Evolution of Force‐Free Magnetic Structure in the Dayside Ionosphere of Mars

Abstract Magnetic fields measured in the ionosphere of Mars by the magnetometers onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) and the Mars Global Surveyor (MGS) spacecraft display a wide range of spatial scales, ranging from global to small‐scale. Magnetic structures advect with the plasma flow from the solar wind into the ionosphere via the magnetosheath and the pile‐up regions. Structures evolve as they move deeper into the ionosphere. Helical force‐free rope‐like structures are frequently observed at lower altitudes in the ionospheres of both Venus and Mars. Force‐free means that the magnetic forces on a structure are zero (or are very small). MAVEN data and simple magnetohydrodynamic (MHD) analysis are used to study structure evolution in the dayside ionosphere. The Kelvin‐Helmholtz instability is shown to be possible for altitudes greater than about 300 km and the Rayleigh‐Taylor instability possible for altitudes below about 250 km. The study also explains how structures can become force‐free due to the Hall and ambipolar electric field terms in the generalized Ohm's law. Timescales for structure evolution are estimated. The role of force‐free ropes and associated field‐aligned currents in linking different regions of the ionosphere is discussed.