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| Titel |
MESSENGER Observations of Cusp Plasma Filaments at Mercury |
| VerfasserIn |
Gangkai Poh, James Slavin, Xianzhe Jia, Gina DiBraccio, Jim Raines, Suzanne Imber, Daniel Gershman, Brian Anderson, Haje Korth, Ralph McNutt, Sean Solomon |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250103183
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| Publikation (Nr.) |
 EGU/EGU2015-2582.pdf |
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| Zusammenfassung |
| The MESSENGER spacecraft, in orbit about Mercury, has documented highly localized,
~1-2-s-long reductions in the dayside magnetospheric magnetic field of the planet with
amplitudes up to 90% of the ambient intensity. These magnetic field depressions which we
have termed cusp filaments are observed from just poleward of the magnetospheric cusp to
mid-latitudes, i.e., from ~55 to 85oN. Minimum variance analysis and superposed epoch
analysis of the Magnetometer (MAG) data indicate that the filaments are simple two
dimensional flux tubes. If the filaments move over the spacecraft at the polar convection
speed, then these filaments have a mean diameter of ~230km, which is an order of magnitude
larger than the gyro-radius of a 1 keV H+ ion, i.e., ~ 23 km. During these events,
MESSENGER’s Fast Imaging Plasma Spectrometer (FIPS) measured H+ ions
with magnetosheath-like energies consistent with the view that the magnetic field
depressions are diamagnetic and most probably the low-altitude extensions of flux transfer
events (FTEs) that form at the magnetopause as a result of reconnection. Here we
analyze 349 filaments identified in MESSENGER magnetic field and plasma data to
determine the physical properties of these structures. MESSENGER observations
during the spacecraft’s final low-altitude campaign confirm that these cusp filaments
extend down to very low altitudes. We calculate an average particle precipitation rate
onto the surface from all of the filaments at any given time of ~ 2x1025 #s-1. This
precipitation rate is comparable to published estimates of the total precipitation rate in the
cusp proper. The existence of these cusp filaments has important implications for
surface sputtering and our understanding of Mercury’s northern cusp. Overall, the
MAG and FIPS observations analyzed here appear consistent with an origin for
cusp plasma filaments by the inflow of magnetosheath plasma associated with the
localized magnetopause reconnection process that produces FTEs at higher altitudes. |
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