MESSENGER observations of plasma ion composition at Mercury through the first 150 days of orbital observations

Details MESSENGER observations of plasma ion composition at Mercury through the first 150 days of orbital observations MESSENGER observations of plasma ion composition at Mercury through the first 150 days of orbital observations

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p44a..07r&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P44A-07

Physics

[2706] Magnetospheric Physics / Cusp, [2756] Magnetospheric Physics / Planetary Magnetospheres, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [5443] Planetary Sciences: Solid Surface Planets / Magnetospheres

Scientific paper

The Fast Imaging Plasma Spectrometer (FIPS) on the MESSENGER spacecraft has taken nearly continuous measurements of Mercury's plasma environment since insertion of the spacecraft into orbit about the innermost planet on March 18, 2011. Though substantial variability is seen, two regular plasma features persist in measurements collected over the first 1.5 Mercury years. On the dayside, a marked plasma population is observed at high northern latitudes, in the region of the magnetic cusp. On the nightside, plasma is regularly observed near the equator. In this work, we examine the composition of these plasma populations in detail, as well as the compositional variability. On average, protons make up 97% of the observed flux. Helium is next in abundance, with the majority in the form of alpha particles (2.6%) and small amounts of He+ (0.09%). Heavy ions (mass > 4 amu) are less well resolved and have been grouped together for improved statistics. The two most abundant are Na-group ions (with mass per charge values (m/q) of 21-30 amu/e, 0.2%) and O-group ions (m/q of 14-20 amu/e, 0.06%). Other species, such as S, K, and Ca, are present in only trace amounts (< 0.01%). These orbital ion abundances are very similar to those seen during MESSENGER's first and second flybys of Mercury for the same analysis methods. Substantial departures from these averages and resulting abundance ratios are found to regularly occur, including instances when Na-group abundances exceed those for protons. These results offer insight into plasma ion source and transport processes in Mercury's space environment.

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