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| Titel |
Compositional variations in the distribution of plasma ions in Mercury's magnetosphere: The first Mercury year of MESSENGER observations |
| VerfasserIn |
J. M. Raines, D. J. Gershman, T. H. Zurbuchen, M. Sarantos, J. A. Slavin, J. A. Gilbert, H. Korth, B. J. Anderson, G. Gloeckler, S. Krimigis, D. N. Baker, R. L. Jr. McNutt, S. C. Solomon |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063246
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| Zusammenfassung |
| MESSENGER’s Fast Imaging Plasma Spectrometer (FIPS) has observed planetary ions on
nearly every orbit about Mercury since orbit insertion on March 18, 2011. On average, the
ratio of the flux of Na-group ions (mass per charge m/q = 21-30) to that of alpha particles
(He2+) has been 26%, and the corresponding ratios for O-group ions (m/q = 14-20) and
He+are 9% and 7%, respectively. Other species, such as S and Ca, are present at times, but
only in trace amounts (< 1%). Though variability has been substantial, three dominant
magnetospheric plasma features were regularly observed in measurements collected
during MESSENGER’s first Mercury year in orbit. On the dayside, a large plasma
population has been seen at high northern latitudes, in the region of the magnetospheric
cusp. On the nightside, plasma has regularly been observed near the equator, in the
central plasma sheet. The third regular feature has been an increase in plasma flux
near the magnetopause. In this work, we examine the composition of these plasma
populations in detail, as well as spatial variability within these regions. We find that
the Na-group has the highest flux among planetary ions in all regions. Its flux is
the highest in the magnetospheric cusp, by a factor of 10, consistent with polar
enhancements of observed neutral species. The fluxes of O-group ions and He+,
although lower, also peak in the magnetospheric cusp, indicating that planetary
ions are produced and/or trapped in that location. In the plasma sheet, the fluxes
of He2+and He+ are enhanced on the post-midnight side, in agreement with the
pattern of diamagnetic depressions documented by MESSENGER’s Magnetometer.
However, Na-group and O-group ions show the opposite pattern: enhancement on
the pre-midnight side. This difference suggests a cause other than increased solar
wind flux and may indicate that an alternate source or transport process is involved.
The increased planetary ion flux observed around the magnetopause suggests a
high-magnetic-latitude source or the entry and subsequent trapping of planetary ions from the
magnetosheath. The regional patterns highlighted by this initial survey confirm that
differences in planetary ion composition across Mercury’s magnetosphere offer
new clues to the processes of ion formation and transport at the innermost planet. |
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