The influence of magnetic topology on ionospheric structure at Mars: Observations of localized “magnetic depletions”

The interaction between Mars' crustal magnetic fields and the solar wind produces a variety of magnetic topologies whose characteristics depend upon the plasma regions that the magnetic field is embedded in. We utilize in-situ Mars Atmosphere And Volatile EvolutioN (MAVEN) measurements to identify localized ionospheric structures, observed as the spacecraft flies through this patchwork of different magnetic topologies. Events are characterized by sharp ‘drop outs’ in magnetic field strength that we term ‘magnetic depletions’. The plasma pressure dominates within magnetic depletions, while the magnetic pressure typically dominates outside of them. Abrupt changes in magnetic topology are coincident with the depletion boundaries. A preliminary statistical study spanning 3 months shows that events occur on ∼4% of MAVEN orbits, between altitudes of 170–360 km. Ionospheric electrons are collisionless and thus magnetized at these altitudes, and combined with the fact that magnetic diffusion timescales range from minutes to an hour, these characteristics suggest that such structures can be observed sporadically by MAVEN on its ∼4.5 hour orbit before being smeared out by magnetic diffusion. At lower altitudes high collision rates lead to diffusion timescales of seconds, while at higher altitudes electromagnetic waves, instabilities and other transport processes driven by the Mars-solar wind interaction can distort the magnetic field, making magnetic depletion events difficult to identify. Magnetic depletions highlight the ability of magnetic topology to drive localized ionospheric structure at Mars, a result that stems from the unique interaction between the solar wind, Mars' crustal magnetic fields, and it's ionosphere.