Mars Global Distribution of the External Magnetic Field and Its Variability: MAVEN Observation and MHD Prediction

摘要

We study the average global distribution of the external magnetic field at Mars, and its variability with the upstream solar wind dynamic pressure and interplanetary magnetic field as well as with the ambient crustal magnetic field strength. Our approach involves excluding the intrinsic planetary field from the total magnetic field by applying a crustal field model previously derived using low altitude measurements. The distribution of the average external field that remains is statistically analyzed using nearly 8 years of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations and several global, time-dependent magnetohydrodynamic simulations. Overall consistent results have been obtained from the data and model, which are complementary to each other and cross validate the findings. It is found that the external field is significantly enhanced from the upstream across the bow shock (BS) and further intensifies closer to the planet in the topside ionosphere. It peaks at similar to 170 km altitude near the subsolar point, significantly decreasing with increasing solar zenith angle. There is a strong day-night asymmetry in the external field, with a typical dayside intensity of similar to 15-50 nT and a nightside intensity of similar to 5-15 nT. Under high solar wind dynamic pressures and IMFs, the external field may be enhanced by a factor of similar to 2 everywhere below the BS, on both the dayside and nightside. In addition, our model results suggest that strong crustal fields, which effectively withstand the penetration of the solar wind, reduce the external field at low altitudes.