wind and exosphere conditions. Comparison with previous simulation studies of the Na⁺ ion density and magnetic field line resonance observations give an average Na⁺ density which is on the same order as our estimate. Finally, we model the phase space density (PSD) distribution in four different regions. We find that in three out of four regions only a fraction of the PSD distribution can typically be observed by FIPS. This is mainly due the obstruction of the field-of-view caused by the spacecraft's sun shield, which blocks plasma with a high v_x/v_z ratio from entering the instrument.