Solar wind ions inside the induced magnetosphere of Mars (PhD defence)
The subject of the thesis is analysis and modeling of the entry, transport, and atmospheric precipitation of solar wind ions, H+ and He2+, into the induced magnetosphere of Mars. The solar wind is a flow of charged particles emitted by the Sun. The solar wind carries with it a magnetic field, the interplanetary magnetic field (IMF). The IMF piles up on the dayside of the non-magnetized Mars and is then convected towards the nightside. The solar wind ions can normally not cross the magnetic barrier, formed by the pile up IMF. However, in situ observations by the Mars Express spacecraft reveal that downward moving solar wind H+ and He2+ are sometimes present in the Martian ionosphere, below the magnetic barrier. The gyroradii of shocked solar wind ions may be comparable to the size of the dayside Martian magnetic barrier and for certain circumstances, these ions can gyrate through. Observations by Mars Express are used to analyze H+ and He2+ penetrating through the magnetic barrier and precipitating into the Martian ionosphere, identified by the presence of ionospheric photo-electrons. A case study shows evidence of narrower energy distributions for H+ (with energy ≥ solar wind energy), as the spacecraft moves down in altitude. From this, the study concludes that the magnetic barrier prevents the lower energy H+, from reaching low altitudes. The thesis also describes a statistical study of precipitating H+ fluxes, which indicate that H+ precipitation is rare (detected during 3% of the dayside observation time only) and carries on average 0.2% of the upstream solar wind particle flux. In another statistical study, the thesis shows that the precipitation of H+ and He2+ decreases even further when Mars encounters solar wind pressure pulses. A possible explanation is that the enhanced mass loading of the magnetic field flux tubes by planetary heavy ions, while the tubes drag through the ionosphere at lower altitudes, slows down their velocity and allows more magnetic flux to pile up. The magnetic barrier becomes a more effective obstacle to the solar wind ion precipitation. Furthermore, the thesis describes a model of H+ precipitation onto the Martian upper atmosphere using a hybrid code of the Mars solar wind interaction. The spatial patterns of the precipitation depend on the H+ energy, on the H+ origin (solar wind or generated from the hydrogen corona) and on the altitude. Some features of the observed H+ distributions are reproduced by simulations, while others are not, indicating a more complex physics than in the model. The thesis also describes a model study of transport of H+, fast H atoms and He2+ through the atmosphere using a Direct Simulation Monte Carlo model. This study demonstrates the crucial role of the magnetic field in determining the energy deposition of the solar wind ions in the topside atmosphere. For instance, a horizontal magnetic field with strength of 50 nT backscattered almost all H+, thus preventing these particles to deposit their energy at lower altitudes. The conclusion of the thesis work is that although some solar wind ions do precipitate, the magnetic barrier effectively protects the ionosphere from precipitating solar wind ions.
Publication date Dec 2012
Place of publication Lule√•
Publisher Lule√• tekniska universitet
ISBN (print) 978-91-7439-525-9
Name Doctoral thesis / Lule√• University of Technology
ISSN (print) 1402-1544