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| Titel |
Coherent ULF waves in Mercury's nightside magnetosphere: Their source and impact |
| VerfasserIn |
S. A. Boardsen, B. J. Anderson, J. A. Slavin, J. M. Raines, H. Korth, D. Schriver, 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 |
250061174
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| Zusammenfassung |
| Coherent ultra-low-frequency (ULF) waves at frequencies between 0.2 and 3 Hz in
Mercury’s inner magnetosphere (R < 2 RM, where R is radial distance from
planetary center and RM is Mercury’s radius) are frequently observed by the orbiting
MESSENGER spacecraft. These waves are not seen deeper in the tail (R >
2 RM). The ULF waves usually exhibit banded harmonic structure that drifts in
frequency as the spacecraft traverses the magnetic equator. The waves are observed at
all magnetic local times sampled during equator crossings. However, their rate of
detection drops off strongly on the dayside, because dayside observations are limited by
the presence of broad-band cusp emissions and the spacecraft on the dayside is
usually located in the magnetosheath at low latitudes. For these reasons, we limit our
investigation of these waves to the nightside magnetosphere. On average, the wave
power is maximum at the equator and decreases with increasing magnetic latitude
(MLAT), suggesting an equatorial source. When the spacecraft traverses strong
diamagnetic depressions during its equatorial crossings, wave power is a factor
of 10 larger than for equatorial crossings that do not cross marked depressions.
The waves are predominantly transverse at large magnetic latitudes but tend to
become compressional near the equator. Some events are marked by transverse waves
at all MLATs, including the equator. A few of these events can be interpreted as
ion cyclotron waves. In general, the waves tend to be strongly linear; they have
ellipticity magnitudes < 0.3 and wave normal angles peaked near 90Ë . There are two
hypotheses for the source of these waves. In the first, the waves are field-aligned
resonances modified by the ion cyclotron frequencies, and their source consists of
compressional waves generated in other regions of the magnetosphere and absorbed at the
ion-ion resonances. Under the second hypothesis, the waves are generated near the
magnetic equator in the nightside magnetosphere from highly unstable plasma-sheet ion
distribution functions; the large planetary loss cone angle at Mercury, typically 30Ë , may
play an important role in wave generation and would, in turn, further scatter these
ions and enhance their precipitation onto the surface. It has been recently been
suggested that the first mechanism is highly sensitive to the concentration of heavy ions
such as Na+ and also that variation in the Na+ concentration will contribute to
the drift in wave frequency. A detailed comparison of observed waves with the
predictions of theoretical models provides a basis for further elucidating the source
mechanism of these waves and the roles they play in Mercury’s magnetosphere. |
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