MAGNETOSPHERE OF MARS AT ALMOST RADIAL INTERPLANETARY MAGNETIC FIELD

[ We have studied the structure of the Martian magnetosphere under conditions of an almost radial interplanetary magnetic field (IMF). Under such conditions, the currents driven by the motional electric field $-\frac{1}{c}\mathbf{V}_{sw} \times \mathbf{B}_{IMF}$ are expected to be strongly reduced, and the induced magnetosphere may degenerate. We analyzed Mars Atmosphere and Volatile EvolutioN (MAVEN) data for orbits during which the IMF cone angle was below $ 20^{\circ} $ or larger than $160^{\circ}$. We find that the main features of the Martian magnetosphere, observed for the nominal Parker spiral IMF configuration at Mars’ orbit, remain present even for nearly field-aligned plasma flow. The IMF cone angle begins to change at the bow shock, exhibiting large fluctuations but with a clear tendency for the magnetic field lines to envelop the ionospheric obstacle. This process is accompanied by a pileup of the magnetic field and the formation of a magnetic barrier that balances the thermal pressure of the shocked solar wind. Only on very rare orbits do we observe sunward ejection of ions from the ionosphere and their strong interaction with the solar wind. Not a perfect alignment of the IMF and the solar wind flow and the existence of a strong crustal magnetic field are probably responsible for a creation of a classical induced magnetosphere even at such a geometry. ]