Mars Crustal Magnetic Fields and Large-Scale Dayside Ionospheric Anomaly

Today's Mars has no intrinsic dipole magnetic field but locally concentrated magnetic anomalies in the crust particularly in the southern hemisphere. It has been found by previous satellite observations that the Martian plasma density on the dayside tends to be enhanced in regions of relatively strong crustal fields. However, due to insufficient coverage of these in-situ measurements below 200 km altitudes, the crustal field control of the main ionospheric region remains largely unknown. In this study, we assess the ionospheric effects of the crustal field using a 3-D multispecies MHD model together with observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. The MHD model is run under quiet average solar wind and solar EUV conditions and in a time-dependent manner by considering the continuous rotation of the planet and its attached crustal magnetic fields. Our model results reproduce the observed dayside ionospheric densities above 200 km altitude, which are significantly enhanced over strong crustal field regions. However, an unexpected reverse pattern is revealed in the large-scale density anomaly within the main ionospheric domain; that is, the density below 150 km altitude is significantly depleted where the crustal field is intense and not significantly impacted in the transition region ( 150-200 km). To test our numerical findings, we collected and analyzed 2.5-year electron density measurements at SZA<75º by the Langmuir Probe and Waves instrument onboard MAVEN. The model-data comparison shows a good agreement on the ionospheric density enhancement at >200 km altitudes by strong crustal fields. Within the transition altitude range of 150-200 km, MAVEN data show that the crustal field induced ionospheric anomaly is weak and indiscernible, following a trend consistent with our MHD prediction. At altitudes lower than 150 km, the current data coverage of MAVEN is insufficient for a statistically meaningful investigation. All together, this study suggests that the Mars crustal fields are responsible for a large-scale ionospheric anomaly on the dayside, which involve the density enhancements and depletions over strong crustal field regions at high and low altitudes, respectively.