The induced magnetosphere of Mars: physical processes and consequences


Abstract: Planets and other solar system bodies, including satellites of the gas giant planets, which possess a conducting ionized upper atmosphere but no intrinsic dynamo magnetic field, are typically engulfed in draped magnetic field lines stemming from the surrounding field of the host star or planet. These so-called ‘induced magnetospheres’ modulate the interaction between the upper atmosphere of the planet or moon and the impinging solar wind or host-planetary plasma. Such an induced magnetosphere has been observed at Mars, and many of the features of ion outflow from the planet, plasma heating in the upper atmosphere etc., are intricately coupled to the intensity and geometry of the draped field within the induced magnetosphere in response to solar wind variations. The ‘history’ of the impinging solar wind is then of great importance in governing the dynamics of its interaction with the Martian upper atmosphere, as the induced magnetosphere it generates is constantly evolving. Furthermore, little is understood of the mechanisms by which significant rapid variations in the upstream solar wind influence the planet’s interaction with the surrounding medium, for example through the enhancement of plasma heating, ion acceleration, and three dimensional current flow within the coupled system. The most powerful disturbances are interplanetary coronal mass ejections (ICMEs), intermittent magnetic bubbles containing dense hot solar plasma, and recurrent co-rotating interaction regions (CIRs), in which a density shock and magnetic structure forms between regions of slow and fast solar wind. In contrast to Venus and Titan the topology of the Martian magnetosphere is unique as it is also disturbed by strong magnetic fields of crustal origin, which can extend outside the magnetic pile-up boundary of the magnetosphere induced by the solar wind. A recent study has shown that the ability for ionospheric currents to flow perpendicular to the magnetic field is very different in regions above magnetic anomalies from regions where there is only an induced magnetosphere, implying strong current divergence at any boundary between these regimes. As the draped magnetosphere strongly depends on the solar wind conditions, while the unchanging crustal magnetic fields rotate into and out of the dayside interaction region with the solar wind, one can expect intense dynamics in the ionospheric current flow in response to solar wind variations.


So far it has been difficult to relate any observation of plasma dynamics at Mars to solar wind drivers, but in March and April in 2010 (for the first time) and again in 2012 (for the second time) the members of this team have carried out coordinated campaigns, when all four plasma instruments on Mars Express (ASPERA, MaRS, MARSIS and SPICAM) have been operated in a coordinated manner, either more frequently, at higher cadence or better performance. During both intervals, Mars was located behind Earth on the so-called Parker spiral of heliospheric magnetic field, allowing the use of data from near-Earth satellites, e.g. ACE, to relate any observed dynamics in the upper atmosphere at Mars to solar wind disturbances previously passing Earth. For this most recent study MARSIS observations have been organised with significantly improved cadence giving uninterrupted sequences over several days. During the first campaign we have been able to identify a large number of moderate solar wind disturbances related to CIRs and some intense CMEs, during a period which was mostly characterised by a very inactive, solar minimum like, solar wind, while during the presently ongoing campaign the solar wind is more active as expected for near solar maximum conditions. The overall goal of the work described here will be to take these observations by Mars Express, along with earlier observations by MGS further, and through the use of modelling work, develop our understanding of the rich and complex physical processes of the Martian ionosphere and plasma environment and the consequences for the rest of the Martian atmosphere. The work that we envisage undertaking can be divided into three main parts:


Comparison of the overall behaviour of the Induced Magnetosphere at Mars during solar minimum and solar maximum conditions, focusing on the effects of basic solar wind structures,


Characteristic features of solar-wind/planet interaction caused by magnetic field and density structures in CIRs and CMEs.


The special role of magnetic crustal anomalies on the solar wind coupling, including possibilities for bulk loss processes and reconnection.