MAVEN observations of high-altitude magnetic flux ropes around Mars
Abstract:Magnetic flux ropes are characteristic twisted helical magnetic field structures seen in throughout the solar system. They have been observed even around unmagnetized planets, such as Venus and Mars. In the case of Mars, the origin of Martian flux ropes is owing to not only the interplanetary magnetic field and associated draped magnetic fields, but also crustal magnetic fields. Planetary ions are energized through the direct interaction of the solar wind with the upper atmosphere, resulting in ion escape into interplanetary space. Hence, magnetic flux ropes can contribute to the ion escape rates, because they may be able to trap large amounts of the Martian ionospheric plasmas.
Here we present simultaneous MAVEN plasma and magnetic field observations with respect to a magnetic flux rope around Mars. This event was observed in the post-dusk southern hemisphere of the Martian induced magnetosphere, where is roughly downstream from strong crustal magnetic fields. The observed altitude was approximately 2500 km, where is more than 5 times higher than those previously detected by Mars Global Surveyor. MAVEN observed a pronounced ion composition change across the structure, with solar wind ions dominating outside and planetary ions dominating within. Therefore, multiple planetary ions species (e.g., H+, O+, and O2+) tend to be included inside the flux rope. These results infer that the observed flux rope was formed in a region with access to planetary ions but to not magnetosheath ions One likely such location is on closed magnetic field lines within the Martian ionosphere. We applied the Grad-Shafranov reconstruction (GSR) technique to the observed flux rope, in order to estimate the two-dimensional spatial structure using the MAVEN simultaneous plasma and magnetic field data. The GSR results for this event showed that the estimated axial orientation is oriented approximately parallel to the Mars-Sun line. In this presentation, we will also report the statistical results of the high-altitude magnetic flux ropes observed by MAVEN from the beginning of its primary science phase (November 2014) to date in order to understand the global characteristics of these high-altitude magnetic flux ropes around Mars including their upstream solar wind dependences.