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| Titel |
Cold Ion Escape from the Martian Ionosphere - 2005-2014 |
| VerfasserIn |
Markus Fränz, Eduard Dubinin, David Andrews, Hans Nilsson, Andrei Fedorov |
| 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 |
250106158
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| Publikation (Nr.) |
 EGU/EGU2015-5811.pdf |
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| Zusammenfassung |
| It has always been challenging to observe the flux of ions with energies of less than 10eV
escaping from the planetary ionospheres. We here report on new measurements of the
ionospheric ion flows at Mars by the ASPERA-3 experiment on board Mars Express. The ion
sensor IMA of this experiment has in principle a low-energy cut-off at 10eV but in negative
spacecraft charging cold ions are lifted into the range of measurement but the field of view is
restricted to about 4x360 deg. In a recent paper Nilsson et al. (Earth Planets Space, 64,
135, 2012) tried to use the method of long-time averaged distribution functions to
overcome these constraints. In this paper we first use the same method to show
that we get results consistent with this when using ASPERA-3 observations only.
But then we can show that these results are inconsistent with observations of the
local plasma density by the MARSIS radar instrument on board Mars Express.
We demonstrate that the method of averaged distribution function can deliver the
mean flow speed of the plasma but the low-energy cut-off does usually not allow
to reconstruct the density. We then combine measurements of the cold ion flow
speed with the plasma density observations of MARSIS to derive the cold ion flux.
In an analysis of the combined nightside datasets we show that the main escape
channel is along the shadow boundary on the tailside of Mars. At a distance of about
0.5 R_M the flux settles at a constant value which indicates that about half of the
transterminator ionospheric flow escapes from the planet. To derive the mean escape flux
we include all combined observations of ASPERA-3 and MARSIS from 2005 to
2014.
Possible mechanism to generate this flux can be the ionospheric pressure gradient
between dayside and nightside or momentum transfer from the solar wind via the induced
magnetic field since the flow velocity is in the Alfvénic regime. |
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