Degenerate induced magnetospheres: a distinctive mode of the solar wind interaction with non-magnetized bodies

Induced magnetospheres of non-magnetized atmospheric planets, like Mars and Venus, are formed by the ionospheric currents induced by the solar wind interaction with the gravitationally bound electrically conducting ionospheres. When the interplanetary magnetic field (IMF) is predominantly aligned to the solar wind velocity, the convective electric field diminishes, leading to a degeneration of the induced magnetosphere. We investigate a 4° cone angle case (angle between the solar wind velocity and the IMF) using observations from Mars Atmosphere and Volatile Evolution (MAVEN) and Mars Express (MEX), and a hybrid model of the solar wind interaction with Mars. The measurements and simulations are in agreement, allowing us to characterize the general structure of a degenerate induced magnetosphere. We find that no magnetic barrier is built. The am bipolar field defines the electrodynamics on the dayside and drives ionospheric ions up stream, forming a cloud of planetary ions. Solar wind protons propagates down to low altitudes and are lost through collisions with the neutral atmosphere, the respective magnetic field tubes deplete and a proton void form behind the planet. No bow shock is formed in the subsolar region, but shock-like structure form at the flanks. The most remarkable feature is a large cross-flow plume of planetary ions with a significant asymmetry, extending more than 10 Martian radii, driven by the ExB drift. The degenerate induced magnetospheres represent a distinctive mode in the solar wind interaction with non-magnetized planets and provide insight into the exoplanet-stellar wind interaction.