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| Titel |
Trans-terminator flow in the ionospheres of Mars and Venus |
| VerfasserIn |
Markus Fränz, Eduard Dubinin, Erling Nielsen, Anne Angsmann, Joachim Woch, Stas Barabash, Rickard Lundin, Andrei Fedorov, Tielong Zhang |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250051284
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| Zusammenfassung |
| The upper ionospheres of Mars and Venus are permeated by the magnetic fields
induced by the solar wind. It is a long-standing question whether these fields can put
the dense ionospheric plasma into motion. If so, the transterminator flow of the
upper ionosphere could explain a significant part of the ion escape from the planets
atmospheres. But it has been technically very challenging to measure the ion flow at
energies below 20eV. The only such measurements have been made by the ORPA
instrument of the Pioneer Venus Orbiter reporting speeds of 1-5km/s for O+ ions at
Venus above 300km altitude at the terminator (Knudsen, GRL 1982). At Venus the
transterminator flow is sufficient to sustain a permanent nightside ionosphere, at
Mars a nightside ionosphere is observed only sporadically. We here report on new
measurements of the transterminator ion flows at Mars and Venus by the ASPERA-3 and -4
experiments on board Mars and Venus Express. For Mars we use support from the
MARSIS radar experiment for some orbits with fortunate observation geometry.
Here we observe a transterminator flow of O+ and O2+ ions with a super-sonic
velocity of around 5km/s and fluxes of 0.8 -
109/cm2s. If we assume a symmetric flux
around the terminator this corresponds to an ion flow of 3.1 ± 0.5 Ã 1025-s half of
which is expected to escape from Mars. This escape flux is significantly higher
than previously observed on the tailside of Mars, we discuss possible reasons for
the difference. 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. For Venus there is no observation of the cold plasma density by Venus
Express and we can infer properties of the plasma distribution only using stronger
assumption than for Mars. We discuss the implication of these new observation for
ion escape and possible extensions of the analysis to dayside observations which
might allow us to infer the flow structure imposed by the induced magnetic field. |
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