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| Titel |
Are planetary ECMI emissions driven by loss-cone or shell distributions? |
| VerfasserIn |
Robert Mutel, Donald Gurnett, John Menietti |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250098735
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| Publikation (Nr.) |
 EGU/EGU2014-14439.pdf |
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| Zusammenfassung |
| In all solar system planets with global magnetospheres, the electron cyclotron maser
instability (ECMI) generates intense, highly polarized coherent radio emission. It
is generated as downward-beamed electrons modify their phase distributions in
converging magnetic fields, resulting in -f--v-¥ > 0 and the creation of X and
O-mode radiation near the electron cyclotron frequency. This occurs in exclusively in
density-depleted auroral regions, where the condition Ωp-Ωc -ª 1 is satisfied. Many
characteristics of the emitted radiation, such as polarization ellipticity, angular beaming
pattern, and energy conversion efficiency, depend on the nature of the unstable phase
distribution.
Two types of phase distribution that have been invoked to explain ECMI radiation:
loss-cone and shell (‘horseshoe’) distributions. Since these distributions can result in
very different emission properties, and hence plasma-physical parameters derived
from the observed radiation, it is important to ascertain which distribution is more
effective in driving the instability and whether the dominant distribution varies with
individual planet. Direct measurements of unstable phase space distributions are difficult
because they must be made in situ in the source region with very high time-resolution.
This has proved difficult: Only at Earth and Saturn are there any direct phase space
measurements, and they are often compromised by inadequate time or pitch angle
sampling.
We summarize the empirical evidence for and against loss-cone and shell distributions at
Earth, Jupiter, and Saturn, based on observed emission properties (polarization, mode,
angular beaming). We also examine the ECMI growth rates for a parameterized phase
distribution function that varies continually from shell (zero velocity offset) to loss-cone. We
find that the shell distribution provides a more robust growth rate, is more consistent with in
situ measurements and with emission properties, but that some aspects of the observed
radiation e.g., the angular beaming pattern of Jovian decametric emission, may favor a
loss-cone distribution. |
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