 |
| Titel |
Ion escape from the Martian Ionosphere |
| VerfasserIn |
M. Franz, E. Dubinin, Y. Wei, J. Woch, D. Morgan, S. Barabash, R. Lundin, A. Fedorov |
| Konferenz |
EGU General Assembly 2012
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063542
|
|
|
|
|
|
| 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 et al. 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
ionospheric ion flows at Mars by the ASPERA-3 experiment on board Mars Express. We
use support from the MARSIS radar experiment for some orbits with fortunate
observation geometry. Here we have observed 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 (Fraenz et
al, 2010). This escape flux is significantly higher than previously observed on the
tailside of Mars, we discuss possible reasons for the difference. Since 2008 the
MARSIS radar does nightside local plasma density measurement which sometimes
coincide with ASPERA-3 measurements. A first analysis of the combined nightside
datasets confirms that at leat half of the transterminator ionospheric flow escapes
from the planet. 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.
We discuss the implication of these new observations 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. |
|
|
|
|
|
|