Distribution and Compositional Variations of Plasma Ions in Mercury's Space Environment: The First Three Mercury Years of MESSENGER Variations

We have analyzed measurements of planetary ions near Mercury made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Fast Imaging Plasma Spectrometer (FIPS) over the first three Mercury years of orbital observations (25 March 2011 through 31 December 2011). We determined the composition and spatial distributions of the most abundant species in the regions sampled by the MESSENGER spacecraft during that period. In particular, we here focus on altitude dependence and relative abundances of species in a variety of spatial domains. We used observed density as a proxy for ambient plasma density, because of limitations to the FIPS field of view. We find that the average observed density is 3.9 × 10-2 cm-3 for He2+, 3.4 × 10-4 cm-3 for He+, 8.0 × 10-4 cm-3 for O+-group ions, and 5.1 × 10-3 cm-3 for Na+-group ions. Na+-group ions are particularly enhanced over other planetary ions (He+ and O+ group) in the northern magnetospheric cusp (by a factor of ~2.0) and in the premidnight sector on the nightside (by a factor of ~1.6). Within 30 degrees of the equator, the average densities of all planetary ions are depressed at the subsolar point relative to the dawn and dusk terminators. The effect is largest for Na+-group ions, which are 49% lower in density at the subsolar point than at the terminators. This depression could be an effect of the FIPS energy threshold. The three planetary ion species considered show distinct dependences on altitude and local time. The Na+ group has the smallest e-folding height at all dayside local times, whereas He+ has the largest. At the subsolar point, the e-folding height for Na+-group ions is 590 km, and that for the O+ group and He+ is 1100 km. On the nightside and within 750 km of the geographic equator, Na+-group ions are enhanced in the premidnight sector. This enhancement is consistent with nonadiabatic motion and may be observational evidence that nonadiabatic effects are important in Mercury's magnetosphere.