Global coupling between the solar wind and the oxygen ion escape at Venus
IRAP, CNRS-UPS-CNES, Toulouse, France
Co-authors: Y. Futaana, Swedish Institute of Space Physics, Kiruna, Sweden R. Ramstad, LASP, University of Boulder Colorado, Boulder, CO, USA A. Schillings, Department of Physics, UmeÃ¥ University, UmeÃ¥, Sweden K. Masunaga, LASP, University of Boulder Colorado, Boulder, CO, USA H. Nilsson, Swedish Institute of Space Physics, Kiruna, Sweden A. Fedorov, IRAP UPS CNRS, Toulouse, France S. Barabash, Swedish Institute of Space Physics, Kiruna, Sweden
Understanding the influence of a global magnetic field requires a comparison with the baseline â€“ non-magnetised planets. Venus is a good example because of its lack of both an intrinsic and crustal magnetic fields. An important characteristic for Venus is that today, it has a very dry and thick atmosphere, but that it might have been covered with several hundreds of meters of water on its surface in its early history. One of the mechanisms that could be responsible for the loss of the water is atmospheric escape to space. One of the largest escape channels at Venus at present day is non-thermal ion escape through the magnetotail. The interaction between the solar wind and the Venusian ionosphere causes an energy transfer from the solar wind to the ionospheric particles. When the ionospheric ion reaches above escape energy (~8 eV for O+) it escapes to space. It has been shown that the escape of oxygen ions at Venus increases as the energy in the upstream solar wind increases. In this study, we extend the previous escape study by further investigating how much of the solar wind energy that are transferred to the escaping particles and how the energy transfer changes with the upstream conditions.
We used the Ion Mass Analyser (IMA) sensor, part of the ASPERA-4 instrument package on board Venus Express, to investigate the coupling between the net power of the escaping oxygen ions and that of the upstream solar wind. We show that only about 0.01 % of the available solar wind power is transferred to the escaping ions, and that the percentage decreases as the available energy in the upstream solar wind increases. This means that, today, the Venusian induced magnetosphere is very efficient at protecting its atmosphere from being stripped by the solar wind. A comparison with a similar study made at Mars, which has crustal magnetic fields but no intrinsic magnetic field, shows a similar trend to that at Venus, but the energy transfer from the solar wind to the Martian ionospheric ions is more effective. A similar study at Earth, which has an intrinsic magnetic field, showed a different response of its escape to the variations in the energy of the upstream solar wind, than both Venus and Mars. Here, we will show the coupling between the solar wind and oxygen ion escape for Venus, and compare it with the similar studies made at Mars and Earth.
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Created 2021-06-10 16:04:34 by Mats HolmstrÃ¶m Last changed 2021-06-10 16:04:34 by Mats HolmstrÃ¶m